An improved and stereoselective route to all-cis-2,6-disubstituted 4-hydroxypiperidines from accessible 4-substituted 4-N-benzylaminobut-1-enes
The reaction between allylmagnesium bromide and imines 5a-l leads to the corresponding 4-substituted 4-N-benzylaminobut-1-enes 6a-1, which were oxidized in a regioselective manner to the alkenylnitrones 7a-l. The intramolecular 1,3-dipolar cycloaddition of these nitrones gave 2-spiroannulated or 2-substituted 6-exo-phenyl-1-aza-7-oxabicyclo[2.2.1]heptanes 8a-j. Reductive cleavage of the N-O bond of the obtained bicycles afforded the diverse substituted 4-hydroxypiperidines 9a-h in good yields. This stereoselective approach allowed the preparation of all-cis-4-hydroxy-6-phenyl-2-nonylpiperidine (9i), a close analogue of dendrobatid frog alkaloid 241D.