Penicilloic acid decarboxylation

Penicilloic acids (III) react instantly in aqueous solution with mercuric chloride 19d giving penicillamine (VII) and penaldic acid (VI). Penaldic acids decarboxylate rapidly to form the corresponding penilloaldehyde (VIII). This degradation forms the basis for a sensitive analytical method for benzylpenicillin. ... 

Degradation. Penicillins are rapidly hydrolyzed by aqueous alkali to the corresponding penicilloic acids (30) which are stable as salts, but which decarboxylate on acidification to yield penilloic acids (31). 

Spectra of benzylpenilloic acids (see Figure 5.36) are more complex than those of the penicilloic acids as a result of the non-equivalence of the methylene protons produced at C-6 after decarboxylation. Full analysis is possible, however, from a 400 MHz spectrum, which reveals four 2-methyl resonances and duplicate 3-H, PhCH2, 5-H and fi-Hj signals, all evidence of the isomeric nature of the product (Figure 5.36). The 6-Hj signals are both composed of 8 lines as required by the ABX system (CH2CH), while the 5-H resonances appear as triplets. 

Benzylpenicilloic acid, the main hydrolysis product of benzylpenicillin, elicits wheal and flare skin reactions in some patients and was considered to be one of the so-called minor (in a quantitative sense) determinants by Levine. Decarboxylation of penicilloic acid gives rise to penilloic acid, another of the minor determinants... 

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