The chemistry of heterocyclic compounds is one of the leading areas of organic chemistry. These compounds can serve as the basis for both natural biologically active substances and synthetic ones. In recent years, an interest to the thiazolopyrimidines increased because of their different biological properties that can be used in medicine.

Also, modification of the thiazolopyrimidine platform using various pharmacophore groups such as arylmethylidene and arylhydrazone which have a variety of medicinal properties may become the basis for the design of new generation drugs.

The further functionalization of both arylmethylidene and arylhydrazone derivatives was investigated by our scientific group and two new rearrangements in the thiazolo[3,2-a]pyrimidine derivatives series were opened.

It was established that 2-arylhydrazone derivatives of thiazolo[3,2-a]pyrimidine under microwave activation conditions in a mixture of methanol and pyridine undergo rearrangement into [1,2,4]triazolo[4,3-a]pyrimidines.

The rearrangement of arylmethylidene thiazolo[3,2-a]pyrimidine to imidazo[2,1-b]thiazole under the reaction with NBS in an acetone-water mixture in the presence of ammonium acetate at room temperature was also revealed. Apparently, initially formed by the endocyclic double C=C bond of the thiazolopyrimidine platform bromohydrin rapidly goes through transformation into an unstable oxyrane, which then launches this rearrangement.

The obtained derivatives were characterized by a complex of physico-chemical methods (IR, NMR-¹H and ¹³C spectroscopy, ESI-MS spectrometry, including single-crystal X-ray diffraction analysis).