Academic literature on the topic 'Siphonaria australis'

A highly stereocontrolled synthesis of (2S,3R,5R,6S)-6-[(1E,3R)-1,3-dimethylhex-1-enyl]-2-ethyl-2-hydroxy-3,5-dimethyltetrahydro-4H-pyran-4-one has been achieved in 13 steps and an overall yield of 7.6% from (R)-(+)-4-benzyloxazolidin-2-one. This substrate-controlled asymmetric synthesis utilized Paterson’s lactate-derived chiral ketone (2S)-2-(benzoyloxy) pentan-3-one to generate the 5R and 6S stereocentres and alkylation of an Evans’ auxiliary to generate the remote side-chain 3R stereocentre. Furthermore, a novel, highly efficient, and selective strategy was used to generate an enol trimethylsilyl ether whose subsequent Mukaiyama aldol condensation gave the acyclic precursor to the final product. A thermodynamically controlled cyclization then gave (2S,3R,5R,6S)-6-[(1E,3R)-1,3-dimethylhex-1-enyl]-2-ethyl-2-hydroxy-3,5-dimethyltetrahydro-4H-pyran-4-one with control of the 2S and 3R stereocentres. The NMR spectroscopic data and optical rotation obtained for synthetic (2S,3R,5R,6S)-6-[(1E,3R)-1,3-dimethylhex-1-enyl]-2-ethyl-2-hydroxy-3,5-dimethyltetrahydro-4H-pyran-4-one were consistent with those reported for the hemiacetal isolated from Siphonaria australis, and thus, prove the absolute and relative configuration of the natural product.

Cellana ampla n. sp. from late Eocene rocks in Oregon is the first New World record of a genus and family heretofore considered to have radiated in Australia and the mid-Pacific. Re-examination of some other Eocene limpets from the Pacific Northwest reveals that species previously assigned to Acmaea belong in Patelloida and Siphonaria. Thus, all three major components of the modern Australian limpet guild were present in the late Eocene of western North America along a coastline occupied today by acmaeid limpet guilds. Re-examination of a large Pliocene ‘Patella’ from South America shows the shell structure of a Cellana and constitutes a second documentation of a large nacellid limpet in the New World Tertiary.Although the taxonomy of modern limpets is founded on anatomical characters, shell structure characters are sufficient to identify genera and families directly in the fossil record. Re-evaluation and reclassification of fossil limpets using shell structure contributes not only to a better understanding of the phylogeny of major limpet groups but also provides a powerful tool for elucidating the evolutionary, biogeographic, and ecologic development of the complexly structured limpet guilds of differing taxonomic composition that dominate rocky intertidal habitats throughout the world.

In recent decades the emergence of marine polypropionate natural products as compounds of diverse structural complexity and intriguing biological activity has influenced the advancement of asymmetric synthesis and predicated detailed studies of marine ecology. The introductory chapter of this thesis explores the nature of marine natural products, including their structure, biological activity and biosynthesis. Additionally, a brief review of the aldol reaction is presented. This well established biomimetic chemical transformation underpins polyketide synthesis and was utilised extensively in the research contributing to this dissertation. Chapter Two describes the first asymmetric total synthesis of the two marine polypropionates isolated from specimens of Siphonaria australis by Hochlowski et al. in 1984. Spectroscopic analysis revealed hemiacetal 22 and ester 23 to be identical to the secondary metabolites extracted from the marine pulmonate. The synthetic approach to hemiacetal 22 utilised lactate derived ketone (S)-67 to control the configuration of the C7 and C8 stereocentres and involved the discovery of a mild protocol for the synthesis of trimethylsilyl enol ether 109, which was employed for a Mukaiyama aldol homologation reaction. Additionally, ester 23 was synthesised from hemiacetal 22 via a retro-Claisen fragmentation. The retro-Claisen approach utilised in the synthesis of ester 23 was extended in Chapter Three to serve as the pivotal transformation in an attempted total synthesis of the unusual marine polypropionate dolabriferol (30). The strategy toward dolabriferol (30) involved an iterative homologation of chiral ketone (S)-67 to install all but one of the requisite stereocentres in the natural product. Chemoselective deprotection of acyclic precursor 160 gave the elaborate 2,4,6-trioxaadamantane 167, whose participation as a protecting group mimic lead to the formation of ester 169 after reaction of the polycycle 167 with base. The synthesis of ester 169, which represents a direct precursor to dolabriferol (30), was achieved in 16 steps with an overall yield of 24%. Unfortunately, a robust protecting group on ester 169 prohibited a synthesis of dolabriferol (30), but intriguingly in one deprotection of ester 169 with aqueous hydrofluoric acid, spiroacetal 172 was isolated. Chapter Four describes the first total synthesis of cytotoxic marine polypropionate auripyrone A (78) and establishes the absolute configuration of this important natural product as that depicted for compound 78. The requisite C8-C12 stereopentad of auripyrone A (78) was formulated from Evans� dipropionate equivalent 53 in a double stereodifferentiating aldol reaction, followed by syn-reduction to give diol 206. Differentiation of the secondary alcohols in compound 206 was achieved by migration of the PMB protecting group and protection at C11 with the requisite acyloxy group of auripyrone A (78). Differential protection was critical to achieving selective spiroacetalisation to afford the unique spiroacetal dihydropyrone core of the natural product. The utility of LiHMDS for highly selective double stereodifferentiating aldol homologations of sensitive fragments is also discussed. This mild aldol protocol was pivotal to forming the carbogenic skeleton of auripyrone A, in particular, elaborate adduct 278.