ABSTRACT
High diastereoselectivities were obtained in the heterogeneous catalytic hydrogenation of some chiral pyridine and pyrrole derivatives with complete conversion, in non-acidic medium. The effects of catalytic metals, solvents, temperature and pressure on the conversion and the diastereomeric excess (d.e.) were investigated. The hydrogenation of N-picolinoyl-(S)-proline methyl ester resulted in 79% d.e. over 10% Pd/C, in methanol, at moderate pressure and temperature (50 bar, 50°C). Very high d.e. (94%) was achieved in the saturation of N-nicotinoyl-(S)-proline methyl ester under similar conditions. The hydrogenation of quaternized N-picolinoyl-(S)-proline methyl ester took place already at RT and 20 bar with almost complete diastereoselectivity (98%), over palladium on carbon, in methanol. Similar high d.e. (90%) was obtained in the saturation of N-(1'-methylpyrrole-2'-acethyl)-(S)-proline methyl ester over 5% Rh/C, in methanol, at 20 bar and RT. These are successful examples of diastereoselections in the hydrogenation of prochiral N-heterocyclic compounds.
Introduction
