V. Tsikaris, V. Moussis, M. Sakarellos-Daitsiotis
Oct 28, 2000
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Influential Citations
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Journal
Tetrahedron Letters
Abstract
Abstract In this study we report, for the first time, on the detection of both the carbonyl (CO) and the hydroxyl (OH) 17 O resonances of the carboxylic group by 17 O NMR spectroscopy. Two well separated peaks, at 340.3 and 175 ppm, were detected for the CO and OH groups, respectively, of Boc-[ 17 O]Tyr(2,6-diClBzl)-OH in DMSO- d 6 . This finding is attributed to the participation of the carboxylic group in a rather strong hydrogen-bonded interaction. These resonances disappeared in the presence of a small quantity of TFA (trifluoroacetic acid), which, however, did not cleave the Boc-group and a broad resonance was detected for both oxygens. In fact, this result indicates that the proposed hydrogen-bonded interaction is not stabilized in the presence of TFA, which is a very strong hydrogen bond breaker. Only one broad signal was observed for both carboxylic oxygens at 251.8 ppm for the HCl[ 17 O]Tyr(2,6-diClBzl)-OH in DMSO- d 6 solution, suggesting that the carbonyl oxygen of the Boc-group is probably the proton acceptor group of the carboxylic hydrogen, stabilizing a γ-turn like structure. A single relatively sharp resonance appearing in chloroform, (247.5 ppm) upfield shifted by ∼14.5 ppm, compared to the resonance found in DMSO- d 6 for Boc-[ 17 O]Tyr(2,6-ClBzl)-OH in the presence of TFA, is attributed to a γ-turn fraction being in a fast exchange with the open form.