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

Abstract

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 diastero-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.