The photochemistry of some pyranopyrazoles

Irradiation of 1,3,6-trimethylpyrano [2,3-c] pyrazole-4 (1H)-one (1) or 3,6-dimethyl-1phenylpyrano [2,3-c]pyrazole-4-(1H)-one (2) in acetonitrile solution at 254 nm resulted in the formation of the cis-head-to-tail [2+2] dimers 5 or 13 respectively.When the irradiation was carried out in ethanol solvent 1 or 2 underwent dimerization to yield 5 or 13 respectively and also photocleavage to provide ethyl 5-hydroxy-1,3-dimethyl-1H-pyrazole-4-carboxylate (10) or ethyl 5-hydroxy-3-methyl-1-phenyl-1H-pyrazole-4-carboxylate ( 14) respectively.