Enantioselective Synthesis of α-exo-Methylene γ-Butyrolactones via Chromium Catalysis.

Enantioenriched α-exo-methylene γ-butyrolactones have been obtained via a two-step sequence consisting of a highly enantioselective chromium-catalyzed carbonyl 2-(alkoxycarbonyl)allylation and lactonization. A variety of functional groups are compatible under the mild reaction conditions. The synthetic utility of this methodology was demonstrated by two short derivatization transformations and the enantioselective synthesis of (+)-methylenolactocin.