Solution-dimerization of 4-aryl-1,4-dihydropyridines

On irradiation at lambda greater than or equal to 270 nm solutions of 4-ary l-1,4-dihydropyridines 1 yield cage dimers 2 as the main products beside sm all amounts of anti dimers 3. H-1-NMR data and X-ray crystal structure prov e centrosymmetrical properties for both dimers with axially orientated 4-ar yl substituents. Irradiation with filtered light (lambda > 313 nm) leads to syn and anti dimers 4 and 3 in nearly equal yields. The poor yields of ant i dimers 3 on irradiation with unfiltered light are demonstrated to result from a partial cleavage back to their monomeric starting materials 1.