Gunasekera, Diyan; (2000) Synthetic studies towards chiral analogues of steganone. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is divided into three chapters. In the first chapter, following a brief overview of the isolation, structure and biological properties of steganone, the various strategies which have been adopted for the total synthesis of the racemate are presented. The more challenging problem of preparing the desired target as a single enantiomer is then examined by illustrating the variety of approaches used for the asymmetric synthesis of such lignans. In the second chapter, having introduced the efficient and highly convergent route to racemic analogues of steganone which had evolved within the group, some of the more problematic steps in this existing route are examined; namely, the synthesis of the key ynone utilizing toxic organotin reagents and the low yielding, but absolutely crucial cobalt-mediated [2+2+2] cycloaddition of the tethered diyne with a third alkyne component. Although an approach that obviated the use of tin reagents was successfully accomplished, efforts to improve the cobalt-mediated cyclisation using a novel Trost reagent generated in situ from cobalt tris(acetylacetonate), triphenylphosphine, and triisobutylaluminum failed. In parallel with the above work, two approaches have been investigated to access chiral analogues of steganone. The first of these was based on a variety of modified literature routes to (R)-(+)-paraconic acid as a key intermediate. Although ultimately successful, multigram quantities of (R)-(+)-paraconic acid could not be prepared in this manner. In an entirely different strategy, the use of a chiral acyl anion equivalent developed by Enders in an addition to butenolide was investigated. The third chapter provides a formal description of the experimental results and procedures.

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