Optical Rotatory Studies on Nucleic Acid Derivatives n–π * Transitions and the Relation of the B 2u Cotton Effect to Conformation in Pyrimidine and Azapyrimidine Nucleosides

It is shown by solvent studies that O 2 :2′‐ and O 2 :5′‐cyclouridine derivatives give a small long wavelength Cotton effect which is due to an n–π * transition. The sign of the B 2u Cotton effect (corresponding to the main ultraviolet absorption band) is as predicted by the previously formulated rule for pyrimidine nucleosides. Cotton effects due to n–π * transitions have also been observed in a number of 6‐azauridine derivatives. Optical rotatory dispersion and circular dichroic data for a variety of azapyrimidine nucleosides are presented, and discussed in relation to corresponding data for pyrimidine nucleosides. The sign of the B 2u Cotton effect in azapyrimidine nucleosides (which is negative in 6‐β‐ d ‐azauridine) can be explained on the basis of conformational considerations, or by assuming that the direction of the transition moment is different from that in pyrimidine nucleosides, owing to the presence of an additional nitrogen in the ring. Both of these possible explanations are illustrated by appropriate projection diagrams relating the base‐sugar torsion angle Ψ CN to the B 2u Cotton effect.