Stereoselective Synthesis of syn-Configured α-Allenols by Rhodium-Catalyzed Reaction of Alkynyl Oxiranes with Arylboronic Acids

A rhodium-catalyzed reaction of alkynyl oxiranes with arylboronic acids affords syn-configured alpha-allenols with high diastereoselectivity. The reaction is initiated by addition of an arylrhodium(I) specie; onto the alkyne moiety of the alkynyl oxirane. The resulting alkenylrhodium(I) intermediate undergoes beta-oxygen elimination to open the oxirane ring in a syn-selective fashion. Protonolysis of the rhodium(I) alkoxide with arylboronic acid releases the corresponding alpha-allenol along with the rhodium(I) boronate, Which undergoes beta-aryl elimination to regenerate the arylrhodium(I) species. The utility of this method is demonstrated by an application to a concise synthesis of (+/-)-Boivinianin B.