Peptide cyclols. Crystal structure and molecular conformation of the oxacyclol derived from L-2-hydroxyisovaleryl-L-phenylalanyl-L-proline

Abstract

Cyclization of L-2-hydroxyisovaleryl-L-phenylalanyl-L-proline p-nitrophenyl ester gives the corresponding tricyclic oxacyclol. The X-ray crystallographic analysis of the oxa-cyclol is reported; crystals are monoclinic, space group P2₁, a= 6.794, b= 14.379, c= 9.260 Å, β= 92.75°, Z= 2. The final R and R_w values are 0.039 and 0.054, respectively, for 1 890 independent reflections. The conformation of the rings, the torsion angles, and several conformational details found in the solid state for the oxa-cyclol are discussed and compared with those found for natural ergot alkaloids and related synthetic compounds. A comparison between the conformation of the oxacyclol in the crystal with that found in solution by means of ¹H n.m.r. spectroscopy, is also reported.