A stable isotope labelled mass spectrometry internal standard of the antibiotic doxycycline was prepared to assist in pharmacokinetic analyses. Our approach was to first N-demethylate doxycycline using a non-classical Polonovski reaction and then re-methylate using methyl-[13CD3] iodide, which gave doxycycline-[13CD3] with an isotopic purity of 99%.
