Uncatalysed (thermal) and Lewis acid-promoted asymmetric hetero-Diels-Alder reaction of 1-thiabuta-1,3-dienes (thiochalcones) with di-(-)-menthyl fumarate. Configuration determination by X-ray crystallographic analysis of (2S,3R,4R)-(+)-2,3-bis[(-)-menthoxycarbonyl]-4,6-diphenyl-3,4-dihydro-2H- thiopyran and conversion of cycloadducts into optically pure diols

Article abstract of DOI:10.1039/P19960001897

1-Thiabuta-1,3-dienes 1 (thiochalcones) underwent asymmetric hetero-Diels-Alder reaction with di-(-)-menthyl fumarate 2 to afford a mixture of 3,4-cis- (3 and 4) and 3,4-trans-dihydrothiopyrans (5 and 6) in good-to-excellent chemical yield with fair diastereoselectivities, the stereoisomer 3 predominating. An endo (3,4-cis) : exo (3,4-trans) selectivity was observed in the ratio 98-85:2-15. The uncatalysed reaction at 20-40°C showed 10-48% de in diastereo-π-facial selectivity, while the selectivity was improved to 62-71% de by using a suitable Lewis acid under appropriate reaction conditions. The 3,4-cis-stereoisomer formed favourably in both thermal and Lewis acid-promoted reactions, had the same configuration 2S,3R,4R (3). The configuration was unequivocally established by X-ray crystallographic structural analysis using the isomer 3a (R¹ = R² = Ph). The isomer 3, which could be obtained stereochemically homogeneous by chromatography and recrystallization, was consecutively transformed to optically pure diols 7 and 8 or 9 by LiAlH₄ reduction, followed by reductive desulfurization with Raney Ni.