Synthesis of 2,3-Diarylquinoxaline Carboxylic Acids in High-Temperature Water

Abstract

Aromatic carboxylic acids are prone to decarboxylate in high-temperature water (HTW). While the decarboxylation kinetics of several aromatic carboxylic acids have been explored, studies on their compatibility with organic syntheses in HTW are scarce. Herein, we report the hydrothermal synthesis (HTS) of 2,3-diarylquinoxaline carboxylic acids from 1,2-diarylketones and 3,4-diaminobenzoic acid. A detailed study of the reaction parameters was performed to identify reaction conditions towards minimal decarboxylation. Thirteen 2,3-diarylquinoxaline-6-carboxylic acids are obtained at temperatures between 150–230 °C within 5–30 minutes. The reported conditions feature comparable performance to those of classic syntheses, avoiding volatile organic solvents, strong acids and toxic catalysts. Decarboxylated quinoxalines arise as side products in variable amounts via direct decarboxylation of the 3,4-diaminobenzoic acid. To completely inhibit the decarboxylation, we show that suitable structural analogues of 3,4-diaminobenzoic acid can act as starting compounds. Thus, ester hydrolysis of methyl 3,4-diaminobenzoate and deprotection of di-Boc-protected 3,4-diminobenzoic can be coupled with the HTS of quinoxaline towards quinoxaline carboxylic acids, while fully avoiding decarboxylated side products.