An Enantioselective Entry to Substituted 6-Membered Nitrogen Heterocycles from Chiral Pyridinium Salts via Selective Epoxidation of Tetrahydropyridine Intermediates

Epoxidation reactions of chiral 2- and 2,6-substituted 1,2,5,6-tetrahydropyridines 6, 11, 12 and 21 proceed with good to high stereoselectivity and excellent yields using pertrifluoroacetic acid as reagent. Epoxides such as 15, 16 and 19 are unstable but they can be deprotonated by LDA to give allylic alcohols such as 28, 30 or 32. These alcohols turned out to be potentially useful synthons, whose further oxidation or conversion to enones (29, 31) allows, in principle, introduction of a number of substituents on the piperidine ring. In particular, this is exemplified by a five-steps synthesis, from chiral tetrahydropyridine 6, of tetrasubstituted piperidine 7 in 26% overall yield.

alkaloids - chiral auxiliaries - enones - epoxidations - protonations