NMR of enaminones. Part 8— 1 H, 13 C and 17 O NMR spectra of primary and secondary 1,2‐disubstituted enaminones: configuration, conformation and intramolecular hdydrogen bondingZhuo, Jin‐Cong
doi: 10.1002/(SICI)1097-458X(199808)36:8<565::AID-OMR338>3.0.CO;2-Hpmid: N/A
The 1H, 13C and 17O NMR spectra for four series of C‐2‐substituted enaminones are reported: MeCO(Me)C=CHNHR (1), EtCO(Me)=CHNHR (2), PhCO(Me)C=CHNHR (3) and MeCO(Me)C=CHNHR (4). The 1H, 13C and 17O NMR data for these enaminones show that 1 and 2 exist as mixtures of E‐ and Z‐forms, 3 exists mainly in the E‐form and 4 is in the Z‐form. The E‐ and Z‐forms exist in the E‐s‐E‐s‐E and Z‐s‐Z‐s‐E conformations, respectively. The 17O shift values of the carbonyl groups in the four series of enaminones show that the influence of N substituents is essentially identical and is additive. The shielding of the carbonyl O atom by intramolecular hydrogen bonding (ΔδHB), ca. ‐30 ppm, is dependent on the donor ability of the amino groups and the type of C‐1 and C‐2 substituents. Correlations of the 1H, 13C and 17O NMR data between the E‐ and Z‐forms of enaminones are excellent. © 1998 John Wiley & Sons, Ltd.
17 O, 13 C and 1 H NMR spectra of 1,2‐dialkoxyethenesTaskinen, Esko
doi: 10.1002/(SICI)1097-458X(199808)36:8<573::AID-OMR343>3.0.CO;2-#pmid: N/A
The 17O, 13C and 1H NMR spectra of a number of 1,2‐dialkoxyethenes R1OCH=CHOR2 were recorded. The O atoms, in particular those of the E forms, are strongly shielded relative to the 17O nuclei of the corresponding alkyl vinyl ethers ROCH=CH2. Moreover, in compounds of the type ROCH=CHOMe, the difference δ(17O)Z‐δ(17O)E of the MeO group decreases and that of the RO group increases with increasing bulkiness of R. These trends probably arise from changes, with the size of the alkyl group R, in the stereochemistry of the RO group of the E‐isomer about the O—C(sp2) bond, whereas the stereochemistry of the Z‐form seems to be independent of the size of R. Additional information on the stereochemistry of the title compounds is provided by their 13C and 1H NMR spectra. © 1998 John Wiley & Sons, Ltd.
Structural and conformational study of substituted triazines by 15 N NMR and x‐ray analysisAmm, Michel; Platzer, Nicole; Guilhem, Jean; Bouchet, Jean Paul; Volland, Jean Paul
doi: 10.1002/(SICI)1097-458X(199808)36:8<587::AID-OMR347>3.0.CO;2-Bpmid: N/A
Since the discovery of the multi‐drug resistance (MDR) phenotype, reversant agents of various origins and structures have been extensively studied. In the present work, two series of related 2,4,6‐tris(amino)‐s‐triazines with different MDR potential1 were studied by 15N NMR spectroscopy. The 15N nucleus allows an easy identification of two protonation sites and an estimation of the electronic effects. 15N was further found to be well suited to demonstrate the occurrence at room temperature of restricted rotation around the Ar—N bonds between the amino substituents and the s‐triazine ring and to measure the rotational barriers. Crystal structures were determined by x‐ray analysis of the compounds at various stages of protonation. The effects of the protonation at the sterically less hindered nitrogen of the triazine, detected by the NMR study, were confirmed in the solid‐state structures. In the crystals, the orientation of the N—H and N—C bonds of the NHallyl substituent with respect to the triazine ring does not depend on the protonation state and corresponds to one of the conformations postulated in solution. © 1998 John Wiley & Sons Ltd.
An ω 1 ‐band‐selective, ω 1 ‐homonuclear decoupled ROESY experiment: application to the assignment of 1 H NMR spectra of difficult‐to‐assign peptide sequencesKaerner, Andreas; Rabenstein, Dallas L.
doi: 10.1002/(SICI)1097-458X(199808)36:8<601::AID-OMR342>3.0.CO;2-Cpmid: N/A
Application of an ω1‐band‐selective, ω1‐homonuclear decoupled ROESY (BASHD‐ROESY) experiment to the assignment of 1H NMR spectra of peptides is demonstrated. Band selection in the ω1 dimension is achieved with the double pulsed field gradient spin echo (DPFGSE) technique; homonuclear decoupling in the ω1 dimension is achieved by placing a non‐selective 180° pulse together with the first half of the DPFGSE in the middle of the evolution period. Application of the BASHD‐ROESY experiment is demonstrated with the complete assignment of the proton resonances of the synthetic 19 amino acid peptide N‐Ac–Ala–Glu–Ala–Ala– Ala–Arg–Ala–Ala–Ala–Arg–Arg–Ala–Ala– Arg–Arg–Ala–Ala–Ala–Arg–NH2. Critical to making the assignments was the significantly increased resolution in the CαH–NH region of the ROESY spectrum measured with the BASHD‐ROESY pulse sequence with band selection and homonuclear decoupling in the CαH region. NOEs observed for the peptide indicate it has a helical secondary structure in solution. © 1998 John Wiley & Sons, Ltd.
Four known triterpenoids isolated from three Brazilian plants: 1 H and 13 C chemical shift assignmentsSoares, F. P.; Ronconi, C. A. V.; da Cunha, E. V. L.; Barbosa‐Filho, J. M.; da Silva, M. S.; Braz‐Filho, R.
doi: 10.1002/(SICI)1097-458X(199808)36:8<608::AID-OMR335>3.0.CO;2-Kpmid: N/A
An NMR study of 3‐O‐acetylebelin lactone, 3β,19α,23‐trihydroxyurs‐12‐en‐28‐oic acid, 3‐oxoolean‐18‐en‐28‐oic acid and 7‐oxofriedelin is described. In addition to conventional 1D NMR methods, 2D shift‐correlated NMR experiments (1H×1H‐COSY, 1H×13C‐COSY–1JCH (HETCOR and HMQC), 1H×13C‐COSY–nJCH (n=2 and 3, COLOC and HMBC)) and 2D 1H×1H‐NOESY were used for 1H and 13C chemical shift assignments of these triterpenoids. © 1998 John Wiley & Sons, Ltd.