Characterization of two hydroxytrichloropicolinic acids: application of the one-bond chlorine-isotope effect in 13C NMR

The structures of 4‐hydroxy‐3,5,6‐trichloropyridine‐2‐carboxylic acid (1a) and 6‐hydroxy‐3,4,5‐trichloro‐2‐carboxylic acid (1b) were verified by the NMR analysis of their corresponding methylated and decarboxylated derivatives 2,3,5‐trichloro‐4‐methoxypyridine (5) and 3,4,5‐trichloro‐2‐methoxypyridine (8), respectively. The 6‐hydroxy isomer (1a) was found to be in equilibrium with its pyridinone tautomer as evidenced by the formation of significant amounts of 3,4,5‐trichloro‐1‐methyl‐6‐oxo‐1,6‐dihydropyridine‐2‐carboxylic acid methyl ester (6b) on exhaustive methylation. The one‐bond chlorine‐isotope effect was used and shown to be an effective tool for the identification of chlorinated carbons in 13C NMR spectra providing an additional tool for solving structural problems in chlorinated compounds. Copyright © 2008 John Wiley & Sons, Ltd.