Abstract
The proton nuclear magnetic resonance (1H-NMR) spectra of some 3-(1, 3, 4-oxadiazol-5-ylmethylene)-2-oxo-1, 2, 3, 4-tetrahydroquinoxalines and 3-(1, 2, 4-triazol-5-ylmethylene)-2-oxo-1, 2, 3, 4-tetrahydroquinoxalines were measured in dimethylsulfoxide-d6 (DMSO-d6), trifluoroacetic acid (TFA), and TFA-d1. The spectra in DMSO-d6 indicated that 2, 3, 4, and 5 existed as the two tautomers A and B, and 6, 7, and 8 as a single tautomer. From the spectra of 2, 3, 4, and 5 in DMSO-d6, it was elucidated that tautomer A was predominant over tautomer B at low temperature, while the ratio of the tautomer B gradually increased with increase of the temperature. The spectra in TFA indicated that 2 occurred as the three tautomers C, D, and E, while 3, 4, and 5 existed as a single tautomer, D. Compounds 6 and 7 also appeared to isomerize in TFA, that is, 6 existed as the two tautomers C and E, while 7 existed as two of the three tautomers C, E, and G. Compound 8 was predominantly present in the form of tautomer C or E.
