Synthesis of pyrazinamide analogues and their antitubercular bioactivity

The Yamaguchi reaction is widely and generally applied to synthesize esters and lactones. It involves 2,4,6-trichlorobenzoyl chloride as the Yamaguchi reagent, 4-dimethylaminopyridine, and triethylamine. Pyrazinamide is a crucial first-line drug for tuberculosis treatment, therefore their analogues are interesting in organic synthesis. In general, the synthesis pyrazinamide analogues involve reaction of pyrazine-2-carboxylic acids with thionyl chloride to yield the corresponding acyl chlorides, which on treatment with amines gave pyrazine-2-carboxamides. However, thionyl chloride is listed under the Chemical Weapons Convention and releases toxic sulfur oxide when react with carboxylic acid. We successfully synthesized series of pyrazinamide analogues using the Yamaguchi reaction. The synthesis involved reaction of pyrazine-2-carboxylic acid with various aliphatic and aromatic amines in the presence of Yamaguchi reagent and 4-dimethylaminopyridine. The yield of the pyrazine-2-carboxamides and the reaction time depended on the type of the amine (aliphatic vs aromatic), substitution pattern, and number of substituents on the aromatic amines. N-(4-chlorophenyl)pyrazine-2-carboxamides can be prepared by this method in 81% yield; N-(2-ethylhexyl)pyrazine-2-carboxamide and N-(4-fluorobenzyl)pyrazine-2-carboxamide showed the best activity against Mycobacterium tuberculosis H37Rv (<6.25 μg/mL). This result could lead to find more active pyrazinamide analogues.