64. N-Acylureas in Peptide-Synthesis: An X-Ray Diffraction and IR-Absorption Study

Article abstract of DOI:10.1002/hlca.19900730311

An X-ray diffraction analysis of two N-acyl derivatives of symmetrical dialkylureas, N-α-(benzyloxycarbonyl)-L-valyl>-N,N'-diisopropylurea (1) and N-α-<(tert-butyloxy)carbonyl>-L-valyl>-N,N'-dicyclohexylurea (2), and one N-acyl derivative of an unsymmetrical N,N'-dialkylurea, N-α-(benzyloxycarbonyl)-L-valyl>-N'-(tert-butyl)-N-ethylurea (3), has been performed.It was established that it is the least hindered O-acylisourea N-atom that attacks intramolecularly the carbonyl group of the N^α-protected amino acid activated by the unsymmetrical carbodiimide to form the major rearrangement product.The occurrence and nature of intra- and intermolecularly H-bonded forms of the N-acylureas in the crystal state were also assessed.It was also shown that soluble N-acylureas may complete with ine intermolecular (petptide)N-H...O=C(peptide) H-bonds in CH2Cl2.