Synthesis and structural study of tetrahydroindazolones

Multinuclear magnetic resonance spectroscopy allowed us to characterize four 1(2),5,6,7-tetrahydro-4H-indazol-4-one derivatives (1-4) and establish the most stable tautomer in each case. The crystal structure of 6,6-dimethyl-1(2),5,6,7-tetrahydro-4H-indazol-4-one (2) (orthorhombic space group P2(1)2(1)2(1), a=10.1243(8), b=21.526(2), c=24.992(2) Å, Z=4, 293 K) presents two different trimers, bonded through N-H⋯N hydrogen bonds involving tautomers 1H and 2H. In crystalline 3,6,6-trimethyl-2,5,6,7-tetrahydro-4H-indazol-4-one (4) (monoclinic space group P2(1)/c, a=5.9827(7), b=16.494(2), c=11.012(1) Å, β=93.464(2)°, Z=4, 293 K) only tautomer 2H exists forming a hydrogen-bonded network through the 4-oxo group and a water molecule.