Synthetic approaches to peptides containing the l-Gln-l-Val-D(S)-Dmt motif Original Research Article

The pseudoprolines S-Dmo (5,5-dimethyl-4-oxaproline) and R-Dmt (5,5-dimethyl-4-thiaproline) have been used to study the effects of forcing a fully cis conformation in peptides. Synthesis of peptides containing these (which have the same configuration as l-Pro) is straightforward. However, synthesis of peptides containing S-Dmt is difficult, owing to the rapid cyclisation of l-Aaa-S-Dmt amides and esters to form the corresponding diketopiperazines (DKP); thus the intermediacy of l-Aaa-S-Dmt amides and esters must be avoided in the synthetic sequence. Peptides containing the l-Gln-l-Val-D(S)-Dmt motif are particularly difficult, owing to the insolubility of coupling partners containing Gln. Introduction of Gln as N-Boc-pyroglutamate overcame the latter difficulty and the dipeptide active ester BocPygValOC6F5 coupled in good yield with S-DmtOH. BocPygVal-S- DmtNH(CH2)2C6H4NO2 was converted quantitatively to BocGlnVal-S-DmtNH(CH2)2C6H4NO2 with ammonia, demonstrating the utility of this approach. Two peptide derivatives (CbzSerLysLeuGlnVal-S-DmtNH(CH2)2C6H4NO2 and CbzSerSerLysLeuGlnVal-S- DmtNH(CH2)2C6H4NO2) were assembled, using these new methods of coupling a dipeptide acid active ester with S-DmtOH and introduction of Gln as Pyg, followed by conventional peptide couplings. The presence of the Val caused these peptides to be cleaved very slowly by prostate-specific antigen (PSA) at Leu ↓ Gln, rather than the expected Gln ↓ Val.