Asymmetric aldol reactions between cyclic ketones and benzaldehyde catalyzed by chiral Zn2+ complexes of aminoacyl 1,4,7,10-tetraazacyclododecane: effects of solvent and additives on the stereoselectivities of the aldol products |
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Authors: | Susumu Itoh Takuya Tokunaga Masayuki Kurihara Shin Aoki |
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Affiliation: | 1. Production Technology Laboratories, Kaken Pharmaceutical Co., LTD, 301 Gensuke, Fujieda, Shizuoka 426-8646, Japan;2. Faculty of Pharmaceutical Sciences, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan;3. Center for Technologies against Cancer, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan |
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Abstract: | The direct aldol reaction between cyclic ketones and 4-nitrobenzaldehyde catalyzed by chiral Zn2+ 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/H2O and cyclohexanone/NMP/H2O. 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 H2O 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/H2O with up to 92% ee (anti/syn ratio = 30:70). |
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