Thia-Diels-Alder reactions of hetaryl thioketones with non-activated 1,3-dienes
Abstract Dihetaryl thioketones substituted with thiophen-2-yl and seleophen-2-yl rings react as superdienophilic reagents with non-activated 1,3-dienes such as 2,3-dimethylbuta-1,3-diene, cyclopentadiene and mixtures of isomeric hexa-2,4-dienes to produce the expected 2 H -thiopyrans in good to excellent yields. In the latter case, the corresponding cis -3,6-dihydro-3,6-dimethyl-2,2-dihetaryl-2 H -thiopyrans are formed as the sole products in a stereoconvergent thia-Diels-Alder reaction. A step-wise mechanism via delocalized intermediate diradicals is postulated to rationalize the observed reaction course. Treatment of 3,6-dihydro-4,5-dimethyl-2,2-di(thien-2-yl)-2 H -thiopyran with excess m CPBA at room temperature leads to the oxidation of both the C=C bond and the sulfur atom in the six-membered ring.