Synthesis of substituted tetrahydro-3,3-dimethyl-1H-benzothiazolo[2,3-b]quinazolin-1-ones, well established medicinally important compounds, has been achieved by multicomponent condensation reaction of 2-amino-6-chlorobenzothiazole, substituted benzaldehydes, and 5,5-dimethylcyclohexane-1,3-dione (dimedone) under different reaction conditions and using different energy sources, e.g., microwave irradiation, sonication, and classical heating, for comparison purposes. Use of a monomode oven enabled accurate monitoring of the temperature distribution in the microwave reaction vessel, and revealed a very strong and unexpected thermal heterogeneity. The reaction was facilitated by the presence of DMF, the catalytic role of which is demonstrated.