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