Cyclols, cyclodepsipeptides, and N-acyl-diketopiperazines from linear precursors: Synthesis and crystal structure of 10-membered cyclodepsipeptides

Abstract
In order to investigate the relative formation tendency of different tautomeric ring systems (cyclols, cyclodepsipeptides and corresponding N-acyl-diketopiperazines), 2 linear peptide precursors containing proline as C-terminal residue, were synthesized and subjected to cyclizing conditions. Boc-Ser-Phe-Pro-ONp gave 3 isomeric cyclic compounds: .**GRAPHIC**. N-(Boc-Ser)-cyclo-(Phe-D-Pro) and the corresponding aza-cyclo. Starting from Hyb-Phe-Pro-ONp [Hyb = RS-3-hydroxybutyric acid] 2 epimeric N(3-hydroxybutyryl) diketopiperazines and the corresponding 10-membered cyclodepsipeptide (VIII) could be isolated. Crystal and molecular structure of VIII is reported.