Synthesis of Pyranopyrazol-4(1H)-ones and -4(2H)-ones from Dehydroacetic Acid. Homo- and Heteronuclear Selective NOE Measurements for Unambiguous Structure Assignment

Article abstract of DOI:10.1246/bcsj.60.4425

Cyclization of dehydroacetic acid N-substituted hydrazones furnished 1-substituted 3,6-dimethylpyrano<4.3-c>pyrazol-4(1H)-ones, together with varying amounts of N,N'-disubstituted 5-hydroxy-3-methyl-4-(3-methyl-1H-pyrazol-5-yl)-1H-pyrazoles.The same pyranopyrazolones were obtained regioselectively without side reactions from N-alkylhydrazines and 4-chloro-3-(1-chlorovinyl)-6-methyl-2H-pyran-2-one, ("Dehydroacetchlorid"), while N-arylhydrazines gave the 2-aryl-3,6-dimethylpyrano<4,3-c>pyrazol-4(2H)-ones.NMR NOE methods, including long-range selective heteronuclear 13C<1H> NOE enhancement measurements, allowed unambiguous between -4(1H)-ones and -4(2H)-ones.