Owing to the outstanding applications of pyrimidobenzothiazoles, the development of newer and more efficient methods for the construction of such compounds is a crucial task. Herein, we wish to report an efficient protocol to access new analogues of 4H-benzo [4,5] thiazolo [3,2-a] pyrimidine from MBH ketones and 2-aminobenzothiazoles using a unique environmentally benign Ca(II) catalyst. These above features, combined with our continued interest in MBH ketones, urged us to carry out detailed investigations for this transformation, the results of which are mentioned below.