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 (3 S * ,5 R * ) 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 (3 S * ,5 R * )‐streptenol B.