Synthesis of Specifically Deuterated S-Benzylcysteines and of Oxytocin and Related Diastereomers Deuterated in the Half-Cystine Positions

S-Benzylcysteine derivatives specifically deuterated at the α carbon only, the β carbon only, and at both the α and β carbons have been synthesized. These labeled compounds have been enzymatically resolved and the enantiomers and reacemates have been converted to the N-tert-hutyloxycarbonyl derivatives. The deuterium labels were not exchanged under the conditions of the syntheses. Condensation of the sodium salt of diethyl α-acetamidomalonate with benzyl chloromethyl sulfide followed by hydrolysis with DCl afforded S-benzyl-dl-[α-²H₁]cysteine. Acetylation followed by treatment with hog renal acylase separated the stereoisomers. A Mannich reaction with [²H₂]methylene diacetate, diethyl α-acetamidomalonate, and dimethylamine followed by quaternization of the amino nitrogen with methyl iodide gave diethyl α-acetamido-α-dimethylamino[²H₂]methylmalonate methiodicle (15). Treatment of 15 with sodium benzylmercaptide gave diethyl α-acetamido-α-benzylthio[²H₂]methylmalonate, which was hydrolyzed with HCl to yield S-benzyl-dl-[β,β-²H₂]cysteine or with DCl to afford S-benzyl-dl-[α,β,β-²H₃]cysteine. These compounds were resolved as before. The preparation of S-benzyl-dl-[α,β,β-₂H₃]cysteine required an efficient source of ethanol-d. This deuterated solvent was prepared in quantitative yield in 2 h from tetraethoxysilane, D₂O, and a catalytic amount of thionyl chloride. The protected deuterated amino acids were used in the preparation of several oxytocin analogues in which the specific deuteration appears in either the 1-hemicystine or the 6-hemicystine residues.