| Abstrakt: |
The crystal structures of endo- and exo-adducts of 1-methoxy-2-benzopyrylium-4-olate with N-methylmaleimide have been determined by means of X-ray analysis. The colorless crystals of the endo-adduct are orthorhombic, with the space group P212121and with a=14.432(3), b=11.874(2), c=7.762(1) Å, and Z=4. The colorless crystals of the exo-adduct are monoclinic, with the space group P21/a and with a=13.381(2), b=12.825(1), c=7.4793(4) Å, β=92.13(1)°, and Z=4. The structures were deduced by a direct method and refined by a block-diagonal least-squares technique. The final Rvalues for the endo- and exo-adducts are 0.047 for 1160 observed reflections and 0.067 for 1986 observed reflections respectively. The configurations of the endo- and exo-adducts are coincident with those assigned on the basis of the NMR data. The observed values of the ring-current effect on the N-methyl chemical shift of the endo-adduct and the coupling constants of the vicinal methine protons of both adducts are in good agreement with the ones calculated on the basis of the present X-ray crystal-structure analysis. |
