Alkyl esters of 5-heterocyclic-pyridine-2,3-dicarboxylic acids and methods for the preparation thereof

A method for the preparation of a compound having the formula I structure ##STR39## wherein R is C.sub.1 -C.sub.6 alkyl;

• X and Y are each independently oxygen, sulfur or NR.sub.4 ;
• R.sub.4 is hydrogen, C.sub.1 -C.sub.6 alkyl optionally substituted with C.sub.1 -C.sub.4 alkoxy or 1-3 halogens, SO.sub.2 R.sub.5, COR.sub.5, CO.sub.2 R.sub.5 or CONR.sub.5 R.sub.5 ;
• R.sub.5 is hydrogen, C.sub.1 -C.sub.6 alkyl optionally substituted with 1-3 halogens, or C.sub.2 -C.sub.6 alkenyl;
• M is C.sub.2 -C.sub.5 alkylene optionally substituted with 1 or 2 C.sub.1 -C.sub.4 alkyl groups, C.sub.1 -C.sub.4 alkoxy, halogen, CO.sub.2 R.sub.6 or oxo, and optionally interrupted by one oxygen or one sulfur C.sub.2 alkenylene optionally substituted with 1 or 2 C.sub.1 -C.sub.4 alkyl groups or CO.sub.2 R.sub.6, C.sub.3 alkenylene optionally substituted with 1 or 2 C.sub.1 -C.sub.4 alkyl groups or CO.sub.2 R.sub.6 or oxo, methyleneamino, optionally substituted with C.sub.1 -C.sub.4 alkyl or CO.sub.2 R.sub.6 , or a single bond, with the proviso that both X and Y are NR.sub.4, provided that the ring formed by M, X and Y and the carbon to which they are attached is no more than 8 atoms and provided that when the substituents on M are either alkoxy or halogen the substituted carbon is not bound to X or Y;
• R.sub.6 is hydrogen, methyl or ethyl;
• Z is hydrogen., halogen, C.sub.1 -C.sub.6 alkoxy, or C.sub.1 -C.sub.6 alkyl optionally substituted with C.sub.1 -C.sub.4 alkoxy or halogen
• which comprises reacting a compound having the structure ##STR40## wherein R.sub.1 is C.sub.1 -C.sub.4 alkyl, with at least 2 molar equivalents of a tri(C.sub.1 -C.sub.4)alkylorthoformate in the presence of a C.sub.1 -C.sub.4 alkyl alcohol, ozone and a catalytic amount of an acid to yield a di(C.sub.1 -C.sub.4)alkyl acetal pyridinedicarboxylate having the structure ##STR41## wherein R is C.sub.1 -C.sub.4 alkyl and reacting said di(C.sub.1 -C.sub.4)alkyl acetal with at least one molar equivalent of an agent having the structure
• HX--M--YH
• wherein X, M and Y are as described above in the presence of an acid and a solvent to give the formula I 5-heterocyclic-pyridine2,3-dicarboxylate compound.