Novel Bicyclic P , S -Heterocycles via Stereoselective hetero -Diels-Alder Reactions of Thiochalcones with 1-Phenyl-4 H -phosphinin-4-one 1-Oxide.

Thiochalcones undergo cycloaddition reactions in THF solution at 60 °C with the synthetically unexplored 1-phenyl-4 H -phosphinin-4-one 1-oxide in a highly regio- and stereoselective manner, yielding hitherto unknown bicyclic P , S -heterocycles containing fused thiopyran and phosphinine rings. The stereochemical structures of two of the obtained (4+2)-cycloadducts were unambiguously assigned by means of the X-ray single-crystal analysis. Based on these assignments, a concerted mechanism of the hetero -Diels-Alder reaction via the preferred endo approach of the heterodiene from the less hindered P=O side of the phosphininone molecule is postulated to explain the established rac-(4 RS ,8 SR ,9 SR ,10 SR )-configured (4+2)-cycloadducts isolated as major products.