Stereochemistry of hydride transfer in acid induced disproportionation of thiachromenes and chromenes

Abstract

Acid catalysed disproportionation of 3,4-dimethyl-Δ³-thiachromene (V) involves stereoselective intermolecular hydride transfer to give 85% cis-isomer (VIIA) and 15% trans-isomer (VIIB) of 3,4-dimethyl-thiachroman. In contrast, the oxygen analogue shows no stereospecificity, giving an equal mixture of cis/trans isomers of 3,4-dimethylchroman (XX). The intermediacy of a bridged sulfonium ion A with a predictable steric configuration has been proposed. In the tricyclic series the difference between the thia- and oxa-series is less pronounced presumably because of additional steric factors imposed due to the rigidity of the fused ring system.