Synthesis of stable isotopes of auxinic herbicides 4‐amino‐3,5,6‐trichloropicolinic acid and 4‐amino‐3,6‐dichloropicolinic acid

Abstract Pentachloropyridine serves as a key intermediate in the synthesis of 4‐amino‐3,5,6‐trichloropicolinic acid (picloram) and 4‐amino‐3,6‐dichloropicolinic acid (aminopyralid). An M+3 stable isotope of pentachloropyridine (1, pentachloropyridine‐1‐ 15 N‐2,6‐ 13 C 2 ) was prepared from K 13 C 15 N. Isotopically labeled pentachloropyridine was then carried through a seven‐step synthesis to give an M+3 stable isotope of 4‐amino‐3,5,6‐trichloropicolinic acid (2, picloram‐1‐ 15 N‐2,6‐ 13 C 2 ) in an overall yield of 42%. The chlorine atom in the 5‐position of 2 was selectively removed via electrochemical reduction. Carrying out the electrochemical reduction in water provided an M+3 stable isotope of 4‐amino‐3,6‐dichloropicolinic acid (3, aminopyralid‐1‐ 15 N‐2,6‐ 13 C 2 ), whereas conducting the reduction in deuterium oxide produced an M+4 stable isotope (4, aminopyralid‐1‐ 15 N‐2,6‐ 13 C 2 ‐5‐ 2 H). Copyright © 2009 John Wiley & Sons, Ltd.