Enantioselective Aldol Reaction of Tetrasubstituted Ketene Silyl Acetals with Achiral Aldehydes for the Construction of Asymmetric Tertiary Alcohols: An Application for the Divergent Total Syntheses of Buergerinins F and G

The asymmetric aldol reaction of heteroatom-substituted ketene silyl acetals with achiral aldehydes has been developed by the promotion of tin(II) triflate coordinated with a chiral diamine to afford the corresponding aldols having chiral tertiary alcohols at the α-positions. This reaction has been successfully applied to the construction of the basic skeletons of buergerinins F and G starting from achiral materials. (+)-Buergerinin G, a potentially antiphlogistic and febrifuge agent having a unique trioxatricyclo[5.3.1.0 1,5 ]undecane skeleton, is stereoselectively prepared by means of the enantioselective aldol reactions via ten linear steps from crotonaldehyde in 18% overall yield including an effective intramolecular Wacker-type ketalization of the dihydroxy-γ-lactone as a key step. In addition to the former establishment for the synthesis of (+)-buergerinin F, effective divergent methods for the preparation of buergerinins F and G were developed through a unified optically active aldol-type intermediate, which was generated from the tetrasubstituted ketene silyl acetal with crotonaldehyde by the asymmetric Mukaiyama aldol reaction.