Stereoselective Rhodium-Catalyzed Conjugate Addition of Boronic Acids to Unprotected δ-Hydroxy-γ-butenolides. Synthesis of (-)-7-Oxamuricatacin and β-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 (R-1 = H, R-2 = CH2-O-(C10H21)-C-n) and of new beta-substituted 7-oxamuricatacin analogues (R-1 = aryl, vinyl) is reported.