The reaction between N-alkyl-1,4-dihydropyridines and dimethyldioxirane leads to dimeric tetrahydropyridines having oxygen atoms at the 2- and 3-positions in good yields and with good stereocontrol; these compounds are useful iminium ion precursors, and are satisfactorily transformed into 2-substituted 3-hydroxy-1,2,3,4-tetrahydropyridines.