Labeling of Amino Acids and Peptides With Isotopic Oxygen as Followed by ₁₇O‐N.M.R.*

17O was introduced into the respective alpha- and gamma-COOH groups of Boc-Gly and Boc-Glu by saponification of the corresponding O-methyl esters with 1N NaOH in H2 17O. Other 17O enriched Boc-amino acids were prepared by acid catalyzed exchange into the amino acid alpha-COOH group followed by t-butyloxycarbonylation with t-butyl S-4, 6-dimethylpyrimidin-2-ylthio carbonate. Final enrichment, by approximately three orders of magnitude over natural abundance, was 60-100% of the possible maximum. The synthesis of [17O]-Gly-Ala, [17O]-Gly-Leu and [17O]-Gly-Glu by DCC/HBT mediated coupling of Boc-Gly-[17O]-alpha-COOH with amino acid-O-t-butyl esters followed by deprotection with HCl/EtOAc proceeded without undue loss of the isotope. Boc-[17O]-Pro-Leu-Gly-NH2 was prepared by a similar procedure. [Tyr2-17O]-, [Pro7-17O]- and [Gly4-17O]-oxytocin were synthesized using solid phase support. 17O-chemical shifts of synthetic intermediates and of the final products were as expected for each functional group. Linewidth data correlate with the molecular weights of the compounds prepared.