Stereoselective rhodium-catalyzed conjugate addition of boronic acids to unprotected delta-hydroxy-gamma-butenolides. Synthesis of (-)-7-oxamuricatacin and beta-substituted derivatives.

The chiral delta-hydroxy-gamma-butanolide moiety is widely found among biologically active natural products. We report herein the stereoselective synthesis of beta-substituted analogues of these compounds by the Rh(I)-catalyzed conjugate addition of boronic acids to chiral delta-hydroxy-gamma-butenolides, easily prepared from the chiral pool. The reaction takes place with high trans diastereoselectivity without protection of the hydroxyl group. The three-step syntheses of (-)-7-oxamuricatacin (R1 = H, R2 = CH2-O-(n)C10H21) and of new beta-substituted 7-oxamuricatacin analogues (R1 = aryl, vinyl) is reported.