Pyrone-carboxylic acids, decarboxylation

Pyridine, pyrone and pyridone carboxylic acids undergo decarboxylation when heated, and the general order of reactivity is a > y > p. In pyridine, the carboxylic acids, as expected, exist mainly in the zwitterionic forms and decarboxylation of the a and y isomers under fairly mild conditions is a consequence of the relative stability of the ions of the type (48). 

Azinecarboxylic acids lose C02 significantly more easily than benzoic acid. Pyridinecarboxylic acids decarboxylate on heating with increasing ease in the order (3 < < y < a. 2-Pyridazinecarboxylic acid gives pyrazine at 200°C, and 4,5-pyrimidinedicarboxylic acid forms the 5-mono-acid on vacuum distillation. Pyrone- and pyridone-carboxylic acids also decarboxylate relatively easily thus, chelidonic acid (680 Z = O) at 160°C over copper powder and chelidamic acid (680 Z=NH) at 260°C give (681 Z = 0, NH). 

As in the case of <%-amino carboxylic acids, the zwitterions of N- and O-heterocyclic a-carboxylic acids can be decarboxylated thus <%-picolinic, thiazole-2-carboxylic, quinaldic, and chelidonic acid afford, respectively, pyridine, thiazole, quinoline, and 4-pyrone. Furan is best prepared by removal of carbon dioxide from pyromucic acid (2-furoic acid) by heat in early work this was effected by heating the acid in a sealed tube or with soda-lime, but Wilson19 has described a convenient method that involves only simple apparatus and affords the very good yields of 72-78% ... 

Treatment of MI-220 or MI-221 (R = H, CH3) with aromatic amines in water gives l-aryl-5-hydroxy(or methoxy)-4-pyridone-2-carboxylic acids, which are readily decarboxylated to XII-223, (R = H, CH3) or converted to ethyl esters."" 2-Hydroxymethyl-S-methoxy-4-pyrone, a monomethyl ether of kojic acid, and 2-dialkylaminoethylamines in water give l-(2-dialkylaminoethyl)-2-hydroxymethyl-5-methoxy-4-pyridones (MI-224 R = CH3,C2Hs). Natural maltol glucoside and ammonia or primary amines give the corresponding pyridones (XU-225) when boiled in methanol. ... 

Decarboxylation, of coumalic acid to give a-pyrone, 46,101 of diphenyliodonium-2-carboxylate, Va,m... 

Irradiation of 4,6-dimethyl-a-pyrone immobilized in a guanidinium-sulfonate-calixarene gives rise to a 4,6-dimethyl-Dewar-p-lactone, a carboxyl zwitterion and 1,3-dimethylcyclobutadiene, both in the solid crystalline state and in aqueous solution. Diarylmethyl carbocations have been generated within the cavities of non-protic zeolites (LiY, NaY, KY, RbY, CsY, and NaX) via laser-mediated decarboxylation of diarylacetic acids. 

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