Formal oxygen atom addition to the arylmethylidenethiazolo[3,2-a]pyrimidines by NBS-mediated rearrangement as a concept of molecular skeletal editing.

This work reports a novel, catalyst-free, N-bromosuccinimide (NBS)-mediated rearrangement of thiazolo[3,2-a]pyrimidines leading to the formation of imidazo[2,1-b]thiazole derivatives. The transformation proceeds under mild conditions in an acetone-water system and involves a formal ring contraction of the pyrimidine fragment with simultaneous insertion of an oxygen atom. The reaction demonstrates high diastereoselectivity. A series of 2-arylmethylidene thiazolo[3,2-a]pyrimidines bearing various halogen substituents were successfully converted into the corresponding imidazo[2,1-b]thiazoles in high yields.