CONFORMATIONAL DEPENDENCE OF THE CIRCULAR-DICHROISM OF N-NITROSOPYRROLIDINES

Several substituted N-nitrosopyrrolidines were prepared and their chir optical spectra studied. The circular dichroism of monocyclic compound s depends on substituents, solvents, and temperature changes. Some of these N-nitrosamines exhibit bisignate CD curves in the region of n-pi transition. On the contrary, conformationally restricted bi- and tri cyclic compounds show monosignate Cotton effects, the magnitudes of wh ich are almost solvent independent. Bisignate CD curves result from tw o half-chair conformers of the pyrrolidine ring being in equilibrium a nd contributing with the opposite CD signs. The Cotton effect signs ca n be predicted with our ''lowered symmetry'' sector rule developed ear lier. Molecular geometries were calculated by the molecular mechanics (MM2) method, and the resulting torsional angles were used to estimate the vicinal coupling constants in the H-1 NMR spectra. The crystal st ructure of (R)-N-nitroso-3-phenylpyrrolidine ((R)-6), solved by us, is very close to the calculated minimum energy conformation.