Synthesis and Photochemistry of Pyrano[2,3-c]pyrazoles

Two different synthetic approaches to the synthesis of pyrano[2,3-c]pyrazoles have been investigated. In one approach, dehydroacetic acid derivatives were treated with phenylhydrazine and methylhydrazine led to the formation of the phenylhydrazones and methylhydrazones, which undergo rearrangement in refluxing acetic acid to diketo-phenylpyrazoles and diketo-methylpyrazoles. Upon treatment with a mixture of acetic and sulfuric acid these compounds isomerize to the phenylpyrano[2,3-c]pyrazol-4-one and methylpyrano[2,3-c]pyrazol-4-one derivatives. In a second approach, phenylhydrazine and methylhydrazine reacted with dimethyl(methoxymethylene)malonate (34) to give phenylpyrazole and methylpyrazole ester derivatives which were converted to phenylpyrazolone and methylpyrazolone by hydrolysis and decarboxylation. C-acylation of these compounds with trans-cinnamoyl chloride gave á,â-unsaturated-4-acetyl-5-hydroxypyrazoles. Bromination of these á,â-unsaturated-4-acetyl-5-hydroxypyrazoles with spontaneous cyclization, followed by dehydrobromination led to pyrano[2,3-c]pyrazol-4-one derivatives, respectively. Phototochemical excitation of 1-phenyl and 1-methylpyrano[2,3-c]pyrazol-4-ones in acetonitrile led to the formation of cis-head-to-tail [2+2] cycloaddition products. Irradiation in ethanol solvent led to photodimerization and to photofragmentation to yield pyrazole ethylesters