[Oxidation of nicotine and chelating agent by mercury(II)-compounds]

The dehydrogenation of (S)-nicotine (1) with mercuric acetate in diluted acetic acid yields no cotinine (3), but reacts only to the iminium stage, resulting far predominant the 5'-iminium structure without affecting the chiral center at 2', therefore the reduction with borohydride nearly quantitatively gives rise to (S)-nicotine (1). For the preparation of cotinine (3) the best method proves the oxidation of (S)-1 with the equimolecular complex Hg(II)-EDTA in pure water. With preliminary alkalization of the preparation an oxidation also of the liberated EDTA to iminodiacetic acid (10) and oxalic acid (15) occurs. This side reaction increase with an excess of chelating agent, which makes the precipitation of mercury as measuring system for control of the dehydrogenation invalid. Surprising is the nearly complete failure of the dehydrogenation to the tertiary carbenium ion and the consecutive reaction of the secondary carbenium ion 5, which in equilibrium with its carbinolamine 5a is again dehydrogenated with Hg(II)-EDTA to the lactam 3 with retention of the configuration.