Asymmetric thio-claisen rearrangement induced by an enantiopure alkylsulfinyl group. Unusual preference for a boat transition state in the acyclic series.
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.
Nowaczyk
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Laboratoire de Chimie Moléculaire et Thioorganique (UMR CNRS 6507), ISMRA-Université de Caen, 6 Boulevard du Maréchal Juin, 14050 Caen, France.
Alayrac
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C
Reboul
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Metzner
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P
Averbuch-Pouchot
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