Diastereoselective, Three-Component Cascade Synthesis of Tetrahydrofurans and Tetrahydropyrans Employing the Tandem Mukaiyama Aldol−Lactonization Process

A full account of studies leading to the development of a cascade sequence that generates as many as two C−C bonds, one C−O bond, and three new stereocenters providing substituted tetrahydrofurans (THFs) from simple γ-ketoaldehydes and thiopyridyl ketene acetals is described. The process involves a tandem Mukaiyama aldol−lactonization (TMAL) and accumulated evidence suggests the intermediacy of a silylated β-lactone that is intercepted by the pendant ketone. Formation of a cyclic oxocarbenium is followed by reduction with silicon-based nucleophiles leading to a highly diastereoselective synthesis of tetrahydrofurans. This cascade process has now been extended to the synthesis of tetrahydropyrans from simple δ-ketoaldehydes. The stereochemical outcome of the cascade processes described was determined by NOE correlations, coupling constant analysis, and X-ray crystallography of the derived oxygen heterocycles and is in accord with established and recently proposed models for nucleophilic additions to cyclic 5- and 6-membered oxocarbenium ions. The utility of this process was demonstrated by the synthesis of the tetrahydrofuran fragment of colopsinol B.