Effects of N-oxidation on the N-15 chemical shifts in the Strychnos alkaloids strychnine and brucine

The effects of N-oxidation on the N-15 chemical shifts of the Strychnos alk aloids strychnine and brucine are discussed. The N-15 shifts were determine d using the inverse-detected, long-range GHMBC experiment at natural abunda nce. Following N-oxidation, the N-19 resonance shifted downfield from 35.0 ppm in strychnine to 136.3 ppm in the N-19-oxide. The N-19 resonance of bru cine shifted downfield from 35.9 to 135.5 ppm in the N-19-oxide, Small uphe ld shifts were observed for the N-9 resonances of both N-oxides. The H-1 an d C-13 shifts of both N-oxides were assigned using inverse-detected 2D NMR methods to ensure proper assignments of the long-range H-1-N-15 couplings. Copyright (C) 1999 John Wiley & Sons, Ltd.