REGIOSELECTIVE AND STEREOSELECTIVE ADDITION OF NUCLEOPHILES TO 1-PHENOXYCARBONYL-2,3-DIHYDROPYRIDINIUM SALTS

Several 2-alkyl-1-phenoxycarbonyl-DELTA3-piperidines were prepared by the addition of alkylzinc iodides to dihydropyridinium salt (9). The t reatment of ethyl-1-phenoxycarbonyl-1,2,3,4-tetrahydropyridine (12) wi th allyltrimethylsilane in the presence of a variety of Lewis acids wa s studied and found to give cis- and trans-2-6-dialkyl-DELTA3-piperidi nes (14) and (15) in moderate to good yield. Among the Lewis acids stu died, stannyl chloride gave the best yield with good stereoselectivity (84:16) favoring the cis-isomer (14). The analogous reaction of the 4 -methoxy derivative (13) with organozinc reagents was examined and fou nd to give the cis-isomers (17) as the major products. Using this meth odology, the cis-piperidine alkaloid, (+/-)-dihydropinidine, was prepa red in six steps from 4-methoxypyridine.