Labeled oxytocin and method of manufacture and use

A method of manufacturing a compound of Formula IV, wherein the compound is a 13N-labeled oxytocin peptide comprising the amino acid sequence of SEQ ID NO:

• 1, the method comprising:
• a) converting a compound of Formula (I):
• into a compound of Formula (II):
• using a R-LG reagent, wherein LG is a leaving group selected from a group comprising CN, OH, O-Acyl, OAt, OBt, Cl, 1-imidazolyl;
• and
• wherein R is P(═O)(OR1)(OR2);
• wherein R1 and R2 are independently C1-C6 alkyl or C6-C14 aryl, wherein the C1-C6 alkyl and C6-C14 aryl are independently optionally substituted by halogen, cyano, or C1-C6 alkyl;
• and
• wherein PG1 is an amine protecting group;
• b) reacting a compound of Formula (II) with gaseous 13NH3 to provide a compound of Formula (III):
• c) deprotecting a compound of Formula (III) to provide the compound of Formula IV:
• and
• d) purifying the compound of Formula IV.