Aluminum trichloride acts as readily available, inexpensive, and efficient catalyst for one-pot three-component condensation reaction of aldehydes, dicarbonyl, and 2-amino benzothiazole under the solvent-free conditions to afford the 4H-pyrimido[2,1-b][1,3]benzothiazole derivatives 4 with good yield. The compounds synthesized in this study were evaluated for their antibacterial activities against gram-positive and gram-negative bacteria, viz., Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella typhi, Escherichia coli, Bacillus cereus, and Providencia rettegeri. Compounds 4c, 4d, 4f, 4g, and 4h showed their good activities against tested bacterial species. Pyrimidine derivatives 4d, 4f, and 4g have shown good antifungal activities against tested fungal strains, such as Aspergillus niger, Aspergillus fumigates, Aspergillus flavus, etc.