Recent Advances in Enantioselective Synthesis of γ-Substituted Butenolides via the Catalytic Asymmetric Vinylogous Reactions

The catalytic asymmetric vinylogous-type reactions of γ-butenolides, have received growing attention, because the resulting enantiomerically pure γ-substituted butenolides are versatile building blocks for various natural products and biologically active compounds. The related reactions, such as vinylogous aldol reaction, vinylogous Mannich reaction, vinylogous Michael reactions, as well as allylic alkylation reaction, have been well developed in the past two decades. Both chiral metal complex catalysts and organocatalysts have proven to be useful. Among the plethora of catalytic systems developed so far, vinylogous Mukaiyama-type reactions using 2- silyloxfurans as the nucleophiles are popular for the synthesis of γ-substituted butenolides. On the other hand, the direct version of such reactions using simple butenolides is appreciated in recent years, due to the fact of the synthetic convenience and efficiency. Some advances have also been made in the catalytic direct enantioselective addition of simple butenolides and the analogues. In this microreview, recent progresses regarding the various types of reaction, substrate scope, and probable reaction mechanisms are discussed. In addition, the application of the methodology to the synthesis of natural products and useful intermediates is also presented briefly wherever appropriate. Keywords: γ-substituted butenolide, asymmetric synthesis, chiral Lewis acid catalyst, enantioselectivity, organocatalyst, vinylogous reactions.