RK-397

Our new synthetic strategy for assembling polyacetate structures features efficient cross-couplings of six-carbon modules derived from any stereoisomer of the epoxyalkynol derivative (1). Hydration of the internal alkyne in the coupled products and stereoselective reduction of the resulting ketone intermediate provides a general approach to a library of stereoisomeric 1, 3, 5, ... alternating polyols (2). The strategy is demonstrated in a stereoselective synthesis of the C11-C28 polyol substructure of the natural product RK-397.

The total synthesis of the natural product RK-397 is based on a new synthetic strategy for assembling polyacetate structures, by efficient cross-coupling of nucleophilic terminal alkyne modules with electrophilic epoxides bearing another alkyne at the opposite terminus. The natural product is constructed from four principal modules: a polyene precursor for carbons 3-9, and three alkyne-terminated modules for carbons 10-16, 17-22, and 23-33. Each module is prepared with control of all stereochemical elements, and the alkynyl alcohols obtained from alkyne-epoxide couplings are converted into 1,3-diols by a sequence of hydroxyl-directed hydrosilylation, C-Si bond oxidation, and stereoselective ketone reduction with induction from the beta-hydroxyl group. The highly convergent nature of our synthetic pathway and the flexibility of the modular synthesis strategy for virtually any stereoisomer can provide access to other members of the polyene-polyol macrolides, including stereoisomers of RK-397.

An enantioselective synthesis of the polyene macrolide RK-397 is described. The use of the same eight-carbon building block twice for the construction of the polyol chain allowed for a highly convergent synthesis. The synthesis highlights stereoselective vinylogous aldol addition using a chiral bisphosphoramide as well as a sequential palladium-catalyzed cross-coupling reaction for the preparation of key fragments.