Publication details
Ascorbigen A-NMR identification
| Authors | |
|---|---|
| Year of publication | 2019 |
| Type | Article in Periodical |
| Magazine / Source | Magnetic Resonance in Chemistry |
| MU Faculty or unit | |
| Citation | |
| Web | http://dx.doi.org/10.1002/mrc.4890 |
| Doi | http://dx.doi.org/10.1002/mrc.4890 |
| Keywords | C-13 NMR; C-13-C-13 coupling constants; C-13-H-1 coupling constants; H-1 NMR; H-1-H-1 coupling constants; natural product; NMR; NOE; rotamers; stereochemistry |
| Description | The connectivities of all atoms in ascorbigen A, an important metabolite, were determined unambiguously for the first time. The connectivity between carbon atoms was established by 2D INADEQUATE, and one-bond C-13-C-13 coupling constants were determined for all pairs of directly connected carbon atoms except for two strongly coupled carbon pairs. The C-13-C-13 coupling in one of the pairs was proved by a modification of standard INADEQUATE; however, the signals from the other pair were too weak to be observed. The connectivity within the two strongly coupled C-C pairs was confirmed by a combination of COSY and gHSQC; the latter experiment also identified all C-H bonds. The proton nuclear magnetic resonance (H-1 NMR) spectra in dry dimethyl sulfoxide allowed identification and assignment of the signals due to NH and OH protons. The derived structure, 3-((1H-indol-3-yl)methyl)-3,3a,6-trihydroxytetrahydrofuro[3,2-b]furan-2(5H)-one, agrees with the structure suggested for ascorbigen A in 1966. The density functional theory (DFT) calculations showed that among 16 possible stereoisomers, only two complied with the almost zero value of the measured (3)J(H6-H6a). Of the two stereoisomers, 3S,3aS,6S,6aR and 3R,3aR,6R,6aS, the latter was excluded on synthetic grounds. The nuclear Overhauser effect measurements unveiled close proximity between H2 ' proton of the indole and the H6a proton of the tetrahydrofuro[3,2-b]furan part. Detailed structural interpretation of the measured NMR parameters by means of DFT NMR was hampered by rotational flexibility of the indole and tetrahydrofuro[3,2-b]furan parts and inadequacy of Polarizable Continuum Model (PCM) solvent model. |
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