Novel Approach to Synthesis of meta-Terphenyls

Abstract

In studying the synthetic possibilities for isomerization recyclization of nitropyridinium salts we described earlier in [1], we have established that the reaction can be used to synthesize meta-terphenyls (meta-diphenylbenzenes).Rearrangement of pyridinium salts 1a-d when treated with an aqueous alcoholic base occurs at room temperature, and leads to diphenylbenzenes 2a-d in 41-70% yield. Compounds 2a-d are formed through the open form A, which is formed after attack by the hydroxyl anion on the position ortho to the nitro group. The benzene ring is formed by an intramolecular aldol-crotonic condensation of the benzoyl group (formed after opening of the pyridine ring) at the methyl group of the open form A. A byproduct of recyclization of salt 1a is the pyridone 3a (9% yield), which is formed from the open form B by intramolecular acylation of the ester group of the enamine moiety. Recyclization of the salt 1b occurs regiospecifically; the possible product of heterocyclization with participation of the cyano group [2, 3] is not observed. N X Me Me Ph Ph O 2 N OH N Me Me X Ph Ph O 2 N Me X Ph Ph O 2 N NH O Me X Ph Ph O 2 N NHMe Me O N O 2 N Ph COMe Ph + X = COOEt B A 2a-d 1a-d 3a 1-3 a X = COOEt, b X = CN, c X = COMe, d X = CONHPh + + __________________________________________________________________________________________