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

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‐1‐15N‐2,6‐13C2) was prepared from K13C15N. 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‐1‐15N‐2,6‐13C2) 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‐1‐15N‐2,6‐13C2), whereas conducting the reduction in deuterium oxide produced an M+4 stable isotope (4, aminopyralid‐1‐15N‐2,6‐13C2‐5‐2H). Copyright © 2009 John Wiley & Sons, Ltd. Pentachloropyridine‐1‐15N‐2,6‐13C2 was prepared in three steps from 1,3‐dibromopropane and K13C15N. Isotopically labeled pentachloropyridine served as a key intermediate in the multi‐step synthesis of stable isotopes of auxinic herbicides 4‐amino‐3,5,6‐trichloropicolinic acid and 4‐amino‐3,6‐dichloropicolinic acid. Copyright © 2009 John Wiley & Sons, Ltd.