Abstract
Highly enantioselective alkylation of tert -butyl α-fluoro-β-keto
esters can be effected by the use of N -spiro
chiral quaternary ammonium salt as chiral phase-transfer catalyst,
as a complementary approach to the asymmetric fluorination of α-alkyl-β-keto
esters.
Key words
phase-transfer catalysis - alkylations - asymmetric
catalysis - esters - alkyl halides
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Typical Procedure
for the Catalytic Asymmetric Alkylation of
tert
-Butyl 2-Fluoro-3-oxo-3-phenyl-propanoate
(2b) under Phase-Transfer Conditions
To a solution
of phase-transfer catalyst (S ,S )-1 (0.001
mmol, 0.9 mg) and 2b (0.10 mmol, 23.6 mg)
in mesitylene (2 mL) was added benzyl bromide (0.12 mmol, 14.0 µL),
and the mixture was cooled to 0 ˚C. After adding 10% aq
CsOH (0.3 mL, ca. 0.2 mmol) to this mixture, the reaction mixture
was vigorously stirred until the completion of the reaction. The mixture
was then poured into sat. aq NH4 Cl and extracted with
EtOAc. The organic layer was dried over Na2 SO4 and concentrated
in vacuo. The residue was purified by column chromatography on SiO2 to
give tert -butyl 2-benzyl-2-fluoro-3-oxo-3-phenylpropanoate
(3b ) as a colorless oil [82% (26.8
mg), 85% ee]. Enantiomeric purity was determined
by HPLC analysis [Daicel Chiralpak OD, hexane-2-PrOH
(300:1), flow rate = 0.5 mL/min,
254 nm, t
R = 18.8
min(minor) and 21.7 min(major)]. ¹ H
NMR (400 MHz, CDCl3 ): δ = 7.99-8.02
(2 H, m, ArH), 7.53-7.58 (1 H, m, ArH), 7.41-7.44
(2 H, m, ArH), 7.25-7.30 (5 H, m, ArH), 3.46-3.68
(2 H, m, PhCH2 ), 1.29 (9 H, s, t -Bu).