Regio- and Stereoselective Addition of Nucleophiles to 1-Phenoxycarbonyl-2,3-dihydropyridinium Salts

Several 2-alkyl-1-phenoxycarbonyl-Δ 3 -piperidines were prepared by the addition of alkylzinc iodides to dihydropyridinium salt (9). The treatment of 4-hydroxy-2-methyl-1-phenoxycarbonyl-1,2,3,4-tetrahydro- pyridine (12) with allyltrimethylsilane in the presence of a variety of Lewis acids was studied and found to give cis- and trans-2-6-dialkyl- Δ 3 -piperidines (14) and (15) in moderate to good yield. Among the Lewis acids studied, 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 found to give the cis-isomers (17) as the major products