2-Methylterephthalic acid → 2-carboxy-5-oxo-5H-dibenzo[a,d]cycloheptene

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The starting material (i.e., 5-oxo-5H-dibenzo[a,d]-cycloheptene-2-carboxylic acid) is prepared by esterifying 2-methylterephthalic acid with methanol, in the presence of acid catalyst, to afford the corresponding dimethyl ester which, in turn, is reacted with N-bromosuccinimide to afford 2-bromomethylterephthalic acid dimethyl ester. This diester is reacted with triphenylphosphine to afford 2,5-bis(carbomethoxy)-benzyltriphenylphosphonium bromide which is treated with benzaldehyde and diazabicyclononene to afford, after alkaline hydrolysis, cis and trans-stilbene 2,5-dicarboxylic acid. Hydrogenation of this latter compound with hydrogen over a 5% palladium on carbon catalyst affords 2-(2-phenethyl)terephthalic acid. Treatment with polyphosphoric acid yields 5-oxo-5H-dibenzo[a,d]-cycloheptane-2-carboxylic acid which can be recrystallized from aqueous dimethylformamide.

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