Asymmetric hydrogenation of ketopantolactone was studied on a 5 wt% Pt/Al2O3 catalyst in the presence of cinchonidine and its O-methyl. -ethyl. -phenyl and -trimethylsilyl derivatives. Inversion of enantioselectivity with the latter two bulky substituents proved that in the enantiodifferentiating step cinchonidine adsorbs via the quinoline ring lying approximately parallel to the Pt surface. The striking nonlinear effect observed with cinchonidine-O-phenyl-cinchonidine mixtures is attributed to differences in the adsorption strength and geometry of the modifiers. (C) 2003 Elsevier Ltd. All rights reserved.
A Combined NMR, DFT, and X-ray Investigation of Some Cinchona Alkaloid <i>O</i>-Ethers
Structures and conformational behavior of several cinchona alkaloid O-ethers in the solid state (X-ray), in solution (NMR and DFT), and in the gas phase (DFT) were investigated. In the crystal, O-phenylcinchonidine adopts the Open(3) conformation similar to cinchonidine, whereas the O-methyl ether derivatives of both cinchonidine and cinchonine are packed in the Closed(1) conformation. Dynamic equilibria