Pyridines decarboxylative approach

An alternative approach was used in the synthesis of the herbicide 530, where the sulfoxide 529 was converted to 530 in a single step by heating with lithium chloride in refluxing pyridine <1997TL4339>. The one-pot transformation involves sigmatropic sulfoxide elimination, lithium chloride-induced demethylation of the carbomethoxy group, decarboxylation, and a final isomerization/aromatization step <1997TL4339>. 

In an approach to direct C-functionalization of triazolo[4,5-c]pyridines, shown in Scheme 3, 1-methyl (or phenyl)[l,2,3]triazolo[4,5-c]pyridines (26,33) are alkylated exclusively at C-4 by radicals generated by decarboxylation of carboxylic acids (ammonium persulfate-sulfuric acid-silver nitrate) <90ZOB683>. However, with /-butanol various products are obtained depending on the catalyst employed. For example, with ammonium persulfate-sulfuric acid-silver nitrate, exclusive C(4)-methylation (34) was observed, while ammonium persulfate-sulfuric acid gave exclusively C(4)-/ -hydroxy-/ ,/ -dimethylethylation (cf. (36)). The /-butyl analogue (35) was obtained by decarboxylation of pivalic acid. 

Other procedures for decarbalkoxylation include heating P-keto esters in the presence of boric acid or 4-(//,A -dimethylamino)pyridine (DMAP) in HjO-tolueneP This latter approach is also useful for the decarboxylation of a,a-disuhstituted P-keto esters, which are prone to undergo cleavage via retro-Claisen condensation. 

Similarly to P-diketones a number of other p-oxo esters can also be readily arylated with a variety of aryllead triacetates. This was first reported by Pinhey in the arylation of ethyl 2-oxocyclopentanecarboxylate (153) with 4-methoxyphenyllead triacetate (141) in chloroform and pyridine in high yields (Scheme 4.41). The ideal conditions reported the use of 3.0 equivalents of pyridine to 1.0 equivalent of a-ryllead triacetate [89]. Moreover this approach could be utilised to generate a-aryl ketones following decarboxylation of the P-ketoester. Other examples include the a-arylation of 3- and 4-hydroxycoumarins [90, 91]. 

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