Cycloaddition of Bis[3,3-bis(diethylamino)thioacryloyl] Disulfide with Alkynic Dienophiles

The reaction of bis[3,3-bis(diethylamino)thioacryloyl] disulfide (1) with dimethyl acetylenedicarboxylate gave dimethyl 4-diethylamino-2-thioxo-2H-thiopyran-5,6-dicarboxylate in 69% yield. Similarly, a series of alkynic dienophiles reacted with 1 to give the corresponding 2H-thiopyran-2-thione derivatives in modest-to-reasonable yields. A full experimental description of this 2H-thiopyran-2-thione formation is given and a mechanism involving [4+2]cycloaddition followed by homolytic cleavage of the S-S bond is presented.