A highly efficient photochemical bromination as a new method for preparation of mono, bis and fused pyrazole derivatives.

N-Bromosuccinimide (NBS) was found to afford photochemical bromination of N-substituted 3-methyl-2-pyrazolin-5-one in chloroform solution. The nature of the products formed was found to be highly dependent on the photolysis time and on the type of N-substituted 3-methyl-2-pyrazolin-5-one. The reaction of substituted 2-pyrazolin-5-ones with NBS in the absence of light yielded in all cases a mixture of N-substituted 3-methyl-4-bromo- (and -4,4-dibromo-) 2-pyrazolin-5-ones. The mechanism of photobromination was illustrated. Separation of all of the products was achieved using column chromatography. The chemical structure of all of the products was assigned by elemental analysis, IR, 1H NMR and GC-MS measurements.