H. Brunner, P. Schmidt, Markus Prommesberger
Apr 1, 2000
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Tetrahedron-asymmetry
Abstract
Abstract Amides of 9-amino(9-deoxy)epicinchonine have been used as chiral base catalysts to induce asymmetry in enantioselective decarboxylation reactions. To understand the reaction mechanism and to optimize the bases by variation of the amide substituents, a detailed knowledge of their preferred conformation is necessary. The conformations were investigated by 1H NMR spectroscopy, X-ray analysis and semi-empirical molecular orbital calculations. Principally, cinchona alkaloids may adopt four different conformations (Fig. 1 Download : Download high-res image (168KB) Download : Download full-size image Figure 1 . Schematic drawings illustrating the two closed and open conformations of 9-amino(9-deoxy)epicinchonine derivatives ), of which open b and open b′ are preferred by 2-ethoxy-N-(9-deoxyepicinchonine-9-yl)benzamide 2 in solution. AM1 Calculations confirm these results, and the small energy barrier between the two open conformations explains their simultaneous existence in solution. Crystal structure analyses of various amides show open conformations in the solid state. Local minimum energy conformations were found for open conformations of the compound 2 protonated at the quinuclidine system, which is the active species in the enantioselective decarboxylation reaction.