Compound or salt thereof and preparation method and application of same

A method for preparing a target polypeptide or a salt thereof, wherein the C terminal of the target polypeptide contains the —NHR2 group from Formula (1), the method comprising the following steps:

• attaching the compound of Formula (1) to a starting solid-phase resin to obtain Formula (1)-resin, or directly providing Formula (1)-resin;
• in Formula (1), R1 is an amino-protecting group;
• wherein the amino-protecting group is Fmoc, Boc, Alloc, Dde, ivDe, Trt, Mtt or Mmt;
• R2 is
• X1 and X2 are each independently O;
• R3 is H or —(CH2)mCOOH, m is an integer from 1 to 10, and any one or more CH2 in the (CH2)m may have a substituent;
• R4 is H or an alkyl group;
• in Formula (1)-resin,
• represents the body of the solid-phase resin, G is a linking structure formed by a reaction between a group in R3 in Formula (1) and a group on the side chain of the solid-phase resin, R′3 is the residual structure after the reaction of R3, Z is the residual structure after the reaction of the side chain of the solid-phase resin, and the definitions of the other substituents are the same as those in Formula (1);
• R′3-G-Z is R′3—CONH—Z, or R′3—COO—Z;
• alternatively, Z is directly connected to X1 without R′3-G;
• when R1 is an amino-protecting group, it is removed to expose the NH group;
• preparing a peptide-resin with the peptide chain fully protected by stepwise solid-phase coupling on the exposed NH group;
• cleaving the fully protected peptide chain off the peptide-resin to obtain, directly or after removing the protecting groups, the target polypeptide or a salt thereof;
• optionally, before cleaving the fully protected peptide chain off the peptide-resin, using the fully protected peptide chain as a starting peptide to carryout cyclization to form a cyclic peptide;
• wherein the target polypeptide is PMX-53 or a salt thereof, which is a cyclic peptide, wherein Formula (1)-resin is represented by Formula (ix)-resin;
• and the resin is a CTC resin;
• wherein the peptide-resin with the uncyclized PMX-53 fully protected is prepared by successive solid-phase coupling of the amino acids Fmoc-Trp(Boc)-OH, Fmoc-D-Cha-OH, Fmoc-Pro-OH, Fmoc-Orn(Dde)-OH, and Fmoc-Phe-OH as reaction reagents to the exposed NH2 group, followed by acetylation of the amino group of the last amino acid coupled, thereby obtaining the following peptide-resin with the uncyclized PMX-53 fully protected;
• Ac-Phe-Orn(Dde)-Pro-D-Cha-Trp(Boc)-Formula (ix)-CTC resin;
• wherein the Fmoc-protecting group attached to the peptide on the resin is removed before each coupling step;
• then the All protecting group and the Dde protecting group are removed, followed by a cyclization reaction to obtain the peptide resin with PMX-53 fully protected:
• Ac-Phe-c(Orn-Pro-D-Cha-Trp(Boc)-Arg(pbf))-resin, wherein the resin is a CTC resin;
• finally, the protecting groups are removed by cleavage to obtain PMX-53 or a salt thereof.