“Progress in understanding the N-demethylation of alkaloids by exploiting isotopic techniques” Robins, R.J., Molinié, R., Kwiecien R.A., Paneth, P., Lebreton, J., Bartholomeusz, T.A., Roscher, A., Drager, B., Meier, A.-C., Mesnard, F. Phytochem. Rev. 6, 51-63 (2007)