Comparative Use of Benzhydrylamine and Chloromethylated Resins in Solid-Phase Synthesis of Carboxamide Terminal Peptides. Synthesis of Oxytocin Derivatives

Specifically deuterated derivatives of the peptide hormone oxytocin were synthesized by the solid-phase method of peptide synthesis using either the standard chloromethylated resin or the benzhydrylamine resin as the support for the syntheses, and a comparison of the overall efficiency of the syntheses on the two resins was made. [1-Hemi-dl-[β,β-²H₂]cystine]oxytocin was synthesized using the standard chloromethylated resin, and the two diastereomers were separated and purified by partition chromatography and gel filtration in an overall yield of about 30%. [1-Hemi-dl-[α-²H₁]cystine]oxytocin was prepared using the benzhydrylamine resin to prepare the nonapeptide resin precursor, but otherwise using essentially identical conditions as used for the synthesis on the chloromethylated resin. Again the two diastereomers were separated and purified by partition chromatography and gel filtration. The overall yield of purified diastereomers under the best conditions was about 49%. For the synthesis of the latter compounds, S-3,4-dimethylbenzyl protecting groups were used to introduce the cysteine residues. The overall yields of the peptide hormone derivatives prepared on the benzhydrylamine resin were substantially improved if HF reactions were run at lower temperatures (0 °C rather than 25 °C), and if the S-3,4-dimethylbenzyl rather than the S-benzyl group was used for cysteine protection. Reproducible procedures for preparing benzhydrylamine resins with amino substitution levels of 0.15-0.45 mmol of amino group/g of resin were developed.