An efficient method has been developed for the preparation of 4H-pyrimido[2,1-b]benzothiazole derivatives by the condensation of aldehydes, β-ketoester, and 2-amino benzothiazole under solvent and solvent-free conditions using various catalysts. The reaction uses benzothiazole as a new component, and good yield is obtained at 60–65°C under solvent-free conditions. Atom economies, good yield, environmentally benign, and easy to work-up are some of the important features of this protocol. The present study suggests that acetic acid and metal catalysts follow different mechanism. In acetic acid, 2-amino benzothiazole reacts with benzaldehyde, and resultant intermediate reacts with ethyl acetoacetate to give final product, whereas in the presence of metal catalysts, 2-amino benzothiazole first reacts with ethyl acetoacetate, and resultant intermediate reacts with benzaldehyde to give pyrimido[2,1-b]benzothiazole.