Current interest has been concentrated on the chalcogen atom-containing reactive species generated by the fragmentation of cyclic chalcogenoacetals. Especially, it was expected that oxidation of conformationally flexible eight membered aminochalcogenoacetals would cause transannular Ch-Ch interaction to give 1,5,3,7-dichalcogenadiazabicyclo[3.3.0]octane-type dichalcogena dications which would undergo further fragmentation to give novel cyclic dichalcogenides by the attack of nuclephiles toward the methylene carbons of the dications. In this study, we carried out the investigation of a novel ring contraction of 3,7-Disubstituted 2H,6H-tetrahydro-1,5,3,7-dischalcogenadiazocines 1 to 1,2,4-dichalcogenazolidines 2 by treating with various oxidizing agents.
Treatment of diselenadiazocines and diteluradiazocines with oxidizing agents such as NBS, mCPBA, t-BuOOH, or CuCl_2 afforded the corresponding 4H-dihydro-1,2,4-dichalcogenazolidines in goodyields.
On the other hand, treatment of a CH_2Cl_2 solution of 1,5,3,7-dithiadiazocines with bromine at -78 ℃ afforded unexpected 6H-dihydro-1,2,3,4,5,7-pentathlazocines 3 in low yields along with insoluble products and no 1,2,4-dithiazolidines were obtained. Yields of 6H-dihydro-1,2,3,4,5,7-pentathiazocines were dramatically improved by treating 1,5,3,7-dithiadiazocines with Br_2/S_8 or S_2Cl_2.
It was assumed that 2(Se, Te) were formed from 1 through oxidative ring contraction via dication. Compound 3 would be formed by the sequent reaction of 1,2,4-dithiazolidines 2(S), which were generated by a similar manner, with elemental sulfur.