Method for synthesizing peptide containing n-substituted amino acid

A method of producing a peptide comprising at least one N-alkylated amino acid or proline, wherein the N-alkyl of the N-alkylated amino acid and the proline are optionally substituted with a substituent independently selected from the group consisting of a halogen group, an ether group, and a hydroxyl group;

• wherein the method comprises the steps of:
• 1) preparing an Fmoc-protected amino acid comprising at least one each of following functional groups i) and ii), or an Fmoc-protected peptide comprising the Fmoc-protected amino acid:
• i) a main chain amino group protected by at least one protecting group having an Fmoc skeleton;
• and
• ii) at least one free carboxylic acid group or active esterified carboxylic acid group;
• 2) converting at least one free carboxylic acid group or active esterified carboxylic acid group in the Fmoc-protected amino acid, or the Fmoc-protected peptide prepared in step 1) into an ester group;
• 3) deprotecting the protecting group having the Fmoc-skeleton of the Fmoc-protected amino acid, or the Fmoc-protected peptide, by using a base to expose its amino group;
• 4) forming an amide bond by adding a new Fmoc-protected amino acid, or a new Fmoc-protected peptide;
• and
• 5) converting the ester group into a carboxylic acid group under a condition having weaker acidity than TFA;
• wherein the condition having weaker acidity than TFA is a condition that uses an acid having an aqueous pKa value of 1 to 5.