Perturbations of15N chemical shifts followingN-oxidation of the alkaloid cryptolepinone determined by1H-15N GHSQC and GHMBC at natural abundance

Abstract

Direct and long‐range 1H–15N correlations for cryptolepinone and its N5‐oxide as a mixture in DMSO‐d6 were determined using a combination of 1H–15N GHSQC and GHMBC experiments. The 15N chemical shifts of cryptolepinone in the mixture were comparable to those reported previously for a pure sample of the alkaloid. Following N‐oxidation, the N‐5 resonance shifted from 103.4 ppm in cryptolepinone to 188.1 ppm (+84.7 ppm) in the N5‐oxide. The N‐10 resonance is also shifted from 111.4 ppm downfield by +23.9 ppm to 135.3 ppm in the N‐oxide. The downfield shift observed for N‐10 is attributed to a transfer of electronic effects via the intervening C‐5a—C‐10a carbon–carbon double bond in a manner comparable to that observed for fully aromatic diazines and their corresponding N‐oxides, although the shift of N‐10 is in the opposite direction of that observed for diazine systems and their N‐oxides. The downfield shift of N‐10 is consistent with the calculated partial charges at N‐10 before and after oxidation, which are identical in sense with the partial charges at N‐5, the actual site of oxidation, although much smaller in magnitude. © 1999 John Wiley & Sons, Ltd.

Interconvertible Cationic and Neutral Pyridinylimidazole η3-Allylpalladium Complexes. Structural Assignment by 1H, 13C, and 15N NMR and X-ray Diffraction

“…13 C NMR chemical shift are reported in ppm from residual CD 3 OD (49.00) in a mixture of CD 2 Cl 2 and CD 3 OD. 15 N NMR spectra were obtained by the 1 H− 15 N PFG-HMBC method at natural abundance, and chemical shifts are reported in ppm from N H 4 NO 3 (0 ppm) in DMSO- d 6 as an external reference. Melting points were determined on a Yanaco MP-500 melting point apparatus and were not corrected.…”

Discovery, Isolation, and Structure Elucidation of Dretamycin

“…The presence of the N -methyl- N -oxo functionality in 6 was supported by 15 N-HMBC data, which showed a correlation from the 15 N1 signal at 146.4 ppm to H2 at 8.20 ppm (Figure b). The 15 N chemical shift of N1 is consistent with known 15 N chemical shift data for N -oxides. , The expected NOESY-1D correlations were observed between for H4a/H4b, H5/H4a, and H2/NMe. However, no NOESY-1D correlation was seen for H5/NMe.…”

12 NMR Spectroscopic methods

“…By far the longest experiment, to determine long-range 15 N connectivities, required an overnight run. 82 This group also compared micro (3 mm) and submicro (1.7 mm) probes and saw a 2.3-fold improvement in the signal-to-noise ratio for the submicro probe in a GHSQC experiment. 83 Complexes with transition metals…”