Artículos de revistas sobre el tema "DQ-NMR"

ABSTRACT A series of unfilled and stabilized natural rubber compounds varying in concentration of tetramethylthiuram disulfide (TMTD) was analyzed using rheometry, hardness, dynamic mechanical properties, stress–strain (Mooney–Rivlin), equilibrium solvent swell (Flory–Rhener), and low-field nuclear magnetic resonance (NMR) by the double quantum (DQ) technique. Crosslinking level increased proportionately with TMTD concentration, and the reaction ratio of three TMTD molecules producing one crosslink was generally upheld. Unreacted TMTD acted as a pseudo-plasticizer and lowered the chain entanglement density with increasing TMTD content. DQ NMR confirmed that the elastic network was homogeneous and that the absolute chemical crosslink distributions broaden with increasing curative level. Upon mild heat aging, zinc complexes based on TMTD/ZnO are likely responsible for causing additional crosslinking, explaining the rise in crosslink density by equilibrium solvent swell and DQ NMR. The amine-based antioxidant, the generation of thiocarbamate radicals from TMTD, and the heat stability of the predominant monosulfide crosslinking system helped to limit network breakdown through chain scission. The chain entanglement increase is likely due to reduction of the plasticizing effect caused by unreacted curative. The distribution of crosslinks slightly broadens toward higher total crosslink density because of the generation of additional chemical crosslinks and chain entanglement densification.

Abstract N,N,N-Trimethylchitosan (TMC) represents a rare example of cationic polysaccharides and numerous studies have shown its potential in biological and biomedical applications. TMC with high degrees of quaternization (DQ) were synthesized from N-methylation of N,N-dimethylchitosan (DMC), which was obtained by reductive alkylation of high molecular weight chitosan in a simple step process and in good yields. The effects of base and solvents were evaluated on the quaternization reaction. The N-methylation of DMC was performed selectively by CH3I and carbonate in water where quaternization was achieved quantitatively with a low degree of O-methylation (17 %). Moreover, the greener procedure allows easy recovery and purification by conventional filtration as a carbonate salt, in which the anion can be exchanged by an acid-base reaction. Quantification of DQ involving 1H NMR integration of methyl peaks must be performed on protonated TMC. High field NMR spectra of TMC showed two specific chemical shifts for anomeric peaks (5.0 and 5.4 ppm) that can also be used for the determination of DQ. This latter method avoids the superimposition problems with other pyranosyl peaks.

Melts of ultrahigh molecular weight polyethylene (UHMWPE) entangled significantly, suffering processing difficulty. In this work, we prepared partially disentangled UHMWPE by freeze-extracting, exploring the corresponding enchantment of chain mobility. Fully refocused 1H free induction decay (FID) was used to capture the difference in chain segmental mobility during the melting of UHMWPE with different degrees of entanglement by low-field solid-state NMR. The longer the polyethylene (PE) chain is in a less-entangled state, the harder the process of merging into mobile parts after detaching from crystalline lamella during melting. 1H double quantum (DQ) NMR was further used to obtain information caused by residual dipolar interaction. Before melting, the DQ peak appeared earlier in intramolecular-nucleated PE than in intermolecular-nucleated PE because of the strong constraints of crystals in the former one. During melting, less-entangled UHMWPE could keep disentangled while less-entangled high density polyethylene (HDPE) could not. Unfortunately, no noticeable difference was found in DQ experiments between PE melts with different degrees of entanglement after melting. It was ascribed to the small contribution of entanglements compared with total residual dipolar interaction in melts. Overall, less-entangled UHMWPE could reserve its disentangled state around the melting point long enough to achieve a better way of processing.

The formation and steric structure of the inclusion complex of cucurbit[n]uril (CB[n]; n = 6, 7) and diquat (DQ) were investigated through NMR measurements under the pH conditions of human pseudo-gastric or body fluids, in physiological saline.

Chain-level structure of semicrystalline polymers in melt- and solution-grown crystals has been debated over the past half century. Recently, 13C–13C double quantum (DQ) Nuclear Magnetic Resonance (NMR) spectroscopy has been successfully applied to investigate chain-folding (CF) structure and packing structure of 13C enriched polymers after solution and melt crystallization. We review recent NMR studies for (i) packing structure, (ii) chain trajectory, (iii) conformation of the folded chains, (iv) nucleation mechanisms, (v) deformation mechanism, and (vi) molecular dynamics of semicrystalline polymers.

ABSTRACT The concentrations of triallyl isocyanurate (TAIC) in a peroxide-curable fluoroelastomer terpolymer containing 67 wt% of fluorine were varied to generate compounds of differing crosslink densities. Experimental analysis was undertaken using rheometry, hardness, stress–strain (Mooney–Rivlin), equilibrium solvent swell, and low-field nuclear magnetic resonance (NMR) using the double quantum (DQ) technique. Increasing the TAIC concentration caused a systematic rise in rheometry elastic torque, hardness, and tensile strength, whereas both elongation at break and swelling levels decreased. These results are concurrent with an enhanced overall level of crosslinking, which was confirmed by the steady increase of the Mooney–Rivlin C1 values. DQ NMR analysis using hydrogen and fluorine probes and subsequent application of fast Tikhonov regularization to the corrected intensity data were particularly useful in discerning the inhomogeneous nature of the compound morphology. The spatial distribution of the crosslink density suggests that the compound consists of small, highly crosslinked/entangled polymerized TAIC domains embedded within the elastic crosslinked matrix. A concentration of 3 phr of TAIC is optimal according to compression set testing.

3-(4-X-Phenyl)-3a,4,6,6a-tetrahydrofuro[3,4-d]isoxazoles Ia-If (X = H, CH3, CH3O, Cl, F, C6H5) react with molybdenum hexacarbonyl to give not only the expected cis-3-aroyl-4-hydroxytetrahydrofuran III, but also its trans isomer IV and 3-aroyl-2,5-dihydrofuran II. This paper concerns a new preparation of 3-aroyl-2,5-dihydrofuran derivatives starting from 2,5-dihydrofuran via 1,3-dipolar cycloaddition of nitrile oxides, cleavage with molybdenum hexacarbonyl and dehydratation with p-toluenesulfonic acid. The structure of cis-(III) and trans-(IV) derivatives was deduced from both the γ-effect in the 13C NMR and the DQ COSY experiment in the 1H NMR spectra.

Understanding the structural evolution process after the yielding of networks in polymer nanocomposites can provide significant insights into the design and fabrication of high-performance nanocomposites. In this work, using hydroxyl-terminated 1,4-polybutadiene (HTPB)/organo-clay nanocomposite gel as a model, we explored the yielding and recovery process of a polymer network. Linear rheology results revealed the formation of a nanocomposite gel with a house-of-cards structure due to the fully exfoliated 6 to 8 wt% organo-clays. Within this range, nonlinear rheologic experiments were introduced to yield the gel network, and the corresponding recovery processes were monitored. It was found that the main driving force of network reconstruction was the polymer–clay interaction, and the rotation of clay sheets played an important role in arousing stress overshoots. By proton double-quantum (1H DQ) NMR spectroscopy, residual dipolar coupling and its distribution contributed by HTPB segments anchored on clay sheets were extracted to unveil the physical network information. During the yielding process of a house-of-cards network, e.g., 8 wt% organo-clay, nearly one-fourth of physical cross-linking was broken. Based on the rheology and 1H DQ NMR results, a tentative model was proposed to illustrate the yielding and recovery of the network in HTPB/organo-clay nanocomposite gel.

Polymer networks were prepared by Steglich esterification using poly(sorbitol adipate) (PSA) and poly(sorbitol adipate)-graft-poly(ethylene glycol) mono methyl ether (PSA-g-mPEG12) copolymer. Utilizing multi-hydroxyl functionalities of PSA, poly(ethylene glycol) (PEG) was first grafted onto a PSA backbone. Then the cross-linking of PSA or PSA-g-mPEG12 was carried out with disuccinyl PEG of different molar masses (Suc-PEGn-Suc). Polymers were characterized through nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). The degree of swelling of networks was investigated through water (D2O) uptake studies, while for detailed examination of their structural dynamics, networks were studied using 13C magic angle spinning NMR (13C MAS NMR) spectroscopy, 1H double quantum NMR (1H DQ NMR) spectroscopy, and 1H pulsed field gradient NMR (1H PFG NMR) spectroscopy. These solid state NMR results revealed that the networks were composed of a two component structure, having different dipolar coupling constants. The diffusion of solvent molecules depended on the degree of swelling that was imparted to the network by the varying chain length of the PEG based cross-linking agent.

From the ethereal extract of Melilotus neapolitana Ten., an herbaceous plant living in the Mediterranean macchia, a new aromatic metabolite was isolated. The structure 1-(2,4-dihydroxyphenyl)2-(4-hydroxy-2-methoxyphenyl)1,3-propandiol, was elucidated on the basis of its mass spectrum and 1D and 2D NMR spectroscopic data (DQ-COSY, TOCSY, HSQC, HMBC). The relative configuration was determined from NOESY/ROESY experiments and computational calculations. The compound was tested for its potential allelopathic effect on two wild plant species living in the same habitat as M. neapolitana.

NMR is a powerful spectroscopic method that can provide information on the structural disorder in solids, complementing scattering and diffraction techniques. The structural disorder in solids can generate a dispersion of local magnetic and electric fields, resulting in a distribution of isotropic chemical shift δiso and quadrupolar coupling CQ. For spin-1/2 nuclei, the NMR linewidth and shape under high-resolution magic-angle spinning (MAS) reflects the distributions of isotropic chemical shift, providing a rich source of disorder information. For quadrupolar nuclei, the second-order quadrupolar broadening remains present even under MAS. In addition to isotropic chemical shift, structural disorder can impact the electric field gradient (EFG) and consequently the quadrupolar NMR parameters. The distributions of quadrupolar coupling and isotropic chemical shift are superimposed with the second-order quadrupolar broadening, but can be potentially characterized by MQMAS (multiple-quantum magic-angle spinning) spectroscopy. We review analyses of NMR lineshapes in 2D DQ–SQ (double-quantum single-quantum) and MQMAS spectroscopies, to provide a guide for more general lineshape analysis. In addition, methods to enhance the spectral resolution and sensitivity for quadrupolar nuclei are discussed, including NMR pulse techniques and the application of high magnetic fields. The role of magnetic field strength and its impact on the strategy of determining optimum NMR methods for disorder characterization are also discussed.

A single-crystal X-ray diffraction structure of a 1:1 cocrystal of two fungicides, namely dithianon (DI) and pyrimethanil (PM), is reported [systematic name: 5,10-dioxo-5H,10H-naphtho[2,3-b][1,4]dithiine-2,3-dicarbonitrile–4,6-dimethyl-N-phenylpyrimidin-2-amine (1/1), C14H4N2O2S2·C12H13N2]. Following an NMR crystallography approach, experimental solid-state magic angle spinning (MAS) NMR spectra are presented together with GIPAW (gauge-including projector augmented wave) calculations of NMR chemical shieldings. Specifically, experimental 1H and 13C chemical shifts are determined from two-dimensional 1H–13C MAS NMR correlation spectra recorded with short and longer contact times so as to probe one-bond C—H connectivities and longer-range C...H proximities, whereas H...H proximities are identified in a 1H double-quantum (DQ) MAS NMR spectrum. The performing of separate GIPAW calculations for the full periodic crystal structure and for isolated molecules allows the determination of the change in chemical shift upon going from an isolated molecule to the full crystal structure. For the 1H NMR chemical shifts, changes of 3.6 and 2.0 ppm correspond to intermolecular N—H...O and C—H...O hydrogen bonding, while changes of −2.7 and −1.5 ppm are due to ring current effects associated with C—H...π interactions. Even though there is a close intermolecular S...O distance of 3.10 Å, it is of note that the molecule-to-crystal chemical shifts for the involved sulfur or oxygen nuclei are small.

Schiff base of chitosan was obtained by the reaction of chitosan and aromatic aldehyde, and then Schiff base of chitosan reacted with methyl iodide to get soluble quaternary ammonium salts of chitosan which were N-metoxybenzenemethyl quaternary ammonium salt of chitosan (NM- Chitosan) and N-hydroxybenzenemethyl quaternary ammonium salt of chitosan(NH-Chitosan). The obtained chitosan derivatives were characterized by Infrared Spectra Analysis(FT-IR), Nuclear Magnetic Resonance Spectroscopy(1H NMR) and elemental analysis. Quantitative analysis showed that there was small difference between degrees of quaternization(DQ) and degrees of substitution(DS), and the solubility of chitosan derivatives was demonstrated to be better in the organic solvents.

The mononucleating hydrazone ligand LH3, a condensation product of salicyloylhydrazine and (2-formylphenoxy)acetic acid, was synthesized and its coordination behavior with first row transition metal(II) ions was investigated by isolating and elucidating the structure of the complexes using elemental analysis, conductivity and magnetic susceptibility measurements, as well as IR, 1H-NMR, electronic and EPR spectral techniques. The ligand forms mononuclear metal(II) complexes of the type [CoLH(H 2O)2], [NiLH(H 2O)2], [CuLH] and [ZnLH]. The ligand field parameters, Dq, B and ? values, in the case of the cobalt and nickel complexes support not only the octahedral geometry around the metal ion, but also imply the covalent nature of the bonding in the complexes. The EPR study revealed the presence of a spin exchange interaction in the solid copper complex and the covalent nature of the bonding. The 1H-NMR study of the zinc(II) complex indicated the non-involvement of the COOH group in the coordination. The physico-chemical study supports for the presence of octahedral geometry around cobalt(II), nickel(II) and tetrahedral geometry around copper(II) and zinc(II) ions. .

We report 1-(dimethylamino) pyrrole (PyDMA) as an electrolyte additive for high voltage lithium ion batteries based on LiNi0.6Mn0.2Co0.2O2 (NMC622)//Graphite with an upper cutoff voltage of 4.4 V. Density Functional Theory (DFT) modeling indicates that the unique structure of PyDMA could be effective in preventing the hydrolysis of LiPF6 in a carbonate electrolyte, mitigating issues related to HF formation. The calculations also indicated that the additive would oxidize at lower potentials than typical electrolyte solvents, which could lead to protective films at the cathode surface. These expectations were tested using Nuclear Magnetic Resonance (NMR) and extensive electrochemical characterization. NMR studies confirmed the superb dehydrating capability of PyDMA, which successfully prevents HF formation even at high water content. Addition of 0.5 wt% PyDMA resulted in improved capacity retention in full-cells, and also in lower levels of transition metal dissolution from the cathode. Incremental capacity (dQ/dV) analysis indicates that benefits of PyDMA at low concentration (0.5-1 wt%) are associated with decreased rates of Li+-trapping reactions, and that higher concentrations of the additive can lead to isolation of cathode domains. Our study indicates that PyDMA could be a promising electrolyte additive for high voltage lithium ion batteries at a low concentration.

Biocompatible gels based on poly(3-hydroxyalkanoate)s (PHAs) were developed by radical polymerization in the presence of poly(ethylene glycol) diacrylate (PEGDA). In order to elaborate cross-linked networks based on PEGDA and PHAs, several PHAs were tested; saturated PHAs, such as poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) or poly(3-hydroxyoctanoate) (PHO), and an unsaturated PHA, poly(3-hydroxyoctanoate-co-3-hydroxyundecenoate) PHOU. The PHAxPEGDA1-x networks obtained in this work were studied by FTIR, Raman spectroscopy, DSC, TGA and NMR. The microscopic structure varied according to the mass proportions between the two polymers. Time Domain 1H DQ NMR measurements demonstrated that in the case of the unsaturated PHA, it was chemically crosslinked with PEGDA, due to the presence of double bonds in the lateral groups. The organogels were able to swell in organic solvents, such as THF, up to 2000% and in water up to 86%. It was observed by rheological analysis that the stiffness of the networks was dependent on the content of PHA and on the degree of cross-linking. The biocompatible characters of PHOU and PEGDA were not affected by the formation of the networks and these networks had the advantage of being non-cytotoxic to immortalized C2C12 myoblast cells.

We report 1-(dimethylamino) pyrrole (PyDMA) as an electrolyte additive for high voltage lithium-ion batteries based on LiNi0.6Mn0.2Co0.2O2 (NMC622)//Graphite with an upper cutoff voltage of 4.4 V. Density Functional Theory (DFT) modeling indicates that the unique structure of PyDMA could be effective in preventing the hydrolysis of LiPF6 in a carbonate electrolyte, mitigating issues related to HF formation. The calculations also indicated that the additive would oxidize at lower potentials than typical electrolyte solvents, which could lead to protective films at the cathode surface. These expectations were tested using Nuclear Magnetic Resonance (NMR) and extensive electrochemical characterization. NMR studies confirmed the superb dehydrating capability of PyDMA, which successfully prevents HF formation even at high water content. Addition of 0.5 wt% PyDMA resulted in improved capacity retention in full-cells, and also in lower levels of transition metal dissolution from the cathode. Incremental capacity (dQ/dV) analysis indicates that benefits of PyDMA at low concentration (0.5–1 wt%) are associated with decreased rates of Li+-trapping reactions, and that higher concentrations of the additive can lead to isolation of cathode domains. Our study indicates that PyDMA could be a promising electrolyte additive for high voltage lithium-ion batteries at a low concentration.

The Paleocene Kongdian Formation coarse clastic rock reservoir in Bozhong Sag is rich in oil and gas resources and has huge exploration potential. However, the coarse clastic rock reservoir has the characteristics of a complex pore structure and strong heterogeneity, which restrict the accuracy of evaluating the reservoir’s physical properties, such as porosity and permeability, for field evaluation. Nuclear magnetic resonance (NMR) technology has become a popular methods for unconventional reservoir evaluation because it can obtain abundant reservoir physical property information and because of its ability to identify fluid characteristics information. The transverse relaxation time (T2) cutoff (T2C) value is an important input parameter in the application of NMR technology. The accuracy of the T2C value affects the accuracy of the reservoir evaluation. The standard method for determining the T2C value requires a series of complicated centrifugation experiments in addition to the NMR experiments, and its application scope is limited by obtaining enough core samples. In this study, 14 core samples from the coarse clastic rock reservoir in the southwestern Bozhong sag of the Bohai Bay Basin were selected, and NMR measurements were carried out under the conditions of fully saturated water and irreducible water to determine the T2C value. Based on the multifractal theory, the NMR T2 spectrum of the saturated sample was analyzed, and the results show that the NMR T2 distribution of the saturated sample has multifractal characteristics, and the multifractal parameter Dq and the singular intensity range Δα have a strong correlation with the T2C value. Thus, based on multiple regression analyses of the multifractal parameters with the experimental T2C value of 10 core samples, we propose a method to predict the T2C value. After applying this method to 4 samples that were not used in the modeling, we confirmed that this method can be used to predict the T2C value of core samples. Furthermore, we expanded this method to the field application of a production well in Bozhong sag by adding an empirical index in the model. The new model can be used to directly calculate the T2C value of NMR logging data, and it does not require any other extra data, such as those from core analysis. This method is applicable in fast reservoir evaluations by only using NMR logging data in the field. The research results improve the accuracy of field NMR logging reservoir evaluations.

Three novel biologically active quaternized sodium alginates were synthesized via the reaction of sodium alginate (SA) with 3-chloro-2-hydroxypropyl trimethylammonium chloride, at room temperature for different time intervals (1, 3 and 6 h), to produce quaternized sodium alginates designated as QSA1, QSA3 and QSA6. The percentage degree of quaternization (DQ%) significantly increased with increasing the reaction time. Images from FTIR, 1H-NMR, XRD and SEM have confirmed the chemical structures of the QSA samples. Their antimicrobial activity was investigated against bacteria and fungi using XTT assay, and the results showed that all QSA samples displayed high growth inhibition capacity of the tested microorganisms, compared to zero inhibition for SA, as shown by their lower minimum inhibitory concentration (MIC). The QSA6 was the best antimicrobial composite, displaying the same MIC value as that of the used reference drugs. The developed composites were found to be safe on normal human fibroblast cells (WI-38 cell line), by evaluating them using cytotoxic activity measurement, which makes QSA a promising material in biomedical and food applications.

Chitosan has the potential to act as mediators of DNA transfection targeted to phagocytic cells such as macrophages, and to protect against biological degradation by nucleases as well as enhance gene expression. However, the poor solubility of Chitosan is the major limiting factor in its utilization. 2-hydroxypropyltrimcthyl ammonium chloride Chitosan has be prepared successfully through covalent binding of 2,3-Epoxypropyltrimethylammonium chloride ligands to the polymer’s primary amino groups and the polymer’s structure was verified with FT-IR spectra and NMR spectra. The new polymers were obtained with degree of quaternization (DQ) values around 34%, except in the case of the Phe-derived polymer, and thus possess reduced net positive charge as compared to the parent Chitosan. This study provided the new peptide-Chitosans with full water-solubility over practically the entire physiological pH range and led to more disordered. Globally, the new peptide Chitosans and especially the Asp-derived polymer, possess physico-chemical properties that turn them into promising candidates as novel Chitosan-based vaccine delivery systems.

Background: The effect of different concentrations of the absorption enhancer Trimethyl Chitosan (TMC) to the physicochemical properties of Large Unilamellar Vesicles (LUV) comprised of L-a-Phospahtidyl Choline (PC) were investigated in the current study. Methods: The Degree of Quartenization (DQ) of trimethylchitosan was assessed with nuclear magnetic resonance (1H NMR). The vesicles were characterized by means of Dynamic Light Scattering (DLS), ζ-potential, Differential Scanning Calorimetry (DSC) and Contact Angle Goniometry (CAG) measurements. Results: The data showed that the surface charge of the PC liposomes was significantly altered as a function of the TMC concentration, giving evidence of presence of the polyelectrolyte to the liposome’s membrane. Varying the concentration of TMC affected the phase Transition Temperature (Tm) of the lipid, verifying the miscibility of the polyelectrolyte with the lipid bilayer. The association of the polymer with the liposomes was related to the amount of the polyelectrolyte present, reflecting changes to the wettability of the dispersion as measured by CAG. Conclusion: The results demonstrated that presence of TMC significantly modified the physical properties of liposomes. Such systems might have a potential use for mucosal delivery (e.g. nasal route of administration).

A precise evaluation of the fluid movability of coal sedimentary rock is crucial to the effective and secure utilization of coal measures gas reserves. Furthermore, its complex pore structure and diverse mineral components impact the flow properties of fluids in pore structures, causing accurate evaluation of fluid mobility to be extremely challenging. Nuclear magnetic resonance (NMR) technology is currently a prevalent technique to assess unconventional reservoirs due to its capacity to acquire abundant reservoir physical property data and determine fluid details. The free-fluid volume index (FFI) is a crucial factor in assessing fluid movability in the application of NMR technology, which can only be derived through intricate NMR saturation and centrifugation experiments This research utilized nuclear magnetic resonance (NMR) tests on 13 classic coal-measure sedimentary rock samples of three lithologies to reveal the FFI value. Moreover, the association between mineral components, pore structure parameters, and FFI was then extensively analyzed, and a prediction model for FFI was constructed. The results indicate that the T2 spectra of sandstone and shale own a bimodal distribution, with the principal point between 0.1 and 10 ms and the secondary peak between 10 and 100 ms. The majority of the T2 spectra of mudstone samples provide a unimodal distribution, with the main peak distribution range spanning between 0.1 and 10 ms, demonstrating that the most of the experimental samples are micropores and transition pores. The calculated results of the FFI range from 7.65% to 18.36%, and depict evident multifractal properties. Porosity, the content of kaolinite, multifractal dimension (Dq), and the FFI are linearly positively correlated. In contrast, the content of chlorite, illite, multifractal dimension subtraction (Dmin − Dmax), multifractal dimension proportion (Dmin/Dmax), and singularity strength (Δα) possess a negative linear correlation with the FFI, which can be further used for modeling. On the basis of the aforementioned influencing factors and the FFI experimental values of eight core samples, an FFI prediction model was constructed through multiple linear regression analysis. The accuracy of the prediction model was validated by utilizing this approach to five samples not included in the model development. It was revealed that the prediction model produced accurate predictions, and the research findings may serve as a guide for the classification and estimation of fluid types in coal reservoirs.

When it comes to crystal structure determination, computational approaches such as Crystal Structure Prediction (CSP) have gained more and more attention since they offer some insight on how atoms and molecules are packed in the solid state, starting from only very basic information without diffraction data. Furthermore, it is well known that the coupling of CSP with solid-state NMR (SSNMR) greatly enhances the performance and the accuracy of the predictive method, leading to the so-called CSP-NMR crystallography (CSP-NMRX). In this paper, we present the successful application of CSP-NMRX to determine the crystal structure of three structural isomers of pyridine dicarboxylic acid, namely quinolinic, dipicolinic and dinicotinic acids, which can be in a zwitterionic form, or not, in the solid state. In a first step, mono- and bidimensional SSNMR spectra, i.e., 1H Magic-Angle Spinning (MAS), 13C and 15N Cross Polarisation Magic-Angle Spinning (CPMAS), 1H Double Quantum (DQ) MAS, 1H-13C HETeronuclear CORrelation (HETCOR), were used to determine the correct molecular structure (i.e., zwitterionic or not) and the local molecular arrangement; at the end, the RMSEs between experimental and computed 1H and 13C chemical shifts allowed the selection of the correct predicted structure for each system. Interestingly, while quinolinic and dipicolinic acids are zwitterionic and non-zwitterionic, respectively, in the solid state, dinicotinic acid exhibits in its crystal structure a “zwitterionic-non-zwitterionic continuum state” in which the proton is shared between the carboxylic moiety and the pyridinic nitrogen. Very refined SSNMR experiments were carried out, i.e., 14N-1H Phase-Modulated (PM) pulse and Rotational-Echo Saturation-Pulse Double-Resonance (RESPDOR), to provide an accurate N–H distance value confirming the hybrid nature of the molecule. The CSP-NMRX method showed a remarkable match between the selected structures and the experimental ones. The correct molecular input provided by SSNMR reduced the number of CSP calculations to be performed, leading to different predicted structures, while RMSEs provided an independent parameter with respect to the computed energy for the selection of the best candidate.

V. Rama Subbaraoa* aNatural Products Laboratory, Organic Chemistry Division-I, Indian Institute of Chemical Technology, Habsiguda, Hyderabad 500007, India. Study and optimization of Diels-Alder reaction of piperine in aqueous ionic solutions using Gn.HCl as a catalyst. The semi-synthesis of these products using intermolecular [4+2] cycloaddition reaction has been described. Obtained products were characterized using IR, HNMR, CNMR and Mass Spectroscopy. Introduction An outsized number of phenomena concern to and are conducted in liquid phase involving ionic species (Millions of years ago, Mother Nature discovered the secrets of water molecule) in different biological and other natural processes. Salt present in the oceans, a striking example from Nature, is a multi component salt solution reflecting the distant marine origin of life on earth together with the composition of physiological fluids. In general the ionic solutions play roles in several industrial and geological processes in addition to their deep impact on the biological molecules. This enormous power of ionic solutions is based on the interactions of ion with solvent. In this work, we present some interesting results with comprehensive implications on the application of ion-solvent (i-s) interactions on organic reactions. Ion-Solvent interactions Cohesion among molecules in the liquid phase results from intermolecular forces. These forces include hydrogen-bonding, dipole-dipole, multi polar, dispersion interactions and also interactions emerging from the repulsion between two molecules. The cohesion due to intermolecular forces gives rise to a 'pressure' which is experienced by the solvent molecules. A liquid undergoing a small, isothermal volume expansion does work against the cohesive forces which causes a change in the internal energy, U. The function (∂U/∂V)T, is called as internal pressure (Pi) of a liquid and is supported by the equation of state. Internal pressure increases upon the addition of some solutes like NaCl, KCI, etc. and decreases by salts like of guanidinium salts. Diels-Alder Reaction in aqueous medium For long time water was not a popular solvent for the Diels-Alder reaction. Before 1980 its use had been reported only incidentally. Diels and Alder themselves performed the reaction between furan and maleic acid in an aqueous medium in 1931,27 an experiment which was repeated by Woodward and Baer in 1948. 28 They noticed a change in endo-exo selectivity when comparing the reaction in water with ether. The extreme influence of water can exert on the Diels-Alder reaction was rediscovered by Breslow in 1980, much by coincidence 29,30 while studying the effect of β-cyclodextrin on the rate of a Diels-Alder reaction in water, accidentally. Schem 1. Alternatively, Grieco et al., have repeatedly invoked the internal pressure of water as an explanation of the rate enhancement of Diels-Alder reactions in these solvents. 31 They probably inspired by the well known large effects of the external pressure on rates of cycloadditions. However the internal pressure of water is very low and offers no valid explanation for its effects on the Diels-Alder reaction. The internal pressure is defined as the energy required bringing about an infinitesimal change in the volume of the solvent at constant temperature. Due to the open and relatively flexible hydrogen-bond network of water, a small change in volume of these solvents does not require much energy. A related, but much more applicable solvent parameter is the cohesive energy density. This quantity is a measure of energy required for evaporation of the solvent per unit volume. The reactions in water were less accelerated by pressure than those in organic solvents, which is in line with notion that pressure diminishes hydrophobic interactions. The effect of water on the selectivity of Diels-Alder reactions Three years after the Breslow report on the large effects of water on the rate of the Diels-Alder reaction, he also demonstrated that the endo-exo selectivity of this reaction benefits markedly from employing aqueous media. Based on the influence of salting-in and salting-out agents, Breslow pinpoints hydrophobic effects as the most important contributor to the enhanced endo-exo selectivity. Hydrophobic effects are assured to stabilize the more compact endo transition state more than the extended exo transition state. In Breslow option the polarity of water significantly enhances the endo-exo selectivity. In conclusion, the special influence of water on the endo-exo selectivity seems to be a result of the fact that this solvent combines in it three characteristics that all favors formation of the endo/exo adduct. 1. water is strong hydrogen bond donor 2. water is polar and water induces hydrophobic interactions. Study of salting-out and salting-in reagents towards the Diels-Alder reaction of piperine (1): The special effects of water as solvent for valuable Diels-Alder reaction (Scheme 1) of piperine (1), greatly altered by the addition of ionic solutes (Table 1) such as LiCl, LiBr, LiClO4,- NaCl, NaBr, KF, KCl, KBr, MgCl2, CaCl2, guanidinium chloride, guanidinium carbonate, guanidinium nitrate. Aqueous salts solutions accelerated cycloaddition reactions (Scheme 1) of piperine (1) to give resultant cycloadducts 2, 3 and 4 among them 2 is major ortho-exo cyclohexene type dimeric amide alkaloid and also known as chabamide, which is previously isolated from this plant, isomer 3 is also known adduct and previously isolated from Piper nigrum. 21 Cycloadduct 4 was synthesized from piperine by Diels-Alder reaction by Wei. et al. its physical and spectroscopic data were identical with reported data22 (1H-NMR & Mass spectra). Table 1: Study of different salts towards the Diels-Alder reaction of piperine (1). aOverall yield of adducts after HPLC, un-reacted piperine was recovered in all reactions. Reaction showed good overall yield and more exo selectivity. This reaction showed completely regioselectivity (yield of 2+3>4) due to maximum involvement of α-double bond rather than γ-double bond of 1 during Diels-Alder reaction. Table 2: Comparision of salting-out and salting-in reagents towards the Diels-Alder reaction of piperine (1). Study of Salting-out reagents Increased rate in Diels-Alder reaction (over all yield up to 79 %) of piperine (1) has been attributed to the hydrophobic effect. Owing to the difference in polarity between water and the reactants, water molecules tend to associate amongst themselves, excluding the organic reagents and forcing them to associate together forming small drops surrounded by water. A further method of increasing the rate of Diels-Alder reaction in water is so called ‘salting-out’ effect. Among the salting-out reagents used (Table 1) in this methodology CaCl2 is the best reagent and gave 79 % over all yield. If anion size increases, reaction yield decreases, where as cation size increases, reaction yield increases. Here a salt such as calcium chloride is added to the aqueous solution. In this case water molecules attracted to the polar ions, increasing the internal pressure and reducing the volume. This has the effect of further excluding the organic reagents. For reactions such as Diels-Alder, which have negative activation volumes, the rates are enhanced by this increase in internal pressure in much the same way as expected for an increase in external pressure. This salting-out reagent showed good exo selectivity, due to formation of cycloadduct 2 (ortho-exo) is major up to 69 % (cycloadduct ratio) compare to cycloadducts 3 (21 %, meta-exo) and 4 (10%, meta-exo) are poor in yield. Schem 2. Plausible mechanism of Diels-Alder reaction catalyzed by Gn.HCl. Study of Salting-in reagents Among the tested salting-in reagents used in this methodology (Table 1) guanidinium chloride (Gn.HCL) is the best reagent and gave 81 % overall yield, where as LiClO4 end up with only 15 % overall yield. Gn.HCL reagent exhibited well selectivity towards the Diels-Alder reaction of piperine in given conditions (scheme 1). Formation of cycloadduct 2 in 80 %, 3 in 15 % and 4 in 5 % ratio is clearly indicates this methodology received good attention towards the exo selectivity in Diels-Alder reaction of piperine. Overall yield is also high with salting-in reagents when compare to salting-out reagents. Procedure for aqueous ionic salts catalyzed Diels–Alder reactions of piperine (1): To a stirred mixture of piperine (1) (50.0 mg, 0.175 mmol), 6M aqueous guanidinium. Hydrochloride (2 mL) in a round bottom flask fitted with condenser and refluxed for 70 h in an oil bath. After completion of the reaction, monitored by TLC (dipped in 5% solution of phosphomolybdic acid in methanol and heating), the reaction mixture was cooled to room temperature and diluted with water (3 mL). Then extracted with EtOAc (2x5 mL), the combined organic layers were dried over anhydrous Na2SO4 and concentrated in vacuo. The residue obtained was then purified by reversed-phase (RP) HPLC (column: Phenomenex Luna C18, 250 x 10 mm, 10µ), solvent system: 80% acetonitrile in water, flow rate: 1.5 mL/min, to give pure compounds of adducts (2) 0.065 g, (3) 0.012 g and (4) 0.004 g. Cycloaddition reaction between piperine (1a) and pellitorine (1b): Our aim of this cycloaddition reaction is to explain to study different cycloadducts and selectivity of diene among piperine and pellitorine (Scheme 4). This biomimetic synthesis will explain the probability of diene, which participated in Diels-Alder reaction between piperine (1a) and pellitorine (1b) both were isolated from same plant (P. chaba). Nigramide N, which is formed biosynthetically via cycloaddition reaction between piperine and pellitorine, this adduct previously isolated from roots of P. nigrum 21 by Wei. et. al. Lewis acid catalyzed cycloaddition reactions of piperine (1a) and pellitorine (1b) under organic and aqueous solvent conditions to give resultant cycloadducts 2c, 3c, 4c, 2a and 3b. Cycloadduct 2c and 3c is new cycloadducts and their structures were illustrated by 1D and 2D spectral data. Structure elucidation of compound 2c: Compound 2c was obtained as pale yellow liquid. The molecular formula of 2c was established as C31H44N2O4 by HRESIMS (Fig-18), which provided a molecular ion peak at m/z 509.3381 [M++H], in conjunction with its 13C NMR spectrum (Fig-12). The IR spectrum displayed absorption bands diagnostic of carbonyl (1640 cm-1) (Fig-10). The 300 MHz 1H NMR spectrum (in CDCl3) indicated the presence of two signals at δ 5.86 (dd, J = 15.6, 10.1 Hz) and 6.27 (d, J = 15.6 Hz), which were assigned to trans-olefinic protons by the coupling constant of 15.6 Hz. It also displayed aromatic protons due to two 1, 3, 4-trisubstituted aromatic rings at δ 6.82 (1H, br s), 6.76 (1H, dd, J = 7.8, 1.4 Hz), 6.75 (1H, d, J = 7.8 Hz) (Fig-11), (Table 4). In addition to the above-mentioned moieties, combined inspection of 1H NMR and 1H–1H COSY revealed the presence of cyclohexene ring, one isobutylamide and one pyrrolidine ring. The 13C NMR spectrum (Fig-12) displayed the presence of 31 carbon atoms and were further confirmed by DEPT experiments (Fig-13) into categories of 11 methylenes, 12 methines and 5 quaternary carbons including two carbonyls (δ 173.01 and 172.50). On the basis of these characteristic features, database and literature search led the skeleton of compound 2c as a dimeric alkaloidal framework. A comprehensive analysis of the 2D NMR data of compound 2c facilitated the proton and carbon assignments. 1H–1H COSY spectrum (Fig-16) suggested the sequential correlations of δ 3.51 (dq, J = 5.0, 2.6 Hz)/5.62 (dt, J = 9.8, 2.6 Hz)/6.10 (ddd, J = 9.8, 1.5 Hz)/2.20 (m)/2.72 (ddd, J = 11.1, 10.1, 5.2 Hz)/3.35 (dd, J = 11.1, 9.8 Hz) assignable to H-2-H-3-H-4-H-5-H-3"-H-2" of the cyclohexene ring. Concerning the connections of the n-amyl and 3, 4-methylenedioxy styryl groups, HMBC spectrum (Fig-15) showed correlations of H-4, H-6, H-7/C-5; H-5", H-4"/C-3", which implies that these units were bonded to the cyclohexene ring at C-5 and C-3". Further, HMBC correlations of two methylene protons at δ 5.95 with 147.91 (C-8"), 146.87 (C-9"), confirmed the location of methylenedioxy group at C-8", and C-9". Remaining units, isobutylamine and pyrrolidine (rings) were connected through carbonyl groups at C-2 and C-2", which was confirmed by HMBC correlations of H-2 and H-1' to C-1 (δ 173.01) and H-2" and H-1''' to C-1" (δ 172.50). The assignment of the relative configuration of compound 2c, and confirmation of overall structure were achieved by the interpretation of the NOESY spectral data and by analysis of 1H NMR coupling constants. The large vicinal coupling constants of H-2"/H-2 (11.1 Hz) and H-2"/H-3" (11.1 Hz) indicated anti-relations of H-2"/H-2 and H-2"/H-3" and the axial orientations for these protons. In the NOESY spectrum (Fig-17), the occurrence of the correlations between H-2/H-3" and the absence of NOE effects between H-2/H-2" and H-2"/H-3" supported the above result. This data indicated β-orientation for H-2" and α-orientation for H-2 and H-3". The α-orientation of H-5 was suggested by the coupling constant of H-5/H-3" (5.2 Hz) and the absence of the NOESY correlations between H-3" and H-2". On the basis of these spectral data, the structure of compound 2c was unambiguously established and trivially named as chabamide M. Compound 3a: IR (KBr) nmax: 2923, 2855,1628, 1489, 1242, 1128, 1035 cm-1 d ppm 0.69 & 1.25 (2H, m, H-2'"), 1.15 & 1.23 (2H, m, H-4'"), 1.31 & 1.40 (2H, m, H-3'"), 1.52 (2H, m, H-2'), 1.56 (2H, m, 4'), 1.61 (2H, m, H-3'), 2.94 (1H, td, J = 10.1, 10.1, 5.5 Hz, H-3"), 3.02 & 3.60 (2H, m, H-5'"), 3.09 & 3.32 (2H, m, H-1'"), 3.51 (2H, m, H-1'), 3.61 (1H, m, H-2), 3.61 (2H, m, H-5'), 3.78 (1H, dq, J = 10.0, 2.3 Hz, H-5), 4.07 (1H, t, J = 10.1, H-2"), 5.72 (1H, ddd, J = 9.8, 5.0, 2.7 Hz, H-3), 5.88 (2H, s, H-12), 5.89 (1H, dt, 10.3, 1.8 Hz, H-4), 5.90 (1H, J =15.8, 9.8 Hz, H-4"), 5.92 (1H, s, H-12"), 6.37 (1H, d, J = 15.8 Hz, H-5"), 6.68 (1H, brs, H-7), 6.69 (1H, d, J = 8.0 Hz, H-10"), 6.70 (1H, dd, J = 8.0, 1.4 Hz, H-11), 6.69 (1H, d, J = 8.0 Hz, H-10), 6.74 (1H, dd, J = 8.0, 1.6 Hz, H-11"), 6.79 (1H, brs, H-7"). ESIMS (m/z): 571 [M+ +H] Table 4: 1H & 13C NMR data of cycloadduct 2c in CDCl3 (300 MHz, δ in ppm, mult, J in Hz) Compound 4a: IR (KBr) nmax: 2926, 2857,1627, 1484, 1440, 1240, 1034 cm-1 1H NMR (300 MHz, CDCl3): d ppm 0.81 & 1.35 (2H, m, H-2'), 1.29 & 1.47 (2H, m, H-4'), 1.35 (2H, m, H-2"'), 1.36 & 1.51 (1H, m, H-3'), 1.47 (2H, m, H-4"'), 1.51 (2H, m, H-3"'), 2.92 (2H, m, H-1"'), 2.99 (1H, ddd, J = 12.5, 9.7, 5.5 Hz, H-4"), 3.22 (2H, m, H-1'), 3.29 & 3.71 (2H, m, H-5'), 3.38 (1H, m, H-4"'), 3.44 (1H, dd, J = 12.1, 10.1 Hz, H-5"), 3.59 (1H, t, J = 5.3 Hz, H-5), 3.70 (1H, dq, J = 12.1, 2.1, H-2), 5.65 (1H, dd, J = 15.6, 9.5 Hz, H-3"), 5.70 (1H, dt, J = 9.9, 1.6, H-3), 5.81 (1H, d, J = 15.6 Hz, H-2"), 5.84 (1H, s, H-12"), 5.90-5.92 (2H, brs, H-12), 5.96 (1H, ddd, J = 9.2, 5.8, 2.6 Hz, H-4), 6.55 (1H, dd, J =7.9, 1.5 Hz, H-11"), 6.61 (1H, d, J = 8.2 Hz, H-10"), 6.62 (1H, d, J = 1.4 Hz, H-7"), 6.79 (1H, d, J = 7.9 Hz, H-10), 6.92 (1H, dd, J = 8.0, 1.5 Hz, H-11), 7.01 (1H, d, J = 1.5 Hz, H-7). ESIMS (m/z): 571 [M+ +H] Acknowledgements The authors are thankful to Director IICT for his constant encouragement and CSIR New Delhi for providing the fellowship References Braun, M. Synth. Highlights 1991, 232 Robinson, R. Chem. Soc. 1917, 762. Stork, G.; Burgstahler, A. W. Am. Chem. Soc. 1955, 38, 1890. Johnson, W. S.; Gravestock, M. B.; McCarry, B. E. Am. Chem. Soc. 1971, 93, 4332. Chapman, O. L.; Engel, M. R.; Springer, J. P.; Clardy, J. C. Am. Chem. Soc. 1971, 93, 6696. Bandaranayake, W. M.; Banfield, J. E.; Black, D. St. C. Chem. Soc., Chem Commun. 1980, 902. Nicolaou, K. C.; Zipkin, R. E.; Petasis, N. A. Am. Chem. Soc. 1982, 104, 5558. O.; Alder, K. Ann. 1928, 460, 98. Woodward, R. B.; Hoffmann, R. Chem. 1969, 81, 797. Fakui, K. Chem. Res. 1971, 4, 57. Houk, K. N. Chem.. Res. 1975, 8, 361. Houk, K. N.; Li, Y.; Evanseck, D. Angew Chem., Ed. Engl. 1992, 31, 682. Alder, K.; Stein, G. Chem. 1937, 50, 510. Fotiadu, F.; Michel, F.; Buono, G. Tetraheron Lett. 1990, 34, 4863. Gleiter, R.; Bohm, M. C. Pure Appl. Chem. 1983, 55, 237. Woodward, R. B.; Katz, T. J. Terahedron 1958, 5, 70. Kakushima, M. J. Chem. 1979, 57, 2564. Houk, K. N. Tetrahedron Lett. 1970, 30, 2621. Houk, K. N.; Luskus, L. J. Am. Chem. Soc. 1971, 93, 4606. Otto, S.; Bertoncin, F.; Engberts, J.B. F. N. Am. Chem. Soc., 1996, 118, 7702–7707. Wei, K.; Li, W.; Koike, K.; Chen, Y-J.; Nikaido, T. Org. Chem. 2005, 70, 1164. Wei, K.; Li, W.; Koike, K.; Chen, Y-J.; Nikaido, T. Lett. 2005, 7, 2833–2835. Rukachaisirikul, T.; Prabpai, S.; Champung, P.; Suksamrarn, A. Planta Med. 2002, 68, 850-853. Nagao, Y.; Seno, K.; Kawabata, K.; Miyasaka, T.; Takao, S.; Fujita, Tetrahedron Lett. 1980, 21, 841. Otto, S.; Boccaletti, G.; Engberts, J. B. F. N. Am. Chem. Soc. 1998, 120, 4238–4239. Otto, S.; Bertoncin, F.; Engberts, J. B. F. N. Am. Chem. Soc. 1996, 118, 7702–7707. O.; Alder, K. Ann. 1931, 490, 243. Woodward, R. B.; Baer, H. Am. Chem. Soc. 1948, 70, 1161. Breslow, R.; Rideout, D. C. Am. Chem. Soc. 1980, 102, 7816. Breslow, R.; Guo, T. Am. Chem. Soc. 1988, 110, 5613. Grieco, P.A.; Nunes, J. J.; Gaul, M. D. Am. Chem. Soc. 1990, 112, 4595.

Isolation, characterization of natural products dimeric amide alkaloids from roots of the Piper chaba Hunter. The synthesis of these products using intermolecular [4+2] cycloaddition reaction has been described. Obtained products were characterized using IR, 1HNMR, 13CNMR and Mass Spectroscopy. Introduction The awesome structural diversity and complexity of natural products inspire many chemists to consider how nature creates these molecules. Nature’s biosynthetic enzymes offer a powerful and practical route to many organic compounds, and synthetic chemists sometimes seek to imitate the efficiency and elegance of the biosynthetic machinery by designing biomimetic reactions that approximate natural reaction pathways. Probably the most astonishing biomimetic reactions1 are tandem processes that combine several transformations in sequence and produce complicated structures from comparably simple starting materials in a single laboratory operation. Biosynthesis is described as “the reaction or reaction sequence occurred in organism or its immediate environment will be viewed as biosynthesis” where as biomimetic synthesis describes as “A specific reaction or a sequence of reactions that mimic a proposed biological pathway is defined as bimimetic synthesis. An early example is Sir Robert Robinson’s landmark synthesis of tropinone in 1917.2 Forty-two years later, Gilbert Stork and Albert Eschenmoser independently proposed that the steroid ring system could be formed by tandem cation-π cyclizations of a polyene in an ordered transition state.3 A non-enzymatic version of this reaction type was demonstrated in W. S. Johnson’s classic synthesis of progesterone in 1971.4 Chapman’s synthesis of carpanone is a striking example of the power of biomimetic strategies.5 In 1980, Black proposed that the endiandric acids could arise biosynthetically from linear polyenes.6 In 1982, K. C. Nicolaou gave chemical support to Black’s hypothesis by chemically synthesizing endiandric acids A-G.7 Biomimetic Synthesis of Natural Products which involves, The biomimetic polyene carbocyclizations reaction, The biomimetic cycloaddition reaction, The biomimetic electrocyclization reaction, The polyether biomimetic synthesis, The biomimetic oxidative coupling of phenol, Some other interesting biomimetic synthesis, The present biomimetic synthesis of chabamides or dimeric amide alkaloids involves cycloaddition reactions. The Diels Alder reaction In the Diels-Alder reaction a six membered ring is formed through fusion of a 4 π component, usually a diene and a 2 π component which is commonly referred to as the Figure 1. dienophile. The Diels Alder reaction has proven to be great synthetic value, forming a key-step in the construction of compounds containing six-membered rings. Cyclohexene ring generated all the way through the formation of two new σ-bonds and one π bond with four adjacent stereocenters. The reaction is named after Otto Diels and Kurt Alder, two German chemists who studied the synthetic and theoretical aspects of this reaction in great detail.8 Their efforts have been rewarded with the 1950 Noble prize. Figure 2 Schematic representation of the Diels-Alder reaction. Cis principle In Diels-Alder reactions, the stereoselectivity is generally high due to the “cis principle”, which states that Diels-Alder reactions require a cisoid conformation for the diene and suprafacial-suprafacial mode of reaction, meaning that both ends of the diene attack from the same face of the dienophile in a syn fashion. Frontier Molecular Orbital (FMO) Approach Diels-Alder rections can be devided into, normal electron demand and inverse electron demand additions. This difference is based on the way the rate of the reaction responds to the introduction of electron withdrawing and electron donating substituents. Normal electron demand Diels-Alder reactions are promoted by electron donating substituents on the diene and electron withdrawing substituents on the dienophile. In contrast, inverse electron demand reactions are accelerated by electron withdrawing substituents on the diene and electron donating ones on the dienophile. There also exists an intermediate class, the neutral Diels-alder reaction, which is accelerated by both electron withdrawing and donating substitutents. The way the substituents affect the rate of the reaction can be rationalized with aid of Frontier Molecular Orbital (FMO) theory. This theory was developed during a study of the role of orbital asymmetry in pericyclic reactions by Woodward and Hoffmann9 and, independently, by fukui10 Later, Houk contributed significantly to the understanding of the reactivity and selectivity of these processes.11 The FMO theory states that a reaction between two compounds is controlled by the efficiency with which the molecular orbitals of the individual reaction partners interact. The interaction is most efficient for the reactivity is completely determined by interactions of the electrons that are highest in energy of the of the reaction partners (those in the Highest Occupied Molecular Orbital, the HOMO) with the Lowest Unoccupied Molecular Orbital (LUMO) of the other partner, applied to the Diels-alder reactions, two modes of interaction are possible. The reaction can be controlled by the interaction of the HOMO of the diene and the LUMO of the Dienophile (normal electron demand), or by the interaction between the LUMO of the diene and the HOMO of the dienophile (inverse electron demand), as illustrated in Fig-B. In the former case, a reduction of the diene-HOMO and dienophile-LUMO energy gap can be realized by either raising the energy of the HOMO of the diene by introducing electron donating substituents or lowering the energy of the dienophile LUMO by the introduction of electron donating substituents or lowering the energy of the dienophile LUMO by the introduction of electron withdrawing substituents. A glance at Fig-A confirms that in the formation of two new bonds, orbital symmetry is conserved so that, according to Woodward and Hoffmann, the reaction is concerted. In other words, no intermediate is involved in the pericyclic process such as the Diels-Alder reaction.12 This conclusion is consistent with a number of experimental observations. The cis or trans conformation of the dienophile is fully conserved in the configuration of the cycloadduct, which proves that there is no intermediate involved with a lifetime long enough to allow rotation around C-C bond. Selectivity can arise when substituted dienes and dienophiles are employed in the Diels-Alder reaction. Two different cycloadducts denoted as endo and exo are possible. Under the usual conditions their ratio is kinetically controlled. Alder and Stein already discerned that there usually exists a preference for formation of the endo isomer i.e formulated as tendency of maximum accumulation of unsaturation, (the Alder-Stein rule)13 Indeed, there are only very few examples of Diels-Alder reactions where the exo isomer is major product.14 The interactions underlying this behavior have been subject of intensive research. Since the reactions leading to endo and exo product share the same initial state, the difference between the respective transition-state energies fully account for the observed selectivity. These differences are typically in the range of 10-15 kJ per mole.15 Woodward and Katz16 suggested that secondary orbital interactions are of primary importance. These interactions are illustrated in fig-B for the normal electron demand (HOMO-diene, LUMO-dienophile controlled). The symmetry allowed overlap between π-orbital of the carbonyl group of the dienophile and the diene-HOMO is only possible in the endo activiated complex. Hence, only the endo transition state is stabilized so that the reaction forming the endo adduct is faster than that yielding exo product. This interpretation has been criticized by Mellor, who attributed the endo selectivity to steric interactions. Steric effects are frequently suggested as important in determining the selectivity of Diels-Alder reactions, particularly of α-subsituted dienophiles, and may ultimately lead to exo-selectivity. 17 For other systems, steric effects in the exo activated complex can enhance endo selectivity. 18 In summary, it seems for most Diels-Alder reactions secondary orbital interactions afford a satisfactory rationalization of the endo-exo selectivity. However, since the endo-exo ratio is determined by small differences in transition state energies, the influence of other interactions, most often steric in origin and different for each particular reaction is likely to be felt. The compact character of the Diels-Alder activated complex (the activation volume of the retro Diels-Alder reaction is negative) will attenuate these effects.19 Results and Discussions Chabamides F & G as dimeric amide alkaloids were isolated from this plant Piper chaba Hunter. These two dimers were formed by Diels-Alder reaction employing monomer trichostachine. This hypothesis was further confirmed by the mass spectrum, which showed a significant peak at m/z 294.113 [M++Na], assigned to the trichostachine ion arising by the Retro-Diels–Alder cleavage of molecular ion into two halves. Finally, to confirm the existence of the compounds F and G, we extracted the roots of P.chaba with MeOH at room temperature followed HPLC/electron spray ionization (ESI) MS experiments. In HPLC/ESIMS of the MeOH extract showed the presence of peaks at m/z 563 [M++Na] and 543 [M++1] at about 8.8 min and 10.6 min of LC retention time, respectively. To prove this biosynthetic hypothesis we have carried out the intermolecular [4+2] cycloaddition reaction with the trichostachine under solvent free conditions (Scheme 1). Reaction mixture was analysed by the LC-MS, which clearly indicted the presence of the compounds 1 and 2 (retention time and mass). In HPLC analysis, retention times of the synthetic 1 and 2 were identical to those of chabamide F and G, confirming the structure and stereochemistry are same as that of isolated alkaloids. Based on above result during Diels-Alder reaction of trichostachine, we developed two kinds of methodologies for this biomimetic synthesis of dimeric amide alkaloids based on catalytic. On the basis of a biosynthetic hypothesis (described in Chapter I) by the intermolecular Diels-Alder reaction, we chosen piperine (1a), pellitorine (1c) and trans-fagaramide (1c) as substrates to perform the biomimetic synthesis of the dimeric chabamides (Compound H-K) and this study also identified plausible products between piperine (1a) and pellitorine (1c). This study not only explains formation of cyclo adducts but also explains the different mechanistic aspects in Diels-Alder reaction (endo and exo products) of copper salts in aqueous medium. Under normal conditions only combinations of dienes and dienophiles that have FMO’s of similar energy can be transformed into a Diels-Alder adduct. When the gap between the FMO’s large, forcing conditions are required, and undesired side reactions and retro Diels-Alder reactions can easily take over. These cases challenge the creativity of the organic chemist and have led to the invention of a number of methods for promoting reluctant Diels-Alder reactions under mild conditions.20 Plausible mechanism for Diels-Alder reaction: Sijbren Otto. et. al studied extensively on copper (II) catalyzed Diels-Alder reactions on various moieties. 25, 26 Based on these reports we proposed plausible mechanism for this copper catalyzed Diels-Alder reaction. The first step in the cycle comprises rapid coordination of the lewis acid to the dienophile leading to a complex in which the dienophile is activated for reaction with the diene. The cycloadduct has dissociated from the lewis acid in order to make the catalyst available for another cycle. However we didn’t carry any kinetic studies to prove this mechanism. Plausible mechanism of Diels-Alder reaction catalyzed by copper (II) salts Use of lewis acids in Diels-Alder reaction is to lower LUMO dienophile energy to result in the decrease of the LUMO dienophile-HOMO diene gap (normal electron demand) or reduce LUMO diene energy to result in the decrease of the LUMO diene-HOMO dienophile gap (inverse electron demand). The presence of Lewis acids, the Diels-Alder dimerization of piperine, pellitorine, piperine with fagaramide, peperine with pellitorine, gave much lower combined yields in neat conditions. Wie et al. previously reported 21, 22 Diels-Alder reaction of piperine and in both thermal and by lewis acid of Co(II) Cl2.6H2O/P(Ph)3/Zn (1:10:10 mol %) in 3-octanol at 170oC with isomerised product (24 %) and 77 % over all yield. To find the optimum conditions towards the catalyst, piperine (1a) was taken to perform the Diels-Alder reaction in presence of variety of lewis acids and metal salts (Table 1). The highest catalytic activity was attained for the reaction using 10 mol % of Cu (II) salts. The role of copper salts in this reaction can be attributed to its Lewis acid ability, which enhances both the electron donating capacity of diene and electron withdrawing capacity of the dienophile (required for normal electron demand for Diels-Alder reaction). The The catalytic activitiy of Lewis acids like Cu+2 mainly relies on their coordinating character to assemble both dienophile and diene to such a way that promote the reaction to wards the reaction barrier. To find the optimum conditions towards the solvent several reactions were carried out under the solvents like benzene, toluene, xylene, water and results were tabulated (Table 2). Among organic solvents xylene is better to get considerable yield with copper salts. Later water was found to be the best for both yield and selectivity of this cycloaddition. Cycloaddition reactions of piperine (1a): Lewis acids catalyzed cycloaddition reactions (Scheme 2) of piperine (1a) under organic and aqueous solvent conditions to give resultant cycloadducts 2a, 3a, 4a, 5a and 6a, among them 2a is major ortho-exo cyclohexene type dimeric amide alkaloid and also known as chabamide, which is previously isolated23 from this plant, isomer 3a is previously isolated from Piper nigrum21 Remaining isomers (4a-6a) were synthesized from piperine by Diels-Alder reaction by Kun Wei. et al. its physical and spectroscopic data were identical with reported data22 (1H-NMR, 13C-NMR & Mass spectra). In the cycloaddition of piperine (1a), solvents toluene, xylene and water were used in presence of cuper (II) salts. Reaction showed good overall yield and more exo selectivity in organic solvent like xylene. Water catalyzed reactions were ended with good overall yield and minute decrease in exo selectivity, infinitesimal increase in endo selectivity (Table 2). This reaction showed completely regioselectivity (yield of 2a+3a>4a+5a+6a) due to maximum involvement of α-double bond rather than γ-double bond of 1a during Diels-Alder reaction. Cycloaddition reactions of pellitorine (1b): Same catalytic and solvent conditions were employed for pellitorine (1b) as used in piperine (1a) for the biomimetic synthesis (Scheme 3) of chabamide J & K (Chapter-II). These dimers were plausibly generated by monomer pellitorine by cycloaddtion reactions in biosynthesis. During cycloaddition of pellitorine (1b), solvents like toluene xylene and water were used in presence of cupper (II) salts. In former catalyzed reaction showed good overall yield and more endo selectivity in both organic (xylene) and water. Increase in endo selectivity is more in aqueous medium rather than organic solvent like xylene (Table 2). Cycloaddition of pellitorine under above said catalytic conditions gave corresponding cycloadducts 2b, 3b, 4b and 5b. Physical and spectral data of adducts 2b & 3b are identical with compound J & K (chabamide J & K mentioned in Chapter-II) and all physical and spectral data of adduct 4b is identical with nigramide O which is isolated previously from piper nigrum.21 The structure of 5b a new cycloadduct formed during this biomoimetic synthesis employ pellitorine as monomer, its structure was elucidated by 1D and 2D spectral data. This reaction showed completely regioselectivity (yield of 2b+5b≈3b+4b) due to maximum involvement of α-double bond rather than γ-double bond of 1b during Diels-Alder reaction. Structure elucidation of compound 5b: Compound 5b was obtained as a pale yellow oil, had the molecular formula of C28H50N2O2, as deduced from the HRESIMS (Fig-9) m/z, 447.3958 [M++H]. IR spectrum (Fig-1) implied the presence of carbonyl (1648 cm-1) and NH (3304 cm-1). The 1H NMR spectrum of 5b revealed the presence of a trans double bond at δ 5.28 (dd, J = 15.0, 10.0 Hz, H-4"), 5.63 (m, H-5"), two isobutylamide groups at δ 3.15 (m), 3.17 (m), 3.17 (m, H2-1'), 1.74 (m, H-2'), 0.91 (d, J = 6.7 Hz, H-3'), 0.90 (d, J = 6.7, H- 3'), 5.53 (br t, J = 5.7 Hz, NH) and δ 2.96 (m, H1-1'''), 2.97 (m, H2-1'''), 1.73 (m, H-2'''), 0.87 (d, J = 6.7 Hz, H-3'''), 0.86 (d, J = 6.7 Hz, H-3'''), 3.15 (br t, J = 6.0 Hz, NH), n-amyl group and 1-heptene unit at δ 1.96 (m, H-6), 1.40 (m, H-7), 1.20 (m, H-8), 1.27 (m, H-9), 0.86 ( t, J = 6.5 Hz, H-10) and δ 5.28 (dd, J = 15.0, 10.0 Hz, H-4"), 5.63 (m, H-5"), 1.89 (m, H-6"), 1.30 (m, H-7"), 1.28 (m, H-8"), 1.27 (m, H-9"), 0.88 (t, J = 6.5 Hz, H-10"), respectively (Table 3). The 13C NMR spectrum (Fig-3) displayed the presence of 28 carbon atoms and were further classified by DEPT experiments (Fig-4) into categories of 6 methyls, 10 methylenes, 10 methines and 2 quaternary carbons including two carbonyls (δ 173.80 and 173.04). ' The analyses of the 1H and 13C NMR spectral data of 5b showed a high degree of similarity to dimeric alkaloid, compound J naturally isolated from this plant (Chapter-II) compound is meta-endo while 5b is meta-exo product. Furthermore, the detailed elucidation of the 2D NMR data (COSY, HSQC and HMBC) had determined the planar structure of 5b. The 1H homodecoupling NMR (Fig-7) experiments of 5b revealed the connectivities H-2 (δ 2.45, m) to H-3 (δ 5.56, ddd, J = 10.0, 4.3, 2.6 Hz) to H-4 (δ 5.98, dt, J = 10.0, 1.8 Hz) to H-5 (δ 2.41, m) to H-2"( δ 2.68, dd, J = 11.3, 10.0 Hz) to H-3" (δ 2.82, ddd, J = 10.1, 10.0, 5.0 Hz ) via cyclohexene ring protons. The meta-orientation of the carbonyl and isobutylamide groups were established by HMBC (Fig-6) correlations for δ 2.45 (m, H-2), 5.56 (ddd, J = 10.0, 4.3, 2.6 Hz, H-3), 2.82 (ddd, J = 10.3, 10.0, 5.0 Hz, H-3")/δ 173.80 (C-1) and δ 2.68 (dd, J = 10.3, 10.0 Hz, H-2"), 2.41 (m, H-5), 2.82 (ddd, J = 10.3, 10.0, 5.0 Hz, H-3")/δ 173.04 (C-1"). Furthermore, the 1H-1H COSY (Fig-7) cross-peaks between δ 2.82 (ddd, J = 10.3, 10.0, 5.0 Hz, H-3") and δ 5.28 (dd, J = 15.0, 10.0 Hz, H-4"), and δ 5.63 (m, H-5") and δ 2.41 (m, H-5), 1.96 (m, H-6), 1.40 (m, H-7), coupled with the HMBC correlation for δ 5.63 (m, H-5'') to δ 28.35 (C-7"), δ 1.40 (m, H-7) to δ 37.04 (C-5) established the attachment of the 1-heptene and n-amyl groups at C-3" and C-5, respectively. The analysis of the 1H-1H coupling constants and NOESY (Fig- 8) data allowed us to determine the relative stereochemistry of compound 5b. The coupling constants of H-2"/H-5 and H-2"/H-3" (10.3 Hz) indicated anti relations of H-2"/H-5 and H-2"/H-3". In the NOESY spectrum correlations were observed at δ 2.45 (H-2) δ 2.82 (H-3") and δ 2.41 (H-5) and correlations were not observed at δ 2.68 (H-2") with δ 2.82 (H-3") and δ 2.68 (H-2") with δ 2.41 (H-5). These data were in agreement with the β-orientation for H-2" and α-orientation for H-3" and H-5. Thus, based on these spectral data the stereostructure of 5b was confirmed and trivially named as chabamide L. Cycloaddition reaction between piperine (1a) and pellitorine (1b): Our aim of this cycloaddition reaction is to explain to study different cycloadducts and selectivity of diene among piperine and pellitorine (Scheme 4). This biomimetic synthesis will explain the probability of diene, which participated in Diels-Alder reaction between piperine (1a) and pellitorine (1b) both were isolated from same plant (P. chaba). Nigramide N, which is formed biosynthetically via cycloaddition reaction between piperine and pellitorine, this adduct previously isolated from roots of P. nigrum 21 by Wei. et. al. Lewis acid catalyzed cycloaddition reactions of piperine (1a) and pellitorine (1b) under organic and aqueous solvent conditions to give resultant cycloadducts 2c, 3c, 4c, 2a and 3b. Cycloadduct 2c and 3c is new cycloadducts and their structures were illustrated by 1D and 2D spectral data. Structure elucidation of compound 2c: Compound 2c was obtained as pale yellow liquid. The molecular formula of 2c was established as C31H44N2O4 by HRESIMS (Fig-18), which provided a molecular ion peak at m/z 509.3381 [M++H], in conjunction with its 13C NMR spectrum (Fig-12). The IR spectrum displayed absorption bands diagnostic of carbonyl (1640 cm-1) (Fig-10). The 300 MHz 1H NMR spectrum (in CDCl3) indicated the presence of two signals at δ 5.86 (dd, J = 15.6, 10.1 Hz) and 6.27 (d, J = 15.6 Hz), which were assigned to trans-olefinic protons by the coupling constant of 15.6 Hz. It also displayed aromatic protons due to two 1, 3, 4-trisubstituted aromatic rings at δ 6.82 (1H, br s), 6.76 (1H, dd, J = 7.8, 1.4 Hz), 6.75 (1H, d, J = 7.8 Hz) (Fig-11), (Table 4). In addition to the above-mentioned moieties, combined inspection of 1H NMR and 1H–1H COSY revealed the presence of cyclohexene ring, one isobutylamide and one pyrrolidine ring. The 13C NMR spectrum displayed the presence of 31 carbon atoms and were further confirmed by DEPT experiments into categories of 11 methylenes, 12 methines and 5 quaternary carbons including two carbonyls (δ 173.01 and 172.50). On the basis of these characteristic features, database and literature search led the skeleton of compound 2c as a dimeric alkaloidal framework. A comprehensive analysis of the 2D NMR data of compound 2c facilitated the proton and carbon assignments. 1H–1H COSY spectrum suggested the sequential correlations of δ 3.51 (dq, J = 5.0, 2.6 Hz)/5.62 (dt, J = 9.8, 2.6 Hz)/6.10 (ddd, J = 9.8, 1.5 Hz)/2.20 (m)/2.72 (ddd, J = 11.1, 10.1, 5.2 Hz)/3.35 (dd, J = 11.1, 9.8 Hz) assignable to H-2-H-3-H-4-H-5-H-3"-H-2" of the cyclohexene ring. Concerning the connections of the n-amyl and 3, 4-methylenedioxy styryl groups, HMBC spectrum (Fig-15) showed correlations of H-4, H-6, H-7/C-5; H-5", H-4"/C-3", which implies that these units were bonded to the cyclohexene ring at C-5 and C-3". Further, HMBC correlations of two methylene protons at δ 5.95 with 147.91 (C-8"), 146.87 (C-9"), confirmed the location of methylenedioxy group at C-8", and C-9". Remaining units, isobutylamine and pyrrolidine (rings) were connected through carbonyl groups at C-2 and C-2", which was confirmed by HMBC correlations of H-2 and H-1' to C-1 (δ 173.01) and H-2" and H-1''' to C-1" (δ 172.50). The assignment of the relative configuration of compound 2c, and confirmation of overall structure were achieved by the interpretation of the NOESY spectral data and by analysis of 1H NMR coupling constants. The large vicinal coupling constants of H-2"/H-2 (11.1 Hz) and H-2"/H-3" (11.1 Hz) indicated anti-relations of H-2"/H-2 and H-2"/H-3" and the axial orientations for these protons. In the NOESY spectrum (Fig-17), the occurrence of the correlations between H-2/H-3" and the absence of NOE effects between H-2/H-2" and H-2"/H-3" supported the above result. This data indicated β-orientation for H-2" and α-orientation for H-2 and H-3". The α-orientation of H-5 was suggested by the coupling constant of H-5/H-3" (5.2 Hz) and the absence of the NOESY correlations between H-3" and H-2". On the basis of these spectral data, the structure of compound 2c was unambiguously established and trivially named as chabamide M. Structure elucidation of compound 3c: Compound 3c was obtained as pale yellow liquid. The molecular formula of 3c was established as C31H44N2O4 by HRESIMS (Fig-27), which provided a molecular ion peak at m/z 509.3391 [M++H], in conjunction with its 13C NMR spectrum (Fig-21). The IR spectrum displayed absorption bands diagnostic of carbonyl (1624 cm-1) moiety (Fig-19). The 300 MHz 1H NMR spectrum (in CDCl3) indicated the presence of two signals at δ 4.63 (dd, J = 15.6, 10.0 Hz) and 5.46 (dt, J = 15.6, 6.8 Hz), which were assigned to trans-olefinic protons by the coupling constant of 15.6 Hz. It also displayed aromatic protons due to two 1, 3, 4-trisubstituted aromatic ring at δ 6.75 (1H, br s), 6.73 (1H, d, J = 7.8, 1.4 Hz), 6.71 (1H, d, J = 7.8 Hz) (Fig-20). In addition to the above-mentioned moieties, combined inspection of 1H NMR and 1H–1H COSY revealed the presence of cyclohexene ring, one isobutylamide and one pyrrolidine ring. The 13C NMR spectrum displayed the presence of 31 carbon atoms (Table 5), and were further classified by DEPT experiments (Fig-22) into categories of 11 methylenes, 12 methines and 5 quaternary carbons including two carbonyls (δ 173.34 and 173.88). On the basis of these characteristic features, database and literature searches led the skeleton of compound 3c as a dimeric alkaloidal framework. A comprehensive analysis of the 2D NMR data of compound 3c facilitated the proton and carbon assignments. 1H–1H COSY spectrum (Fig-25) suggested the sequential correlations of δ 2.82 (m)/5.63 (dt, J = 9.7, 1.9 Hz)/5.82 (ddd, J = 9.7, 4.8, 1.9 Hz)/3.94 (dq, J =.10.0, 1.9 Hz)/2.76 (ddd, J = 11.7, 10.0 Hz)/3.36 (dt, J = 11.7, 4.8 Hz) assignable to H-2-H-3-H-4-H-5-H-3"-H-2" of the cyclohexene ring. Concerning the connections of the 3, 4-methylenedioxyphenyl and 1-heptene groups, HMBC spectrum (Fig-24) showed correlations of H-7, H-11, H-3"/C-5; H-5", H-4", H-5/C-3", which implies that these units were bonded to the cyclohexene ring at C-5 and C-3". Further, HMBC correlations of two methylene protons at δ 5.92 with 147.42 (C-8"), 146.49 (C-9"), confirmed the location of methylenedioxy group at C-8", and C-9". Remaining units, pyrrolidine and isobutylamine were connected through carbonyl groups at C-2 and C-2", which was confirmed by HMBC correlations of H-2 and H-1' to C-1 (δ 171.34) and H-2" and H-1''' to C-1" (δ 173.88). The assignment of the relative configuration of compound 3c, and confirmation of overall structure were achieved by the interpretation of the NOESY spectral data and by analysis of 1H NMR coupling constants. The large vicinal coupling constants of H-3"/H-2" (11.7 Hz) and H-5/H-3" (10.0 Hz), indicated anti-relations of H-3"/H-5 and H-3"/H-2" and the axial orientations for these protons. In the NOESY spectrum (Fig-26), the occurrence of the correlations between H-2"/H-5 and the absence of NOE effects between H-3"/H-2" and H-3"/H-5 supported the above result. These data indicated β-orientation for H-2" and α-orientation for H-2 and H-3". The α-orientation of H-2 was suggested by the coupling constant of H-2/H-2" (4.8 Hz) and the occurrence of the NOESY correlations between H-2" and H-2. On the basis of this spectral data, the structure of compound 3c was unambiguously established and trivially named as Chabamide N. Cycloaddition reaction between piperine (1a) and E-fagaramide (1c) Lewis acid catalyzed cycloaddition reactions (Scheme 5) of piperine (1a) and trans-fagaramide (1c) under aqueous solvent conditions to give resultant cycloadducts 2d, 3d and 2a. To carry this biomimetic synthesis to explain compound H and I (mentioned in chapter-II), we taken piperine (1a) which is isolated from same plant and trans fagaramide was synthesized by reported method.24 Cycloaddition reaction between 1a and 1c end up with overall yield 70% in xylene and 75% in water. In both solvents ortho products were formed dominantly compared with meta products. Spectral data 1D and 2D of cycloadducts 2d & 3d were identical with compound H & I (see chapter I, compound H & K). Cycloadduct 2a is identical with chabamide. This cycloaddition reaction practically proved as biomimetic synthesis for compound H and I. Acknowledgements: The authors are thankful to Director IICT for his constant encouragement and CSIR New Delhi for providing the fellowship References and Notes Braun, M. Synth. Highlights 1991, 232 Robinson, R. Chem. Soc. 1917, 762. Stork, G.; Burgstahler, A. W. Am. Chem. Soc. 1955, 38, 1890. Johnson, W. S.; Gravestock, M. B.; McCarry, B. E. Am. Chem. Soc. 1971, 93, 4332. Chapman, O. L.; Engel, M. R.; Springer, J. P.; Clardy, J. C. Am. Chem. Soc. 1971, 93, 6696. Bandaranayake, W. M.; Banfield, J. E.; Black, D. St. C. Chem. Soc., Chem Commun. 1980, 902. Nicolaou, K. C.; Zipkin, R. E.; Petasis, N. A. Am. Chem. Soc. 1982, 104, 5558. O.; Alder, K. Ann. 1928, 460, 98. Woodward, R. B.; Hoffmann, R. Chem. 1969, 81, 797. Fakui, K. Chem. Res. 1971, 4, 57. Houk, K. N. Chem.. Res. 1975, 8, 361. Houk, K. N.; Li, Y.; Evanseck, D. Angew Chem., Ed. Engl. 1992, 31, 682. Alder, K.; Stein, G. Chem. 1937, 50, 510. Fotiadu, F.; Michel, F.; Buono, G. Tetraheron Lett. 1990, 34, 4863. Gleiter, R.; Bohm, M. C. Pure Appl. Chem. 1983, 55, 237. Woodward, R. B.; Katz, T. J. Terahedron 1958, 5, 70. Kakushima, M. J. Chem. 1979, 57, 2564. Houk, K. N. Tetrahedron Lett. 1970, 30, 2621. Houk, K. N.; Luskus, L. J. Am. Chem. Soc. 1971, 93, 4606. Otto, S.; Bertoncin, F.; Engberts, J. F. N. Am. Chem. Soc., 1996, 118, 7702–7707. Wei, K.; Li, W.; Koike, K.; Chen, Y-J.; Nikaido, T. Org. Chem. 2005, 70, 1164. Wei, K.; Li, W.; Koike, K.; Chen, Y-J.; Nikaido, T. Lett. 2005, 7, 2833–2835. Rukachaisirikul, T.; Prabpai, S.; Champung, P.; Suksamrarn, A. Planta Med. 2002, 68, 850-853. Nagao, Y.; Seno, K.; Kawabata, K.; Miyasaka, T.; Takao, S.; Fujita, Tetrahedron Lett. 1980, 21, 841. Otto, S.; Boccaletti, G.; Engberts, J. B. F. N. Am. Chem. Soc. 1998, 120, 4238–4239. Otto, S.; Bertoncin, F.; Engberts, J. B. F. N. Am. Chem. Soc. 1996, 118, 7702–7707. O.; Alder, K. Ann. 1931, 490, 243. Woodward, R. B.; Baer, H. Am. Chem. Soc. 1948, 70, 1161. Breslow, R.; Rideout, D. C. Am. Chem. Soc. 1980, 102, 7816. Breslow, R.; Guo, T. Am. Chem. Soc. 1988, 110, 5613. Grieco, P.A.; Nunes, J. J.; Gaul, M. D. Am. Chem. Soc. 1990, 112, 4595.

Spatial displacements of spins between radio frequency pulses in a Double-Quantum (DQ) NMR pulse sequence generate additional terms in the effective DQ Hamiltonian. We derive a simple expression allowing the estimation and control of these contributions to the initial rise of the DQ build up function by variation of experimental parameters in systems performing anomaleouse diffusion. The application to polymers is discussed.

AbstractThe objective of the present study is the introduction of well known and readily accessible analytical methods as FTIR, thermogravimetric (TGA) and evolved gas analysis (EGA) for quantitative determination of the degree of polymerization (DP) and degree of quaternization (DQ)/betainization (DB) of amino-based amphiphilic block copolymers. For this purpose a series of amphiphilic poly(dimethylsiloxane)-b-poly[2-{dimethylamino)ethyl methacrylate] (PDMS-b-DMAEMA) diblock copolymers with DP of the PDMAEMA-block ranging from 22 to 162 was synthesized by atom transfer radical polymerization and analyzed by 1H NMR, ATR-FTIR and TGA-co-EGA. The determined composition results have shown linear correlation between the FTIR or TGA and the 1H NMR data and equations allowing the quantitative calculation of PDMAEMA-block DP were found. The errors estimated by ATR-FTIR were less than 1.5%. Further, the PDMAEMA-block was quaternized (or betainized) with DQ (or DB) from 25 % to 100 % and analyzed by TGA-co-EGA. Again, a linear correlation between the quaternized (betainized) PDMAEMA-block mass fraction and the DQ (DB) degrees was obtained by 1H NMR.

ABSTRACT Butyl and bromobutyl inner tubes, specified by the Aerospace Standard AS50141 for military aircraft, were thermally aged from 40 to 120 °C for varying lengths of time and then their hardness and mechanical properties were measured. 1H double quantum nuclear magnetic resonance (DQ NMR) was used to elucidate crosslink density and distribution changes. Time–temperature superposition of the aged data coupled with the Arrhenius approach was used to determine an approximate shelf life. High (80–120 °C) and low (40–80 °C) temperature oxidation processes were occurring for both rubbers. Below 80 °C, an increase in crosslink density, hardening, stiffening, and loss of elongation was observed. Plasticizer and volatile loss contributes to compound stiffening. Sulfur crosslink network modifications during thermal aging can explain ultimate property loss and stiffness increase. Diffusion limited oxidation was taking place above 80 °C, with the development of a thin oxidized layer composed of ionic crosslinking that affected both hardness and mechanical properties. For butyl rubber, the hardness rise stabilizes as do the ultimate properties, likely due to the proliferation of chain scission reactions, whereas crosslinking reactions prevailed over chain scission events for bromobutyl rubber. Crosslink density and defect fractions B and C as measured through DQ NMR were in agreement with the physical property testing results. The degree of heterogeneity of the network as perceived visually through DQ NMR regularization increases upon exposure to higher temperatures and longer aging times due to the broadening of the crosslink density distribution. Similar Arrhenius activation energies were calculated for the low and high temperature oxidation process for butyl and bromobutyl rubbers. The projected shelf life for the butyl and bromobutyl inner tubes was 10 and 20 yr, respectively. For the first time, DQ NMR testing results (crosslink density and its distribution, defect level) have been successfully applied to support a shelf life determination.

ABSTRACT A bromobutyl tire inner liner compound was prepared and subjected to aerobic and anaerobic heat aging at a temperature of 100 °C for seven aging times up to 8 weeks. Hardness and mechanical properties were monitored, and the evolution of the crosslink density was followed using equilibrium solvent swell and low field double quantum (DQ) nuclear magnetic resonance (NMR). The hardness and the 300% tensile stress increased with heat aging, while both tensile strength and elongation at break dropped. Both chain scission and crosslinking reactions were taking place. Equilibrium swelling and DQ NMR results confirmed that a larger crosslink density increase was seen under aerobic versus anaerobic aging conditions. The network distribution consisting of a dominant low crosslinking zone and small areas of higher crosslinking slowly broadened and shifted toward higher crosslink densities upon heat aging. The compounds aged heterogeneously. Attenuated total reflectance–Fourier transform infrared spectroscopy confirmed the presence of an oxidized surface layer, and therefore diffusion-limited oxidation effects, but only under aerobic aging conditions. Reaction mechanisms are proposed to explain the net crosslink rise with heat aging.

AbstractMaterials based on ureidopyrimidinone (UPY) dimers and Adenine (A) / Thymine (T) derivatives were synthesized and characterized by advanced solid state NMR (Nuclear Magnetic Resonance) techniques. Silylated UPY molecules were used as model compounds, leading to structured organic-inorganic materials after hydrolysis and condensation processes (sol-gel reactions). High resolution 1H solid state NMR has been extensively used for the in-depth description of the H-bond networks, including very fast MAS (Magic Angle Spinning) experiments at very high field and DQ (double quantum) recoupling experiments. The chemical nature of the organic-inorganic interface has been illuminated by such techniques. In, particular, it has been demonstrated that H-bond networks were preserved during sol-gel reactions and were comparable to those observed in the UPY crystalline precursors.