Unexpected nitrilimine cycloaddition of thiazolo[3,2-a]pyrimidine derivatives

•Unexpected isomers were synthesized by 1,3-dipolar cycloaddition reaction of DPNI.•The structures of products were solved by single crystal X-ray analysis.•A cycloaddition/ring-opening/rearrangement mechanism was proposed. 1,3-Dipolar cycloaddition of 2-arylidene-6,7-dihydro-5H-thiazolo[3,2-a]pyrimidine-3(5H)-ones to diphenylnitrilimine (generated in situ by triethylamine dehydrohalogenation of the N′-phenylbenzohydrazonoyl chloride) proceeded regio- and site-selectively affording a mixture of two unexpected isomer products at reflux temperature. The cycloaddition/ring-opening/rearrangement/substitution, mechanism of the reaction was also discussed.