Synthesis of a new dipeptide template, its X-ray structure, and modeling studies on its conformational features

The diastereoselective synthesis is reported of a dipeptide template which is closely related to H-Gly-Trp-OH. Intramolecular bond formation between alpha-C of Gly and ring position 2 of the Trp unit has been achieved by a Pictet-Spengler-type electrophilic aromatic substitution. The absolute configuration of the N-Moc protected dipeptide template 9.2H2O was determined by single-crystal X-ray crystallography and found to be (2S,5S). The cis orientation of the amino and carboxy termini prompted us to investigate the potential of 9 as a beta-turn mimic. MD calculations on the model pseudopeptide Ac-Ala-Gly-Trp-Ala-NHMe 11 suggest that an unusually tight turn should be favoured rather than a beta-turn. The proper protective situation as a pre-requisite for the incorporation of the template into a peptide has been established, and comments about its chemical properties are given.