The regiochemistry of the product was further confirmed by single crystal X-ray structure analysis of an analogue, and the results unequivocally corroborated the formation of the 4H-isomer rather than the 2Hisomer (Figure ). Several methods for constructing these scaffolds have been reported. Among these findings, the Biginelli reaction has proven to be the most capable route for creating a huge library of pyrimidobenzothiazoles under variable conditions in a single pot. These Biginelli reactions were carried out between aldehydes, βketo ester/1,3-diketones, and 2-aminobenzothiazoles on heating or with the aid of various catalysts. A few isolated reports have also been published, such as that of Deniaud and co-workers, who described a [4+2] annulation protocol for the synthesis of these scaffolds using amidines as heterodienes and acrylates as dienophiles on reflux.. Furthermore, Li and co-workers have utilized N-(benzothiazole-2yl)amide, dialkyl acetylenedicarboxylates, and isocyanides for the synthesis of these via [3+2+1] cyclization. Recently, Li and co-workers have employed solid calcium carbide as an alkyne source in combination with 2-aminobenzothiazole and aldehydes to render such scaffolds. The above-mentioned methods have their own merit in the synthesis of pyrimidobenzothiazoles; however, some of these protocols also have drawbacks such as a long reaction time, low yield, high catalyst loading, expensive catalyst and toxicity.