S to N alkyl migration of substituted S-(1(3H)-isobenzofuranon-3-yl)isothiuronium bromide to N,N′-dimethyl-N-(3-oxo-1,3-dihydro-2-benzofuran-1-yl)thiourea provided evidence for the existence of an unusual double displacementmechanism involving two consecutive back-side SN2 reactions where a carboxylate anion has a crucial role both as a leaving group as well as an internal nucleophile. The thiazetidine