Page 68 - Catalysts for Fine Chemical Synthesis Vol 1 - Robert & Poignant

52 hydrolysis, oxidation and reduction

A new method of asymmetric epoxidation of a, b-unsaturated ketones using
a stoichiometric amount of N-methylpseudoephedrine as a chiral source in the
presence of diethylzinc and oxygen to afford the a, b-epoxy-ketones with good
[9]
yield and enantiomeric excess was developed by Enders and co-workers .
Shibasaki [10] reported an efficient catalytic asymmetric epoxidation of enones
using lanthanoid complexes, which give epoxides with enantiomeric excesses
between 83 and 94 %. This last method will be reported in another volume of
this series.
New methods for asymmetric epoxidation of alkenes, bearing no functional-
ity to precoordinate the catalyst, have also been developed successfully in the
past few years [11] . Among these methods, Jacobsen et al. [12] were able to epox-
idize monosubstituted, disubstituted Z- and trisubstituted alkenes with good
asymmetric induction, using cationic (salen)manganese(III) complexes. Shi et
al. [13] reported a method of epoxidation using dioxirane generated in situ from
potassium peroxomonosulfate and a chiral fructose-derived ketone as catalyst.
Using this method high enantioselectivity can be obtained for the epoxidation of
unfunctionalized E-alkenes.
Other methods of epoxidation were described; for example in 1979, Groves
et al. [14] reported the first example of alkene epoxidation by a chloro-
ferritetraphenylporphyrin catalyst. By adding an optically active group on to
this catalyst, they obtained optically active chiral epoxides but generally with a
low enantiomeric excess [14] . A number of metalloporphyrins have been used for
the epoxidation of unfunctionalized alkenes [15] (see Chapter 6.3). Asymmetric
epoxides can also be obtained using enzymes. Peroxidases [16, 17] and monoox-
ygenases [18±20] catalyse the synthesis of nonracemic chiral epoxides. A kinetic
resolution of racemic epoxides can be catalysed by epoxide hydrolases. [21±23]
Those methods (using enzymes) will not be described in this chapter since
enzymatic epoxidation has been reviewed previously [24] .
Chapters 4±6 present an overview and a comparison between the various
existing strategies for asymmetric epoxidation of unfunctionalized alkenes,
a, b-unsaturated ketones and allylic alcohols.

REFERENCES

1. Besse, P., Veschambre, H. Tetrahedron, 1994, 50, 8885.
2. Katsuki, T., Sharpless, K.B. J. Am. Chem. Soc., 1980, 102, 5974.
3. Johnson, R.A., Sharpless, K.B. Catalytic Asymmetric Epoxidation of Allylic Alcohols;
VCH, Ed.; Ojima, 1993, pp S 103.
4. Julia Â, S., Masana, J., Vegas, J.C. Angew. Chem. Int. Ed. English, 1980, 11, 929.
5. Colonna, S., Molinari, H., Banfi, S., Julia Â, S., Mansana, J., Alvarez, A. Tetrahedron,
1983, 39, 1635.
6. Julia Â, S., Guixer, J., Mansana, J., Rocas, J., Colonna, S., Annuziata, R., Molonari,
H. J. Chem. Soc., Perkin Trans. I, 1982, 1317.