Reaction of 2-hydrazino-3-methylquinoxaline with aryl-1,3-diketones: A structural reinvestigation

Treatment of 2-hydrazino-3-methylquinoxaline ( 1) with phenyl-1,3-butanedione (2b) in THF affords regioisomeric 1-(3'-methylquinoxalin-2'-yl)-3(5)-methyl-5(3)-phenylpyrazoles (3b and 4b) as major products along with the formation of small amount of 1,4-dimethyl-1,2,4triazolo[4,3-a]quinoxaline ( 5) rather than the reported exclusive formation of 5. Several other regioisomeric 1-(3'-methylquinoxalin-2'-yl)-3,5-disubstituted pyrazoles have similarly been synthesized using other aryl- and heteroaryl-1,3-diketones. The identity of the regioisomeric pyrazoles is based upon NMR (H-1 & C-13) spectral data and an unambiguous synthesis.