From Thiocarbonylamino-acid Silyl Esters

From Thiocarbonylamino-acid Silyl Esters. Thiazolidine-2,5-diones (306) are accessible in high yield and purity by the cyclization of iV-(alkoxythio-carbonyl)amino-acid trimethylsilyl esters (305) by phosphorus tribromide. The reaction proceeds by an electrophilic attack of the acyl group on the sulphur atom, presumably involving intermediates of type (307). The 

Further treatment of (306) with trimethylchlorosilane-triethylamine at 0 °C causes substitution at the ring-nitrogen, yielding (308). The stability of the thiazolidine system prevents the occurrence of an equilibrium with the isomeric linear trimethylsilyl a-isothiocyanatoesters of the kind that is observed with the less stable iV-(trimethylsilyl)oxazolidinediones [(309) (310)]. The greater stability of the thiazolidine-2,5-diones is also reflected by their higher resistance to thermal decomposition and polymerization.  

---