Peptide Conformation, IX Conformational Studies on Cyclotripeptides Containing N‐Benzyl‐Glycine

Synthesis and NMR spectra of the cyclic tripeptides of the general structure cyclo[Pro_xBzl.Gly_3‐x] (x = 0–2; Bzl.Gly. = N‐Benzylglycyl) are reported. Two slowly interconverting conformations of these tripeptides containing only cis peptide bonds have been observed in solution by NMR spectroscopy. Structural proof and assignments to a flexible boat conformation and a rigid crown conformation, respectively, have been achieved by interpretation of the NMR parameters and the correlation with the crystal structure of one compound. The conformational behaviour of cyclic tripeptides is discussed.