Synthesis and backbone conformations of cyclic hexapeptides cyclo-(Xxx-Pro-D-Gln)2

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Abstract
The solution synthesis of cyclo-(Xxx-Pro-D-Gln)2, where Xxx.dbd.Gly, Ala, Leu, Phe and Val are described. Several routes were examined, the most successful involving the intermediate Z-Xxx-Pro-D-Gln-O-tBu [Z = benzyloxy carbonyl] and proceeding to cyclization of H-Xxx-Pro-D-Gln-Xxx-Pro-D-Gln-OH using diphenylphosphoryl azide. The N.sbd.H regions of the PMR spectra of aqueous solutions of these peptides were examined, and in the Xxx.sbd.Leu and Val peptides an unsymmetrical backbone, presumably with 1 cis Xxx-Pro peptide bond, was important. Previous reports of cyclo-(Xxx-Pro-D-Yyy)2 peptides have shown only C2-symmetric forms.