Enantioselective syntheses of isotopically labelled α‐amino acids. Preparation of (ϵ‐13C)‐L‐α‐aminoadipic acid and five isotopomers of L‐lysine with 13C, 15N and 2H in the δ‐ and ϵ‐positions

An asymmetric synthesis of the six‐carbon α‐amino acids (ϵ‐13C)‐L‐α‐aminoadipic acid and various isotopomers of L‐lysine is described. The synthesis is based on a general method starting from the bis‐lactim ether of cyclo(D‐Val‐Gly) and simple labelled reagents like K13CN, K13C15N, 13CH3CN and LiAl2H4. Using this route (ϵ‐13C)‐L‐α‐aminoadipic acid was prepared in 31% yield based on the labelled potassium cyanide. Five different isotopomers of L‐lysine were prepared in high overall yield (45% based on the labelled starting compound): (ϵ‐13C)‐L‐lysine, (ϵ‐13C,ϵ‐15N)‐L‐lysine, (δ‐13C)‐L‐lysine, (ϵ‐2H2)‐L‐lysine and (ϵ‐13C,ϵ‐2H2)‐L‐lysine. The isotopomers were characterized using various spectroscopic techniques, e.g., 1H NMR, 2H NMR, 13C NMR, 15N NMR and mass spectrometry. The 31.2‐ppm and the 27.6‐ppm peak in the 13C NMR spectrum of lysine could be unambiguously assigned to the β‐ and δ‐carbons, respectively. This means that the assignment previously reported is incorrect.