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Perturbations of 15 N chemical shifts following N ‐oxidation of the alkaloid cryptolepinone determined by 1 H– 15 N GHSQC and GHMBC at natural abundance
Author(s) -
Martin Gary E.,
Hadden Chad E.,
Blinn James R.,
Sharaf Maged H. M.,
Tackie Albert N.,
Schiff Paul L.
Publication year - 1999
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(199901)37:1<1::aid-mrc391>3.0.co;2-h
Subject(s) - chemistry , chemical shift , diazine , oxide , resonance (particle physics) , carbon 13 nmr , nitrone , analytical chemistry (journal) , crystallography , stereochemistry , organic chemistry , catalysis , physics , particle physics , cycloaddition
Direct and long‐range 1 H– 15 N correlations for cryptolepinoneand its N 5 ‐oxide as a mixture inDMSO‐ d 6 were determined using a combinationof 1 H– 15 N GHSQC and GHMBC experiments.The 15 N chemical shifts of cryptolepinone in the mixturewere comparable to those reported previously for a pure sample of thealkaloid. Following N ‐oxidation, the N‐5resonance shifted from 103.4 ppm in cryptolepinone to 188.1 ppm(+84.7 ppm) in the N 5 ‐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. Thedownfield shift observed for N‐10 is attributed to a transferof electronic effects via the interveningC‐5a—C‐10a carbon–carbon double bond in amanner comparable to that observed for fully aromatic diazines andtheir corresponding N ‐oxides, although the shift ofN‐10 is in the opposite direction of that observed for diazinesystems and their N ‐oxides. The downfield shift of N ‐10 is consistent with the calculated partial chargesat N‐10 before and after oxidation, which are identical insense with the partial charges at N‐5, the actual site ofoxidation, although much smaller in magnitude. © 1999 John Wiley& Sons, Ltd.