Condensation of 2‐amino‐5‐phenyl‐5H‐thiazolo[4,3‐b] [1,3,4] thiadiazoles (1) with some carboxylic acid derivatives furnished corresponding compounds 2–4, respectively. Alkylation of 1 with benzoylchloride and 4‐chlorobenzyl chloride afforded thiazolo[4,3‐b][1,3,4]thiadiazole derivatives 5 and 6, respectively. Similarly, transformation of 1 with chloroacetyl chloride yielded chloroacetamide derivative 7. The later compound was subjected to react with potassium thiocyanate or piperazine whereby, the binary thiazolidinone derivative 8 and N1,N4‐disubstituted piperazine 9 were produced, respectively. Also, the reactivity of 1 toward various active methylene reagents was investigated. Accordingly, our attempts to synthesize the tricyclic heterocyclic system 10, 11′, 12′ by reaction of 1 with chloroacetonitrile, 4‐oxo‐4‐phenylbutanoic acid and/or diethylmalonate in presence of acetyl chloride was furnished 10, 11, and 12. The newly synthesized compounds were screened as antimicrobial agent.

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