Qian-Yi Zhao et al.
UPDATES
1
given in units of 10 degÀ1 cm2 gÀ1. H NMR spectra were re- References
corded on Bruker AM-300 and AM-400 spectrometers for
solutions in CDCl3 or (CD3)3CO; coupling constants J are
given in Hz. 19F and 31P NMR spectra were recorded on
Bruker AM-300 and AM-400 spectrometers with complete
proton decoupling. Infrared spectra were recorded on a
Perkin–Elmer PE-983 spectrometer with absorptions given
in cmÀ1. Flash column chromatography was performed using
300–400 mesh silica gel. For thin-layer chromatography
(TLC), silica gel plates (Huanghai GF254) were used. Chiral
HPLC was performed on a Shimadzu SPD-10 A vp series
with chiral columns [Chiralpak AD-H, OD-H, PA-H and
IC-H columns 4.6ꢃ250 mm, (Daicel Chemical Ind., Ltd.)].
Mass spectra were recorded by EI, ESI, MALDI and HR-
MS were measured on an HP-5989 instrument. Organic sol-
vents used were dried by standard methods when necessary.
Ligands L1, L4 and L5 are known compounds and were syn-
thesized according to the previous literature. Compounds
1c–1o were commercially available or prepared according to
the previous literature. Fluorinated aldimines 2a, 2b, 2f, 2aa,
2bb, 2cc and 2dd were prepared according to the previous
reports. Racemic 4aa, 4bb, 4cc and 4dd are known com-
pounds.
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Typical Procedure
A dried Schlenk tube was charged with AgOAc (2.50 mg,
0.015 mmol), L1 (9.30 mg, 0.017 mmol), and 4 ꢂ MS
(50 mg). Then, freshly distilled THF and optically active flu-
orinated aldimine 2 (2a–2j) (0.15 mmol) were added succes-
sively, and following addition of CH3CH2OH (16 mL,
0.27 mmol), the resulting solution was allowed to cool to
À788C with stirring. At À788C, compound 3 (3a or 3b)
(0.27 mmol) was added and the resulting solution was kept
at À788C for 7 h before addition of HOAc (15.4 mL,
0.27 mmol) in MeOH (0.2 mL) to quench the reaction. The
quenched reaction solution was allowed to stir at À788C for
an additional two hours, and then it was allowed to warm to
room temperature (228C). A saturated aqueous solution of
NaHCO3 was added and the aqueous layer was washed with
EtOAc (3ꢃ5 mL), dried over anhydrous MgSO4, and the
volatiles were removed under reduced pressure and the resi-
due was purified by flash column chromatography on silica
gel (elution with petroleum ether/EtOAc=4:1) to give the
corresponding compound 4 (4a–4o).
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Supporting Information
Experimental procedures and spectroscopic data for all new
compounds, X-ray crystal structure and CIF data for 4g
areavailable free of charge as Supporting Information.
Acknowledgements
We thank the Shanghai Municipal Committee of Science and
Technology (08dj1400100-2), National Basic Research Pro-
[10] For reviews on a,a-difluoro-b-amino acids, see: a) D.
Seebach, J. Gardiner, Acc. Chem. Res. 2008, 41, 1366–
1375; b) T. Kimmerlin, D. Seebach, J. Pept. Res. 2005,
65, 229–236; c) G. Lelais, D. Seebach, Biopolymers
gram of China ACHTUNGTRENNUNG(973)-2010CB833302, and the National Natu-
ral Science Foundation of China for financial support
(20902019, 20872162, 20672127, 20821002, 20732008 and
20702059) is greatly acknowledged.
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 637 – 643