Potentially tautomeric 1,2,3,4-tetrahydro-1,4-dioxo-5H-pyridazino[4,5-bindole

The tautomeric 1,4-dioxygenated pyridazinoindole 1 exists in aqueous solution as a mixture of all four tautomeric forms 1a - 1d. The predominant tautomeric form is the 4-hydroxy-1-oxo hydroxy-1-oxo compound 1b. The relative abundance of the four forms 1a 1b 1c 1d at equilibrium is 10^4.93:10^8.03:10^3.61:1.