Synthesis of (-)-streptenol A, (+/-)-streptenol B, C and D

2-(2,2-Dimethyl-1,3-dioxan-4-yl)acetaldehyde (3) was used for the preparation of streptenol A and B (Scheme 1) via a Grignard reaction with 1-bromopent-3-ene. Hereby optically pure (4'R)-3 gave the antipode of Streptenol A. Reaction with lithiated 1-pentyne opened access to streptenol C and D. To obtain the dienone structure of streptenol C and D, a palladium-catalyzed alkynone isomerization was induced. Kinetic differences in the acid-mediated cleavage of the 1,3-acetonide protected 1,3,5-triol system caused the stereoselectivity in the natural products. So only the (3S*,5R*) acetonide of streptenol B reacted under mild hydrolytic conditions and gave after transacetalization first a 3,5-protected streptenol B with pure relative stereochemistry and finally (3S*,5R*)-streptenol B.