Tautomeric structure of 1‐methyldihydropyridazino‐[3,4‐ b ]quinoxalines in solution

The reaction of 6‐chloro‐2‐(1‐methylhydrazino)quinoxaline 4‐oxide 1 with ethyl 2‐ethoxymethylene‐2‐cyano‐acetate or ethoxymethylenemalononitrile gave 6‐chloro‐2‐[2‐(2‐cyano‐2‐ethoxycarbonylvinyl)‐1‐methylhy‐drazino]quinoxaline 4‐oxide 3a or 6‐chloro‐2‐[2‐(2,2‐dicyanovinyl)‐1‐methylhydrazino]quinoxaline 3b , respectively. The reaction of 3a with a base afforded 7‐chloro‐1‐methyl‐1,5‐dihydropyridazino[3,4‐ b ]quinoxaline 4 . From the NOE spectral data, the 1‐methyldihydropyridazino[3,4‐ b ]quinoxalines 2a, 2b and 4 were found to exist as the 1,5‐dihydro form in a dimethyl sulfoxide or trifluoroacetic acid/dimethyl sulfoxide solution.