Enantio- and Diastereoselective Synthesis of Tricyclic Lactones Bearing Three Quaternary Stereocenters

An efficient method for the enantio- and diastereoselective synthesis of tricyclic γ-butyrolactones bearing three quaternary stereocenters has been developed through transformations of achiral alkenoic acid substrates. The target optically active tricyclic lactones were obtained as single diastereomers with good enantioselectivities through a sequence of catalytic asymmetric bromolactonization, azidation, and subsequent 1,3-dipolar cycloaddition. The origin of the complete diastereoselectivity observed in the formation of the tricyclic γ-butyrolactone products was elucidated in this study.