| Abstract
| - An approach to the pyranopyran ring system that is found in many natural products, includingthyrsiferol, is described. The route entails the assembly of an α,β-unsaturated ketone (11) fromgeraniol and dihydropyran (23) from acetyl acetaldehyde dimethyl acetal (19) and their titanium(III)-promoted coupling to afford a respectable 60% yield of keto alcohol 26. The σ-bond formed inthis process corresponds to the pro-C9−C10 bond of thyrsiferol (4). Attempts to invert thestereochemistry at the pro-C11 center were thwarted by the congestion imparted by the presence ofthe vicinal TBS-ether. Consequently, cyclization of the coupling adduct under conditions developedby Olah and Prakash and co-workers led to the cis-fused pyranopyran 27. X-ray analysis of thiscrystalline material confirmed each of the stereochemical assignments. After much effort, it wasdetermined that the hydroxyl group at C12 could be removed by treating the derived methyl xanthatewith a tri-n-butylphosphine−borane complex under radical-forming conditions. The reactionsequence worked well, despite the hindered working environment and the presence of a potentiallylabile C−Br bond.
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