Stereoselective synthesis of syn-configured α-allenols by rhodium-catalyzed reaction of alkynyl oxiranes with arylboronic acids

(Chemical Equation Presented) A rhodium-catalyzed reaction of alkynyl oxiranes with arylboronic acids affords syn-configured α-allenols with high diastereoselectivity. The reaction is initiated by addition of an arylrhodium(I) species onto the alkyne moiety of the alkynyl oxirane. The resulting alkenylrhodium(I) intermediate undergoes β-oxygen elimination to open the oxirane ring in a syn-selective fashion. Protonolysis of the rhodium(I) alkoxide with arylboronic acid releases the corresponding α-allenol along with the rhodium(I) boronate, which undergoes β-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.