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-methylhydrozino]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.