Studies on 2, 5-Diaryl-2, 4-dihydro-3H-pyrazol-3-ones. I. Synthesis of Highly Substituted 1H-Indazoles Using Tautomeric 2, 5-Diaryl-2, 4-dihydro-3H-pyrazol-3-ones

The reaction of 2-aryl-5-phenyl-2, 4-dihydro-3H-pyrazol-3-ones (1) with acetone gave 2-aryl-4-(1-methylethylidene)-5-phenyl-2, 4-dihydro-3H-pyrazol-3-ones (2) in nearly quantitative yields, and these products reacted further with acetone in the presence of triethylamine (NEt3) as a catalyst to give 1-aryl-4, 6, 6-trimethyl-3-phenyl-1, 6-dihydropyrano [2, 3-c] pyrazoles (3). Reaction of 3 with dimethyl acetylenedicarboxylate (DMAD) or diethyl acetylenedicarboxylate (DEAD) in dimethylformamide (DMF) at reflux gave 1-aryl-4-methyl-3-phenyl-1H-indazole-6, 7-dicarboxylate (7) in high yields. Thus, a new and convenient route for the synthesis of highly substituted 1H-indazoles has been developed.