Carboxylation of the aromatic ring system

The Ji-electron system of the aromatic ring is electron rich, but this is subjected to the electron-withdrawing effect of the carboxylic acid. 

A common characteristic of all polystyrene/divinylbenzene-based ion-exclusion phases is the high retention of aromatic carboxylic acids. This is due to 7t-7t interactions between the aromatic ring systems of the polymer and the solute. The separation of aromatic carboxylic acids, therefore, is more elegantly accomplished by the various procedures of reversed-phase chromatography. 

It follows from the above evidence that one of the hydroxy-groups in iodinin (180) is present before formation of the phenazine ring system. Accordingly, a highly efficient incorporation of 6-hydroxy-[7,9- H2]phenazine-l-carboxylic acid (as 179) into iodinin (180) was found. It thus appears probable that B. iodinum converts (170) and (179) by the same or similar enzyme systems into (178) and iodinin (180), respectively, N-oxide formation occurring either before or after hydroxylative decarboxylation. The two acids, (170) and (179), may well arise from the same (non-aromatic) precursor, by alternative aromatization reactions. 

A concerted elimination-cyclization mechansim, involving a sulfenyl halide in a 1,3-butadiene-1-thio system, is the most probable mechanism for the formation of benzo[6 Jthiophenes from cinnamic acids or 4-aryl-2-butanones by treatment with thionyl chloride. The reactions shown in Scheme 5 have been carefully worked out, and the intermediates isolated (75JOC3037). The unique aspect of this synthesis is the reduction of the sulfinyl chloride (a) by thionyl chloride to form the sulfenyl chloride (b). The intermediate (b) was isolated and converted in pyridine to the 3-chlorobenzo[6]thiophene-2-carbonyl chloride in 36% yield (73TL125). The reaction is probably initiated by a sulfenyl ion attack on the aromatic ring, since it is promoted by electron-releasing groups para to the site of ring closure. For example, when X in (36) was N02, a 23% yield of (37), a mixture of 5-and 7-nitro derivatives, was obtained, but when X in (36) was OMe, a 54% yield of (37) was obtained, contaminated with some 3,4-dichloro-5-methoxybenzo[6]thiophene-2-carboxylic acid. 

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