Peptidic cyclols. Synthesis, and crystal and molecular structure of a tricyclic thia-cyclol

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Abstract
By following a three-step procedure, the linear peptide [(RS)-2-tritylthiopropionyl]-L-phenylalanyl-L-proline (6) has been converted into the cyclic derivative (9). On the basis of spectroscopic data and X-ray crystallographic analysis, compound (9) is shown to be a this-cyclol whose tricyclic system is related to the peptidic portion of the ergot alkaloids. Properties of the new compound are compared to those of previously studied peptidic aza- and oxa-cyclols. The thiazolidinono ring of (9) adopts in the crystal an approximate envelope conformation, whereas the pyrrolidine ring assumes a half-chair conformation. The benzylic side chain of the phenylalanine residue adopts in the crystal a folded conformation which seems to be preferred even in chloroform solution.