STEREOSELECTIVE REDUCTIONS OF 2-KETO SULFOXIDES WITH HYDRIDES

The results of reductions of some acyclic 2-keto sulfoxides (ArC(o)CH2S(o)R; Ar = Ph, 2-Py, 4-Py, and R = Me, p-Tol) and flexible and rigid 2-sulfmylcyclohexanone diastereomers with various metal hydrides are reported. The configurational assignment and conformational preferences of these substrates have been inferred from their NMR parameters. The high stereoselectivity observed in i-Bu2A1H 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 ZnCl2 the hydride transfer evidently takes place intermolecularly on the half-chair conformation adopted by the chelated species formed by 2-keto sulfoxides and ZnCl2. The reductions with LiAlH4 involve lithium chelates to which hydride is intramolecularly transferred from associated A1H4~ through the most stable transition state. In all cases, an important role of the stereoelectronic effects on the favored hydride approach is proposed. © 1990, American Chemical Society. All rights reserved.