PREPARATION OF TRANS AND -2-ALLYL-6-ALKYL(ARYL)-1,2,3,6-TETRAHYDROPYRIDINES BASED ON THE REDUCTIVE TRANS-2,6-DIALKYLATION OF PYRIDINE - SYNTHESIS OF (+ -)-EPIDIHYDROPINIDINE AND (+/-)-DIHYDROPINIDINE/

A general method for the preparation of unsymmetrical s-2-allyl-6-alky l(aryl)1,2,3,6-tetrahydropyridines 6 based on a combination of 1,2-add ition of RLi to pyridine and trans-6-allylation with triallylborane in the presence of methanol was elaborated. It was shown that trans-pipe rideines 6 (R = Alk, Ph) isomerize into the corresponding cis-2-allyl- 6-alkyl(phenyl)-3-piperideines 14 on heating with triallylborane follo wed by deboronation of aminoborane (16) with methanol and an alkali. T he stereochemistry of compounds 6 and 14 was determined by two-dimensi onal NOE spectroscopy. A possible mechanism of the formation of trans- amines 6 and their transformation into cis-isomers 14 is discussed. Al kaloids (+/-)-epidihydropinidine (trans-2-methyl-6-propylpiperidine 2a , 70%) and (+/-)-dihydropinidine (cis-2-methyl-6-propylpiperidine 1d, 71%) were synthesized by hydrogenation of compound 6a (R = Me) and 14a (R = Me), respectively, over Raney nickel.