Stereoselective reductions of 2-keto sulfoxides with hydrides

Abstract

The results of reductions of some acyclic 2-keto sulfoxides (ArC(O)CH 2 S(O)R; Ar=Ph, 2-Py, 4-Py, and R=Me, p-Tol) and flexible and rigid 2-sulfinylcyclohexanone diastereomers with various metal hydrides are reported. The high stereoselectivity observed in i-Bu 2 AlH reactions can be explained by assuming its association with the sulfinyl oxygen and intramolecular hydride transfer via chair-like transition states. In the presence of ZnCl 2 the hydride transfer evidently takes place intermolecularly on the half-chair conformation adopted by the chelated species formed by 2-keto sulfoxides and ZnCl 2 . The reductions with LiAlH 4 involve lithium chelates to which hydride is intramolecularly transferred from associated AlH 4 − through the most stable transition state. In all cases, an important role of the stereoelectronic effects on the favored hydride approach is proposed