Pyridine Hammick reaction

These compounds all closely resemble the corresponding benzene compounds in their reactivity because the carbonyl group cannot interact mesomerically with the ring nitrogen. The pyridine 2- (picolinic), 3- (nicotinic), and 4- (isonicotinic) acids exist almost entirely in their zwitterionic forms in aqueous solution they are slightly stronger acids than benzoic acid. Decarboxylation of picolinic acids is relatively easy and results in the transient formation of the same type of ylide which is responsible for specific proton a-exchange of pyridine in acid solution (see section 5.1.2. ). This transient ylide can be trapped by aromatic or aliphatic aldehydes in a reaction known as the Hammick reaction. As implied by this mechanism, quaternary salts of... 

Concerning the pyridine-carboxylic acids Table 6.2), there has already been mentioned their tautomerism (p. 154), betaine and ester formation with alkylating reagents (p. 182), and the Hammick reaction (p. 163). 

Carboxylation of the potassium salt of 3-hydroxy-6-methylpyridine (145) gave the picolinic acid 146(84MI16). The Hammick condensation reaction of picolinic acid with benzaldehyde has been studied with regard to the effect of solvent, temperature, and molar ratio of reactants (85MI5). Benzoyl chloride, or benzaldehyde, and l-benzyl-4,6-diphenylpyridinium-2-carboxylate afford 2-benzoyl-4,6-diphenylpyridine (85JCS(P1)2167). Sulfur and 1,4,6-triaryl-pyridinium-2-carboxylates 147 in xylene at 140°C give the corresponding pyridine-2-thiones 148(838149). 

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