Perturbations of 15N chemical shifts following N-oxidation of the alkaloid cryptolepinone determined by 1H–15N GHSQC and GHMBC at natural abundance

✍ Scribed by Gary E. Martin; Chad E. Hadden; James R. Blinn; Maged H. M. Sharaf; Albert N. Tackie; Paul L. Schiff Jr

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 121 KB
• Volume: 37
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199901)37:1<1::aid-mrc391>3.0.co;2-h

✦ Synopsis

Direct and long-range 1HÈ15N correlations for cryptolepinone and its N5-oxide as a mixture in were determined using a combination of 1HÈ15N GHSQC and GHMBC experiments. The 15N chemi-DMSO-d 6 cal 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 downÐeld by ]23.9 ppm to 135.3 ppm in the N-oxide. The downÐeld shift observed for N-10 is attributed to a transfer of electronic e †ects via the intervening C-5aÈC-10a carbonÈcarbon 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 downÐeld 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.