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

Autor: Maged H. M. Sharaf, Albert N. Tackie, James R. Blinn, Gary E. Martin, Paul L. Schiff, Chad E. Hadden
Rok vydání: 1999
Předmět:
Zdroj: Magnetic Resonance in Chemistry. 37:1-6
ISSN: 1097-458X
0749-1581
DOI: 10.1002/(sici)1097-458x(199901)37:1<1::aid-mrc391>3.0.co;2-h
Popis: 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.
Databáze: OpenAIRE
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