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

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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)benzyltriphenyl phosphonium bromide which is treated with benzaldehyde and a non-nucleophilic base, such as diazabicylononene, 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 recystallized from aqueous dimethylformamide. 5-oxo-5H-dibenzo[a,d]cycloheptene-2-carboxylic acid is prepared by successively treating 5-oxo-5H-dibenzo[a,d]cycloheptane-2-carboxylic acid with diazomethane, N-bromosuccinimide, and dimethylformamide/diazabicyclononene, followed by base hydrolysis and acidification.

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