Cyclization of all-L-pentapeptides by means of 1-hydroxy-7-azabenzotriazole-derived uronium and phosphonium reagents

Due to their restricted conformational flexibility, cyclic peptides are of great interest in connection with structure-activity relationships, especially the elucidation of bioactive conformations. For linear peptides that do not contain turn structure-inducing amino acid residues, the cyclization reaction may be an inherently improbable or slow process, and side reactions, such as cyclodimerization and epimerization at the C-terminal residue, may dominate. A number of 1-hydroxy-7-azabenzotriazole-based onium salts were examined for cyclization of thymopentin-derived pentapeptides and the results compared with data from more conventional coupling reagents. The azabenzotriazol-derived coupling reagents stood out as being the most effective by far. The cyclizations proceed extremely rapidly, and in contrast to other coupling reagents, C-terminal epimerization was generally less than 10%. C-terminal D-amino acid residues favor the formation of monocyclic pentapeptide rings. A similar effect was observed for cyclization of linear N-methylamino acid-containing peptides, suggesting that reversible amide bond alkylation such as Hmb-modification should be useful in promoting the cyclization of pepitdes devoid of turn-inducing amino acid residues.