Synthesis of isotopomers of N-(tert-butoxycarbonyl)-4-cyano-l-phenylalanine methyl ester: choice of cyanation solvent

N-(tert-butoxycarbonyl)-4-cyano-l-phenylalanine methyl ester and three isotopomers (C¹⁵N, ¹³CN, and ¹³C¹⁵N) were successfully synthesized in two steps to expand the utility of the nitrile symmetric stretch vibration of this modified amino acid as a vibrational reporter of local environments. The choice of cyanation solvent directly impacted the level of isotopic enrichment of the isotopomers. The commonly used solvent acetonitrile resulted in an isotopic enrichment of only ∼80% with a cyanation reaction time of 4.5 h, however, the cyanation solvent N,N-dimethylformamide afforded the isotopomers with >98% isotopic enrichment.