Synthesis of Oxytocin and Related Diastereomers Deuterated in the Half-Cystine Positions. Comparison of Solid-Phase and Solution Methods

Four derivatives of the neurohypophysial hormone oxytocin deuterated at the α and β positions of the two half-cystine residues have been synthesized. The substituted amino acid Boc-S-benzyl-dl-[α,β,β-²H₃]cysteine was used to prepare [1-hemi-dl-[αβ,β-²H₃]cystine]oxytocin and [6-hemi-dl-[αββ-²H₃]cystine]oxytocin. The diastereomeric mixtures were separated and purified by partition chromatography and gel filtration to give [1-hemi-l-[α,β,β-²H₃]cystine]oxytocin, iri[1-hemi-d-[α,β,β-²H₃]cystine]oxytocin, [6-hemi-l-[α,β,β-²H₃]cystine]oxytocin, and [6-hemi-d-[α,β,β-²H₃]cystine]oxytocin. The former two compounds were prepared by both solid-phase and solution techniques of peptide chemistry, and the two methods were compared in the synthesis of these derivatives. The solid-phase method was considerably faster and gave better overall yields, while the solution method permitted a slightly more conservative use of deuterated amino acid. It was found that much shorter deprotection and coupling times and much smaller excesses of amino acid were compatible with the solid-phase methodology.