Synthesis and characterization of 5‐methyl‐1‐phenyl‐3,4,5,6‐tetrahydro‐1H‐2,5‐benzoxazocine‐1,3,4,6‐13C hydrochloride (Nefopam‐13C hydrochloride)

Abstract Nefopam hydrochloride (I), an analgesic agent, was labelled with carbon‐13 at C‐1,3,4 and 6 positions for metabolic studies. The starting materials for the synthesis included commercially available phthalic acid‐carboxyl‐13C1 (II) and 2‐methylaminoethyl‐1,2‐13C2 alcohol. The extent of isotope enrichment was determined by nuclear magnetic resonance and mass spectroscopy. The overall yield of I was 34% from phthalic acid‐carboxyl‐13C.