15N NMR spectroscopy. 19—spectroscopic characterization of cyclodipeptides (2,5‐dioxopiperazines)

Abstract Various cyclodipeptides containing glycine, alanine, leucine, valine, phenylalanine, phenylglycine and sarcosine units were synthesized by cyclization of dipeptide pentachlorophenyl esters. The 13 C and natural abundance 15 N NMR spectra of these heterocycles were measured in trifluoroacetic acid and compared with the spectra of the corresponding amino acids and polypeptides. The 13 C NMR carbonyl signals of all cyclodipeptides show a 1.5–4.0 ppm upfield shift relative to the corresponding polypeptides. The 15 N NMR signals show no such consistent relationship. The substituent effects and the neighbouring residue effects observed in the 15 N NMR spectra of the cyclodipeptides are different from those of polypeptides, while the one bond NH coupling constant of cis and trans amide groups was almost identical. The nitrogen and the carbonyl signal of the Gly units in cyclo‐Gly‐Phe show an extraordinary downfield shift, reflecting the interaction of the phenyl group with the 2,5‐dioxopiperazine ring.