Polypeptide liquid-phase synthesizing method of oxytocin

The invention discloses a liquid-phase synthesizing method of oxytocin for the first time. The method which is mild in reaction conditions is characterized in that three oxytocin fragments which include Boc-Cys(Acm)-Tyr(tBu)-OH, H-Ile-Gln(Trt)-Asn(Trt)-Cys(Acm)-Pro-OMe and H-Leu-Gly-NH2 are synthesized for the first time, the fragments are assembled to synthesize an all-protected oxytocin amino acid sequence, iodine is used to remove Acm while cyclization is performed to form a disulfide bond to obtain protected cyclic oxytocin, trifluoroacetic acid is used to remove residual protecting groups to obtain crude oxytocin, ethyl acetate is used to perform recrystallization, and reversed-phase chromatography purification is performed to obtain high-purity (crude product purity reaches 95%) and high-titer oxytocin (588IU/mg). The liquid-phase synthesizing method has the advantages that a Boc polypeptide synthesizing method and an Fmoc polypeptide synthesizing method are used in a combined manner, all reactions are