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

Direct and long-range H-1-N-15 correlations for cryptolepinone and its Ns-o xide as a mixture in DMSO-d(6) were determined using a combination of H-1-N -15 GHSQC and GHMBC experiments. The N-15 chemical shifts of cryptolepinone in the mixture were comparable to those reported previously for a pure sam ple 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 N-5-oxide. The N-10 resonance is also shifted from 111.4 ppm downfield by +23.9 ppm to 13 5.3 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 aromat ic 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 part ial charges at N-10 before and after oxidation, which are identical in sens e with the partial charges at N-5, the actual site of oxidation, although m uch smaller in magnitude. (C) 1999 John Wiley & Sons, Ltd.