Quinoxalines XIV. Synthesis, 1H, 13C, 15N NMR spectroscopic, and quantum chemical study of 1H-pyrazolo[3,4-b]quinoxalines (flavazoles)

The synthesis of a series of 1H-pyrazolo[3,4-b]quinoxalines (flavazoles) by acylation, alkylation, halogenation, and aminomethylation of the parent compound is reported and their structure is investigated by 1H, 13C, and 15N NMR spectroscopy. The restricted rotation about the partial C, N double bond of the N-acyl derivatives is studied by dynamic NMR spectroscopy and the barriers to rotation are determined. In order to assign unequivocally the 15N chemical shifts of N-4 and N-9, in case of 3-substituted flavazoles, exemplary the 1H, 13C, and 15N NMR chemical shifts of , , and are also theoretically calculated by quantum chemical methods [ab initio at different levels of theory (HF/6-31G* and B3LYP/6-31G*)].