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being sufficient enough to furnish the aldol products in
excellent yields and enantioselectivities. Further expansion
on broadening the scope of the siloxyserine organocatalyst
is currently in progress in our laboratory and will be
reported in due course.
Acknowledgement
´
Commun. 2006, 2801; Cordova, A.; Zou, W.; Dziedzic, P.;
Ibrahem, I.; Reyes, E.; Xu, Y. Chem. Eur. J. 2006, 12, 5383;
We would like to thank the National Institute of Educa-
tion, Nanyang Technological University for their generous
financial support.
´
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Dziedzic, P.; Zou, W.; Ibrahem, I.; Sunden, H.; Cordova, A.
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9. Representative procedure for the asymmetric direct aldol reac-
tion: Preparation of (2R,10S)-2-[hydroxy-(4-nitro-phenyl)-
methyl]-cyclohexanone 3a. A catalytic amount of siloxyserine
(0.0172 g, 0.05 mmol, 0.1 equiv) was added to a vial containing
4-nitrobenzaldehyde (0.0760 g, 0.5 mmol, 1.0 equiv), cyclohex-
anone (0.26 mL, 2.5 mmol, 5 equiv) and water (0.065 mL,
3.6 mmol, 7.2 equiv) under air in a closed system. The reaction
mixture was stirred at room temperature for 16 h and
subsequently poured into an extraction funnel that contained
brine (5 mL) and water (5 mL). The reaction vial was also
washed with 10 mL of ethyl acetate. The aqueous phase was
extracted with ethyl acetate (3 · 15 mL). The combined
organic extracts were dried with anhydrous MgSO4 and the
solvent removed under reduced pressure. The crude aldol
product was purified by silica-gel column chromatography
(hexane–ethyl acetate 4:1) to afford 3a as a white solid
(0.1183 g, 95% yield). The diastereomeric anti–syn ratio was
determined by 1H NMR analysis of the reaction mixture:
d 5.48 (d, 1H, J = 1.8 Hz, syn, minor), 4.89 (d, 1H, J = 8.8 Hz,
anti, major). The enantiomeric excess was determined by
HPLC with a Chiralcel AD-H (hexane/i-PrOH = 95:5, 1 mL/
min, k = 254 nm, 20 ꢁC): tR = 44.8 min (minor) and 60.5 min
(major).
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