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Efficient preparations of 4- and 5-phenyl-4,5-dihydropyridazin-3(2H)-ones have been developed, the main reactions of these compounds have been studied, and the synthetic routes have been used to give analogues with substituents in the phenyl rings.In the best synthesis of the 4-phenyldihydropyridazinone (72% overall yield) the product was obtained from phenylacetic acid by three simple stages. This approach was applied in preparations of the 2- and 4-hydroxyphenyl compounds and, in conjunction with a recent method for amine protection, the 4-aminophenyl analogue. A four stage synthesis starting from benzaldehyde gave the 5-phenyl-dihydropyridazinone in 47% overall yield; hydroxybenzaldehydes were similarly converted into 5-(allyloxyphenyl)dihydropyridazinones.Oxidation to phenylpyridazinones occurred more readily with the 4- and 5-phenyldihydropyridazin-ones than with the 6-phenyl isomer. The 4- and 5-dihydropyridazinones were smoothly reduced to tetrahydropyridazinones by hydrogenation over platinum but were unaffected by palladium in the presence of hydrazine or cyclohexene.