Page 82 - Catalysts for Fine Chemical Synthesis Vol 1 - Robert & Poignant

epoxidation of a, b-unsaturated carbonyl compounds 67

peroxide [10] (99.6 mL) was added and the whole mixture was stirred for 30
minutes. To the resulting faintly green suspension was added
a tetrahydrofuran (0.9 mL) solution of (E)-benzylideneacetophenone
(93.5 mg) and the mixture was stirred vigorously for 15 minutes at room
temperature.
2. The reaction was monitored by TLC (SiO 2 , eluent: hexane±ethyl acetate,
4:1), where both the substrate and the product were detected by a UV lamp
and also visualized by 10 % ethanolic phosphomolybdic acid dip: chalcone,
R f 0.46; the epoxide, R f 0.37.
3. After completion of the reaction, silica gel (ca. 100 mg) was added while
stirring. The reaction mixture was filtered through Celite (ca. 300 mg) and
washed with ethyl acetate (ca. 5 mL).
4. The filtrate was concentrated using a rotary evaporator and the residue was
purified by column chromatography on silica gel (eluent: hexane±ethyl
acetate, 30:1) to give (2S, 3R)-epoxychalcone (99 %) as a colourless solid.
The ee (99.7 %) of epoxychalcone was determined by HPLC (DAICEL
CHIRALCEL OB-H, eluent 2-propanol±hexane 1:2, flow rate 0.5 mL/min);
(2S, 3R)-enantiomer: R t 23.4 min, (2R, 3S)-enantiomer: R t 30.6 min (detec-
tion at 254 nm with D 2 lamp).
1
H NMR (400 MHz, CDCl 3 ): d 8.03±8.01 (m, 2H); 7.65±7.61 (m, 1H);
7.52±7.28 (m, 7H); 4.31 (d, J 1.96 Hz, 1H, H a ); 4.09 (d, J 1.96 Hz, 1H, H b ).
Notes
Ê
The amount of molecular sieves 4 A largely influences the product's ee [11] .
Usually 100 mg (for the CMHP oxidation) or 1 g (for the TBHP oxidation) of
Ê
MS 4A for 1 mmol of substrate is enough; however, in the case where chemical
Ê
yield and/or optical yield are not high, use of excess MS 4A often improves them.
The addition of achiral ligands such as tributylphosphine oxide, tri-o-tolyl- and
tri-p-tolylphosphine oxides, hexamethylphosphoric triamide, triphenylpho-
sphate, lutidine N-oxide, and 1,3-dimethyl-2-imidazolidinone were found to be
less effective than that of triphenylphosphine oxide in the epoxidation of chal-
cone.
The method is applicable to a wide range of substrates. Table 4.4 gives various
a, b-enones that can be epoxidized with the La-(R)-BINOL-Ph 3 PO/ROOH
2
1
system. The substituents (R and R ) can be either aryl or alkyl. Cumene hydro-
2
peroxide can be a superior oxidant for the substrates with R ˆ aryl group
whereas t-butyl hydroperoxide (TBHP) gives a better result for the substrates
2
1
with R ˆ R ˆ alkyl group.
O O H
R 1 R 2
H