In summary, a distinct reactivity of MBH ketone has been described, involving the MBH ketone acting as a 1,3-dielectrophile and 2-aminobenzothiazole as a 1,3-dinucleophile to bring about a [3 + 3] annulation. Accompanying the fascinating reactivity of MBH ketone, it also provides an efficient Ca(II) catalyzed protocol for the synthesis of new analogues of pyrimidobenzothiazoles from the reaction with 2aminobenzothiazoles. Different functional groups were tolerated by MBH ketone and 2aminobenzothiazole, leading to the formation of desired products in yields ranging from good to excellent.