Morita-Baylis-Hillman chemistry has received a lot of attention for its capacity to produce beneficial outcomes in organic synthesis. Morita-Baylis-Hillman (MBH) adducts have been applied broadly in multiple synthetic transformations owing to the presence of various functional groups in close proximity. One offspring of the adduct is the MBH ketone, obtained by the straightforward oxidation of the allylic alcohol moiety. MBH ketones are 1,1-disubstituted olefins having two electron-withdrawing groups conjugated to a terminal olefin. Owing to its versatile reactivity, it has been utilized as a crucial subject in valuable synthetic applications. In continuation of the growth of this topic of study. Our research group has also investigated MBH ketones to build various significant scaffolds. In this context, we previously reported an intriguing reactivity of MBH ketone to produce benzohydrazides via insertion of the hydrazine into the MBH ketone farmwork (Scheme 1, eq 1). Going from the hydrazines, which are 1,2-dinucleophiles, to the 1,4-dinucleophilic ethylene diamine/o-PDA, an interesting degradative dimerization of MBH ketone was observed (Scheme 1, eq 2). Furthermore, replacing one of the -NH2 groups in o-PDA with -SH gave another different and interesting outcome in the form of oxidative cyclization, leading to the formation of bioactive 2,2-disubstituted dihydro-1,4-benzothiazines (Scheme 1, eq 3). The contrasting reactivity patterns of MBH ketones observed above encouraged us to explore other nucleophilic systems in pursuit of novel transformations. As the earlier studies involved 1,2-&1,4nucleophiles, we imagined that it would be interesting to investigate a 1,3-dinucleophile. Therefore, 2aminobenzothiazole, a highly reactive, inexpensive, and easily synthesizable molecule capable of reacting with bis electrophilic substrates, was selected. It has been extensively employed as 1,3-dinucleophiles to construct the isolated or fused heterocycles in recent trends. We wished to examine the reactivity of 2aminobenzothiazoles to access novel analogues of fused heterocycle pyrimidobenzothiazole via a possible [3+3] annulation with MBH ketones (Scheme 1, eq 4). It must be mentioned at the outset that a [3 + 3] annulation of MBH ketones with 2-aminobenzothiazole has the possibility of delivering two different products -the 2H-and 4H-regioisomers of the pyrimidobenzothiazole. Towards our aim, the study was initiated by setting up a reaction of MBH ketone 1a with 2-aminobenzothiazole (2a) in toluene at room temperature, which produced a fluent reaction. Upon isolation of the observed product, NMR and HRMS spectroscopic analysis revealed it to be the aza-Michael adduct, formed by a simple addition of the endocyclic nitrogen of 2-aminobenzothiazole to the olefinic carbon of the MBH ketone. Subsequently, to obtain the fused heterocyclic product, a reaction was carried out in toluene at 110 C, which progressed well; upon isolation and characterization of the product, we were pleased to decipher it as the 4H-isomer of the fused heterocycle 3a (Scheme 2), formed by a successful [3 + 3] annulation; the formation of the 2H-isomer 3a' was not observed.