Synthesis and Mechanistic Study of Triheterocyclic 4H‐Pyrimido[2,1‐b]benzothiazole derivatives, One‐Pot Three‐component Reaction under Solvent‐Free Conditions

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.