Conversion of pyridine-2,3-dicarboxylic acid esters to cyclic anhydrides

A process for the production of a pyridine-2,3-dicarboxylic anhydride compound which comprises (1) synthesizing and recovering a pyridine-2,3-dicarboxylic diester which contains more than about 10 weight percent of organic impurities, and which corresponds to the formula:

• ##STR5## where each of R, R.sub.1 and R.sub.2 is hydrogen, halogen, C.sub.1 -C.sub.6 alkyl, phenyl or C.sub.1 -C.sub.4 alkyl-substituted phenyl group, and each of R.sub.3 and R.sub.4 is a C.sub.1 -C.sub.12 organic group;
• (2) cycling the diester through a thin film evaporator and distillation column system to provide a diester distillate of at least 90 weight percent purity;
• (3) hydrolyzing the purified diester in an alkaline aqueous medium, and acidifying the reaction medium to form a corresponding 2,3-pyridine-dicarboxylic acid having the formula:
• ##STR6## where R, R.sub.1 and R.sub.2 are as previously defined;
• (4) extracting the aqueous reaction medium with an organic solvent to obtain a solvent solution of the pyridine-2,3-dicarboxylic acid;
• (5) removing any residual water by azeotropic distillation;
• and (6) reacting the pyridine-2,3-dicarboxylic acid with at least an equimolar quantity of alkanoic anhydride in an organic solvent medium under anhydrous conditions to form pyridine-2,3-dicarboxylic anhydride product.