On the detection of both carbonyl and hydroxyl oxygens in amino acid derivatives: a 17O NMR reinvestigation

The hypothesis and the conclusions of previous 17O NMR studies on the detection of both oxygens of the carboxylic group of Boc–[17O]Tyr(2,6-diClBzl)–OH in DMSO-d6 solution (Tetrahedron Lett. 2000, 41, 8651) are reconsidered. The appearance of two discrete resonances at 340 and 175 ppm of this protected amino acid is not now attributed: (a) to the reduction of the intramolecular conformational exchange rate, due to the effect of intramolecular hydrogen bonding of the hydroxy part of the carboxyl with the carbonyl oxygen of the Boc-group, and (b) to the effect of solvent viscosity, suggested in the mentioned study. The cause of this phenomenon is now attributed to a strong hydrogen bonding of the polar proton acceptor solvent DMSO with the carboxy group, which effectively reduces the proton exchange rate, thus becoming slow on the 17O NMR time scale.