Asymmetric thio-claisen rearrangement induced by an enantiopure alkylsulfinyl group. Unusual preference for a boat transition state in the acyclic series.

Abstract

Ketene aminothioacetals 2a-c and 5a-b bearing an enantiopure vinylic alkylsulfinyl substituent were readily prepared from (R)-2-cyclohexylsulfinyl-N,N-dimethylethanethioamide 1 with full control of the geometry of their double bonds. They underwent a Claisen rearrangement upon heating at THF reflux to afford alpha-sulfinyl gamma-unsaturated thioamides 3a-c and 6a-b. With all substrates the asymmetric induction of the sulfinyl group was excellent. The determination of the absolute configurations of thioamides 3a-c and 6a-b was achieved either by X-ray crystallographic analysis or by chemical correlation. The stereochemical course of this [3,3] sigmatropic transposition was explained by an electronic model. Interestingly the Claisen rearrangement of the (ZE)-cinnamyl substrates 5b was shown to proceed through a boat transition state rather than a chair transition state; such a preference is quite unusual for acyclic systems.