Bioorganic stereochemistry. A study of the peptide oxazolones from Z-(Aib)n-OH (n = 2–4) in the solid state*

An investigation of the preferred conformations and modes of self-association of the peptide oxazolones from Z(-Aib-)_n-OH (n = 2–4) in the solid state has been performed by infrared absorption. More detailed information on the peptide oxazolone from Z(-Aib-)₃ OH (2) has been obtained using X-ray diffraction. In this compound the conformations of the first two Aib residues differ substantially, only the N -terminal one being found in the usual 3_10- (or α-) helical region of the Ramachandran map. The C=N-bond of the oxazolone group is not conjugated with the lactone moiety. A very weak intermolecular interaction occurs between the urethane N-H and the carbonyl group of the oxazolone ring.