After investigating the substrate scope, two control experiments were conducted to evaluate the resulting selectivity of the product for this transformation, as presented in Scheme 5. An experiment of MBH ketone 1a with 2a was carried out in toluene at room temperature at the outset of this transformation, which produced the aza-Michael adduct 5 in good yield (Scheme 5 (i)). The regioselectivity of the addition was defined based on the literature reports on 2-aminobenzothiazole as a Michael donor utilizing the endocyclic nitrogen atom, including Biginelli reactions and other transformations. Subsequently, the isolated aza-Michael adduct 5 was further subjected to the standard reaction conditions, which afforded Subsequently, a calcium-promoted intramolecular condensation of 5 through the transition state TS may be expected to afford the desired pyrimidobenzothiazole 3. Scheme 6. Proposed mechanism to synthesis of 3a