Synthesis of linear and cyclic opioid‐based peptide analogs containing multiple N‐methylated amino acid residues

A series of six novel opioid peptide analogs containing one to three N‐methylamino acid residues, and six cyclic counterparts of these peptides were prepared by the solid‐phase method. Introduction of two consecutive N‐methylated amino acids, as well as cyclization of such conformationally constrained sequences, turned out to be challenging. The use of a recently reported triazine‐based coupling reagent, 4‐(4,6‐dimethoxy‐1,3,5‐triazin‐2‐yl)‐4‐methylmorpholinium toluene‐4‐sulfonate, enabled the synthesis and cyclization of the designed analogs in acceptable yields and with a lesser amount of by‐products than observed with the standard coupling reagents such as TBTU or HATU.Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.