Six l(2)//-3-aryl-4-/?-chlorophenyl-dihydropyrazolo [3,4-b]pyridin-6-ones have been prepared from the corresponding arylaminopyrazoles.The NMR study of these compounds reveals that all of them exist as 2//-tautomers in solution (DMSO-dg) as well and in the solid state. IntroductionThe reaction of 3(5)-aminopyrazoles with benzaldehydes and Meldrum's acid directly or preparing first the Meldrum's acid benzylidene derivative (3), has been studied by some of us in the case of 5-amino-l-aryl-3-methylpyrazoles. 1 With N-unsubstituted pyrazoles (1) or (2), the reaction, in principle, can afford two derivatives: a dihydropyrazolo[3,4-i»]pyridin-6-one (4) or (5) (similar to that obtained previously with 1-aryl derivatives) 1 or a dihydropyrazolo[l,5-a]pyrimidine (6), like in the case of the compounds prepared from methyl orthoformate, Meldrum's acid and NH-pyrazoles. 2e absence of H4 excludes structure 6; it remains to determine the tautomerism and properties of the 3-aryl-4-p-chlorophenyl-dihydropyrazolo[3,4-b]pyridin-6-ones (4) or (5).The starting aminopyrazoles (1/2) present an interesting case of tautomerism: depending on the substituent X (a H, b CH3, c OCH3, d CI, e Br, f NO2) the most stable tautomer is the 3-amino-5-aryl (1) or the 5-amino-3-arylpyrazole (2). 3 Therefore, it was interesting to determine the tautomeric structure, (4) or (5), of the resulting cyclic compound and to compare it with that of the starting material, (1) or (2). Results and Discussion SynthesisCompounds (4/5) were prepared by reaction of the corresponding l//-3(5)-amino-5(3)arylpyrazoles (la/2a)-(lf/2f) with p-chlorobenzaldehyde and Meldrum's acid in ethanol.Vol. 5, No. 2, 1999 Synthesis,molecular structure and tautomerism of 1 (2)H-dihydropyrazol of[3,4-b]pyridin-6-ones γ X X X 2 Scheme 1 Ή and 13 C NMR spectroscopy in solutionThe iH and 13 C NMR spectra are gathered in Tables 1 and2.The 13C results will be discussed in relation with the tautomerism.The first order analysis of the ABC system formed by the protons of positions 4 and 5 yield three coupling constants, a geminal (15.8-15.9Hz) and two vicinal ones (7.4-7.6 and 1.8 Hz).The application of Karplus equations [ 3 / v ic = 8.5 cos 2 <|> -0.3 (for 0° < φ < 90°) and 3/ vic = 9.5 cos2$ -0.3 (for 90° < φ < 180°)] 4 • 5 leads to two dihedral angles 80° (for 1.8Hz) and 140° (for 7.5 Hz).These values identified H-5a and H-5e and established that the 4-pchlorophenyl group is in a pseudo-equatorial position.13 C NMR spectroscopy in the solid state (CPMAS) ^