The reaction between N-alkyl-1,4-dihydropyridines and dimethyldioxiran
e leads to dimeric tetrahydropyridines having oxygen atoms at the 2- a
nd 3-positions in good yields and with good stereocontrol; these compo
unds are useful iminium ion precursors, and are satisfactorily transfo
rmed into 2-substituted 3-hydroxy-1,2,3,4-tetrahydropyridines.