Synthesis of Oxytocin by the Nitrophenyl Ester Method

Abstract

AFTER the completion of work on the degration of oxytocin¹, this compound was synthesized in this laboratory². Since that time several other synthetic methods leading to biologically active solutions of the crude hormone have been reported³. Recently, an improved synthesis of oxytocin was elaborated⁴ by us. All these syntheses of oxytocin have the common feature of linking peptides together. It is well known, however, that formation of the peptide bond with a protected peptide as the acid component can lead to racemization⁵, the extent of which depends on the components, the conditions and the method. No racemization was observed with the use of the azide method⁶. Nevertheless, the latter procedure can involve the formation of by-products other than diastereoisomers. To avoid racemization it is possible to take advantage of the protecting groups, for example, the carbobenzoxy group, which protects the amino-acid acylated by it not only against unwanted acylation but also against racemization. This protection is effective only if the carbobenzoxy group is joined to the amino-acid, the carboxyl group of which takes part in the peptide bond-forming reaction. Therefore it becomes necessary to start the synthesis with the C-terminal amino-acid, to link to it the next (protected) amino-acid, to remove the protecting group from the resulting dipeptide derivative, to couple to the free amino component thus formed the third (protected) amino-acid, etc. This manner of peptide-chain synthesis is practicable only if the method used for the formation of the peptide bond is unambiguous, leads to only one peptide, and gives high yields. All the mixed anhydride methods were eliminated as they usually lead to two acylation products, the separation of which, for example by crystallization, can be difficult, especially in cases of peptides with several amino-acids. After the elimination of many other unsuitable possibilities the nitrophenyl esters of acylamino-acids were found to be suitable for this purpose.