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Conformational Analysis by Nuclear Magnetic Resonance. Nitrogen-15 and Carbon-13 Spectra of Lactams

Williamson, Kenneth L. and Roberts, John D. (1976) Conformational Analysis by Nuclear Magnetic Resonance. Nitrogen-15 and Carbon-13 Spectra of Lactams. Journal of the American Chemical Society, 98 (17). pp. 5082-5086. ISSN 0002-7863. doi:10.1021/ja00433a005.

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The 5-, 6-, 7-, 8-, 9-, and 13-numbered lactams have been studied as simple models for the amide linkage. ^(15)N and ^(13)C spectra have been determined in various solvents and assignments made for the majority of the resonance lines. The ^(15)N and ^(13)C carbonyl chemical shifts show no apparent relationship to the amide ring size or to each other. The 9-membered lactam, azacyclononanone, exists as an equilibrium mixture of cis and trans conformers, the relative amounts of which depend on solvent and temperature. Carbon-13 NMR spectra of pure trans-zacyclononanone have been obtained from solutions of the solid at -45 ºC in CDCl_3. At -35 ºC two trans conformers are seen; at room temperature approximately equal quantities of the cis and trans isomers are present in ethanol, dimethyl sulfoxide, and tetrachloroethylene, whereas the cis conformer predominates in chloroform. Further increases in temperature cause rapid (on the NMR time scale) equilibrium between the two isomers such that a single averaged spectrum is seen at 120 ºC. From the coalescence temperature of the carbonyl peaks the free energy of activation for the interconversion of cis- and trans-azacyclononanone is found to be 17 kcal/mol. This value is discussed in terms of the twofold periodic potential function for the distortion of amides from a coplanar conformation.

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Additional Information:© 1976 American Chemical Society. Received December 8, 1975. Supported by the Public Health Service, Research Grants No. GM 10224 and GM 11072, from the Division of General Medical Sciences, and by the National Science Foundation.
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U.S. Public Health Service (USPHS)GM-10224-13
U.S. Public Health ServiceGM-11072
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Caltech Gates and Crellin Laboratories of Chemistry5229
Issue or Number:17
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Official Citation:Conformational analysis by nuclear magnetic resonance. Nitrogen-15 and carbon-13 spectra of lactams Kenneth L. Williamson and John D. Roberts Journal of the American Chemical Society 1976 98 (17), 5082-5086 DOI: 10.1021/ja00433a005
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:60915
Deposited By: Ruth Sustaita
Deposited On:10 Oct 2015 12:22
Last Modified:10 Nov 2021 22:41

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