A straight forward method has been developed for the synthesis of novel heterocycles such as benzo[4,5]thiazolo[3ˈ,2ˈ:1,2]pyrimido[4,5-d][1,3]thiazine derivatives (5a-i) by condensation of 6-imino-4-(methylthio)-2-phenyl-6H-1,3-thiazine-5-carbonitrile (3) with 2-amino 1/2/3/4-substituted benzothiazoles (4a-i) in DMF by using anhydrous potassium carbonate as catalyst. Compound (3) was prepared by reaction of benzothioamide (1) and bis(methylthio)methylene malononitrile (2) with same reaction condition which is used for title compounds. The chemical structures of newly constructed derivatives were corroborated by IR, 1H-NMR 13C-NMR and Mass spectral analysis. Synthesized compounds were screened for antimicrobial as well as antioxidant activity.