Asymmetric aldol reactions between cyclic ketones and benzaldehyde catalyzed by chiral Zn~(2+) complexes of aminoacyl 1,4,7,10-tetraazacyclododecane: effects of solvent and additives on the stereoselectivities of the aldol products

摘要

The direct aldol reaction between cyclic ketones and 4-nitrobenzaldehyde catalyzed by chiral Zn~(2+) complexes of aminoacyl 1,4,7,10-tetraazacyclododecane is reported. The anti-aldol products were mainly formed in cyclohexanone/N-methylpyrrolidone(NMP)/MeOH with good diastereo- and enantioselectivity, while syn-aldol adducts were obtained as major products with good enantioselectivity in cyclohexanone/H_2O and cyclohexanone/NMP/H_2O. The fact that the UV/vis spectra of 2,6-diphenyl-4-(2,4, 6-triphenyl-1-pyridinio)phenolate (Reichardt's dye) were nearly identical in these solvent systems suggests that the switch in the relative configuration of the aldol products is induced by a large excess of H_2O rather than the polarity of the solvent system. Furthermore, the addition of a small amount of TFA improved the enantioselectivity of the syn-aldol adducts produced in cyclohexanone/H_2O with up to 92% ee (anti/syn ratio = 30:70).