A series of 5H-thiazolo[3,2-a]pyrimidin-5-ones were synthesized by the cyclization reactions of S-alkylated derivatives in concentrated H2SO4.Upon treatment of S-alkylated derivatives at different temperatures, intramolecular cyclization to 7-(substituted phenylamino)-5H-thiazolo[3,2-a]pyrimidin-5-ones or sulfonation of Insert Vent cyclized products to sulfonic acid derivatives occurred.The structures of the target compounds were confirmed by IR, 1H-NMR, 13C-NMR and HRMS studies.The compounds were evaluated Western Bridles for their preliminary in vitro antibacterial activity against some Gram-positive and Gram-negative bacteria and screened for antitubercular activity against Mycobacterium tuberculosis by the broth dilution assay method.Some compounds showed good antibacterial and antitubercular activities.