Method for producing peptide compound comprising highly sterically hindered amino acid

A method for producing a peptide compound having an N-substituted-α,α-disubstituted amino acid residue at the N-terminus and comprising a dipeptide residue in which the N-substituted-α,α-disubstituted amino acid residue is linked to an N-substituted amino acid residue, a salt thereof, or a solvate of these, the method comprising the following steps of:

• Step A:
• reacting (1) an N-substituted amino acid, a salt thereof, or a solvate of these, or a peptide compound having an N-substituted amino acid residue at the N-terminus, a salt thereof, or a solvate of these, wherein the main-chain amino group of the N-substituted amino acid or the N-substituted amino acid residue is represented by —NHR, wherein R is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, optionally substituted aralkyl, and optionally substituted cycloalkyl, or R represents a carbon chain bonded to the N atom, which together with the carbon atom at the α-position form an optionally substituted ring,
• with (2) an α,α-disubstituted amino acid having an amino group protected with an electron-withdrawing protecting group, a salt thereof, a dehydrated product thereof, or a solvate of these, wherein the amino group is the amino group on the main chain of the α,α-disubstituted amino acid and is represented by the formula —NHR′, wherein R′ is the electron-withdrawing protecting group, wherein the electron-withdrawing protecting group is a protecting group with which the pKa in water of the NH group to which the protecting group is bonded is 6 to 11,
• in the presence or absence of a condensing reagent to obtain a peptide compound having an α,α-disubstituted amino acid residue having an amino group protected with an electron-withdrawing protecting group at the N-terminus and comprising a dipeptide residue in which the α,α-disubstituted amino acid residue is linked to an N-substituted amino acid residue, a salt thereof, or a solvate of these;
• and
• Step B:
• introducing a substituent to the amino group of the α,α-disubstituted amino acid residue protected with the electron-withdrawing protecting group at the N-terminus in the presence of a base and a substituent-introducing agent to obtain a peptide compound having an α,α-disubstituted amino acid residue having an amino group on the main chain of the α,α-disubstituted amino acid substituted with the substituent and protected with the electron-withdrawing protecting group at the N-terminus and comprising a dipeptide residue in which the α,α-disubstituted amino acid residue is linked to the N-substituted amino acid residue, a salt thereof, or a solvate of these.