Preparative electroreduction of trans-2-allyl-6-methyl(phenyl)-1,2,3,6-tetrahydropyridine

Electrocatalytic hydrogenation of trans-2-allyl-6-phenyl-1,2,3,6-tetrahydro pyridine and trans-2-allyl-6-methyl-1,2,3,6-tetrahydropyridine in 40% aqueo us DMF in the presence of AcOH on a nickel cathode gave trans-6-phenyl-2-pr opylpiperidine and trans-2-methyl-6-propylpiperidine ((+/-)-epidihydropinid ine), respectively. Direct electroreduction of trans-2-allyl-6-phenyl-1,2,3 ,6-tetrahydropyridine in anhydrous DMF on a mercury cathode afforded a 7 : 5 mixture of trans- and cis-2-allyl-6-phenylpiperidine. The structure of th e latter compound was confirmed by 2D NOESY spectroscopy. The possible mech anism of formation of the cis-isomer is discussed.