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

Abstract

The synthesis of a series of 1 H-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 7– 10 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 34 , 35 , and 39 are also theoretically calculated by quantum chemical methods [ab initio at different levels of theory (HF/6-31G* and B3LYP/6-31G*)].