O11-enol/O10-keto (b) and O10,O11-diketo (c) tautomeric forms were investigated in the gaseous phase, solution and crystalline state. Spectroscopic studies 1H, 13C NMR and IR allowed for the identification of the compounds 1–6 but they did not indicate explicitly their tautomeric forms present in solution and in the solid state. The X-ray analysis showed that the molecules of all investigated compounds exist