Metal-free C3-alkoxycarbonylation of quinoxalin-2(1H)-ones with carbazates as ecofriendly ester sources

Quinoxaline-3-carboxylates and analogues are prevalent key structural motifs in bioactive natural products and synthetic drugs. However, the practical protocol for preparation of these motifs from simple raw materials under mild conditions remains rare. In this article, we report a facile protocol for the efficient preparation of various quinoxaline-3-carbonyl compounds (30 examples, 63%-92%) through oxidation coupling of quinoxalin-2(1H)-ones with readily available carbazates (or acyl hydrazines) in the presence of K2S2O8 as an oxidant in metal- and base-free conditions. When tert-butyl carbazate was used as the coupling reagent, the decarboxylation product 3-(tert-butyl)-1-methylquinoxalin-2(1H)-one was obtained. The application of this process into a gram-scale synthesis can be easily accomplished. Mechanistic investigations reveal that the functionalization of quinoxalin-2(1H)-ones via a free-radical pathway.