الفهرس 
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Abstract
Treatment of the hydrazones 146-151 with acetic anhydride under reflux afforded either a mixture of the angular and linear regioisomers III and IV, respectively or only one regioisomer III or IV) as deduced from their spectral data.Moreover, under the acidic medium of the reaction, Dimroth rearrangement has been taken place depending on the nature of the substituent on the phenyl ring. Electron withdrawing groups as chloro, bromo and nitro influenced Dimroth rearrangement in order to obtain a mixture of the two regioisomers, the kinetically controlled angular isomers N,N’-diacetyl-3-aryl-5-methyl-7-oxo [1,2,4]triazolo[4,3-a]pyrimidine 153-154 and the thermodynamically linear isomers N,N’-diacetyl 3-aryl-7-methyl-5-oxo-[1,2,4]triazolo[4,3-a]pyrimidine 156-157. An observation of Dimroth rearrangement in presence of light was reported, where exposure of a solution of the angular isomers 153 or 154 in ethanol to light afforded the rearranged pure more stable thermodynamically linear isomers 156 or 157, respectively. However, no changes were observed when 153 or 154 was irradiated using ultrasound in dark at room temperature. Most remarkable, when the nitro derivative 155 was exposed to light, a mixture of both isomers was obtained in ratio 2:3 (kinetic isomer 155: thermodynamic isomer 158).