Ting-ting Jiang, Ning Liu, Yi-wen Jiang
Sep 3, 2017
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Organic Preparations and Procedures International
Abstract
Indole-2-carboxylic acid (4) is a versatile intermediate in the preparation of many pharmaceutically active agents. A review of the literature and patents indicates that several synthesis procedures already exist for this useful molecule. Among these, the reduction of 3-(2-nitrophenyl)-2-oxopropanoic sodium salt (3) prepared from 1-methyl-2-nitrobenzene (1) and diethyl oxalate (2) is generally used. For example, Wang et al. developed a method using ferrous sulfate and ammonium hydroxide as the reducing agents to obtain indole2-carboxylic acid. But the crude product needed to be recrystallized because of the precipitation of a large amount of iron mud. In addition, Kong et al. developed a ferrous hydroxide-catalyzed method with 80% (w/w) hydrazine hydrate as a reductant to obtain indole-2-carboxylic acid. And Kong et al. also developed a Raney-Ni catalyzed hydrogen reduction process to obtain indole-2-carboxylic acid, albeit in only moderate isolated yield. However, environmental concerns have led to increased interest in alternate processes. As a continuation of our interest in the study of medicinal compounds and the need for the title compound, we have developed a practical hydrogen reduction process with Pd-loaded Al-MCM-41 mesoporous catalyst (RS001) for the synthesis of indole-2-carboxylic acid (4) (Scheme 1).