Nmr study of the dynamic equilibria among isomeric species in quinoxalin-2-one derivatives

The understanding of the prototropic lactam/lactim/enol equilibrium and the E/Z isomerism around the N-C(O) bond in amides is important for the determination of different physicochemical properties in compounds, moreover in compounds with more than one equilibrium. In this work, these equilibria were studied for N-acyl and N-carbamoyl-3,4-dihydroxyquinoxalin-2(1H)-ones using mono- and bi-dimensional NMR experiments, at different temperatures. For both types of derivatives, the lactam form was the only tautomer detected. The results obtained from molecular modelling experiments agree with the experimental results. For the N-acyl derivative, the E isomer is the predominant, while for the N-carbamoyl derivative both geometric isomers are present in a similar ratio. The theoretical calculations allowed the signal assignment for each isomer from the chemical shift values estimated for both geometric isomers corresponding to the studied N-acyl and N-carbamoyl derivatives.