Synthesis, Structure, and Hypochromism of Pyrimidinopurinophanes

Abstract

Abstract Seventeen pyrimidinopurinophanes in which a pyrimidine and a purine ring are fixed with different mode of stacking were prepared. The synthesis was carried out by stepwise introduction of two bridging chains. In the final ring closure reaction, two isomers, i.e., isomers bridged at 9-position of a purine ring and those having the bridge at 7-position of a purine ring were obtained. The structures of the isomers were determined on the basis of 1H NMR, IR, and UV spectra and X-ray analysis. In 1H NMR spectra the bridge protons of the pyrimidinopurinophanes show complex multiplets, in contrast to the first order splittings in singly bridged reference compounds. This shows clearly the fixation of the conformations of the present cyclic compounds at room temperature. All of the pyrimidinopurinophanes show relatively large hypochromism (H) values. Even the compounds 2a–2d, 4, and 6 where the two base rings are not stacked in parallel, but inclined with the dihedral angle of around 50°, show H values of 10–20%. The H values did not change among the compounds 1c, 1e–1g, and 3 which have similar parallel-stacking structures of the two base rings. The results are well explained by the simplified equation of Ts′o et al.