3 -AMINO-2- hydroxy-7-methyl- thiazolo[3,2-a]pyrimidin-5-one(5) ….was synthesized by one-pot reaction from 2-bromo cyanoacetamide, ethyl acetoacetate  and thiourea. The new 3-amino-2-hydrazino derivatives (15,18), 2-pyrazol- thiazolo [3,2-a] pyrimidin -5-ones )14,17,20) and 2-(4-arylidene) -pyrazol-thiazolo [3,2-a] pyrimidin-5-ones (22a-d) were synthesized through facile condensing procedures starting from 3-amino-2-hydrazino-7-methyl-thiazolo[3,2-a]pyrimidin-5-one (12). The newly synthesized products were characterized by their IR, ESI-MS, NMR and micro analytical data.