STUDY OF AMINO-IMINO TAUTOMERISM IN DERIVATIVES OF 2-AMINONICOTINIC, 4-AMINONICOTINIC AND 6-AMINONICOTINIC ACID

C-13 NMR spectra of p-nitrobenzoyl 2-, 4-, and 6 -aminopyridine-3-carb oxylates, their hydrochlorides, trifluoroacetates and 1-benzyl derivat ives were studied. As found from the chemical shifts of pyridine carbo n atoms C-2, C-4 and C-6, the free bases exist in the amino form where as hydrochlorides and 1-substituted pyridinium derivatives in the imin o form. Trifluoroacetates of the 2- and 6-amino derivatives have struc ture similar to that of amidiniumcarboxylates (parallel hydrogen bonds and partially ionic character) whereas trifluoroacetate of the 4-amin o derivative is structurally close to the corresponding hydrochloride. The found structures were confirmed by H-1 NMR and IR spectroscopy.