Extended Solution-phase Peptide Synthesis Strategy Using Isostearyl-Mixed Anhydride Coupling and a New C‑Terminal Silyl Ester-Protecting Group for <i>N</i>‑Methylated Cyclic Peptide Production

Herein, we present a new and efficient convergent solution-phase synthetic strategy for producing peptides containing N-methyl amino acids. Specifically, we have synthesized a model cyclic octapeptide with two N-methyl amino acids, utilizing an isostearyl-mixed anhydride coupling methodology and a novel silyl ester-protecting group, cyclohexyl di-tert-butyl silyl (cHBS). This newly developed method uses an isostearic acid chloride (ISTA-Cl) and silylation reagent that allows coupling between N- and C-terminally unprotected amino acids with sterically hindered N-methyl amino acids. High yields of four dipeptide fragments are efficiently synthesized by omitting the traditional C-terminal deprotection step. The silyl ester-protecting group at the C-terminus is stable during general peptide synthesis, and is selectively cleaved by fluoride ions. This group further suppresses diketopiperazine formation during the deprotection of the Nα-amino-protecting group. The linear octapeptide precursor is convergently synthesized utilizing protection and selective deprotection of the silyl ester-protecting group, and the cyclic octapeptide can be obtained with high purity using this novel methodology, via a route shorter than conventional solution-phase peptide synthesis strategies