3,4-Diaminobenzoic acid → 2,3-Bis(4-methoxyphenyl)quinoxaline-6-carboxylic acid

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Briefly, the AB2 monomer, 2,3-bis(3-amino-4-hydroxyphenyl)quinoxaline-6-carboxylic acid dihydrochloride, is synthesized by condensing 3,4-diaminobenzoic acid and 4,4′-dimethoxybenzil to afford 2,3-bis(4-methoxyphenyl) quinoxaline-6-carboxylic acid, followed by demethylation in hydrobromic acid in acetic acid to form 2,3-bis(4-hydroxyphenyl) quinoxaline-6-carboxylic acid. The latter is then nitrated using nitric acid (70% conc.) in acetic acid at room temperature, yielding 2,3-bis(3-nitro-4-hydroxyphenyl) quinoxaline-6-carboxylic acid. The desired monomer is prepared by catalytic reduction in the presence of palladium catalyst in 10% hydrochloric acid.

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