A. Armstrong et al. / Tetrahedron: Asymmetry 12 (2001) 2779–2781
2781
Since the a-acetoxyketone 4a appeared to be the best of
the esters examined, we investigated its use in the
epoxidation of a wider range of alkenes (Table 3). The
results are highly encouraging given that they were
obtained at room temperature and without optimiza-
tion of solvent or reaction pH. Importantly, catalyst 4a
does not appear to undergo Baeyer–Villiger decomposi-
tion under the reaction conditions. Very high e.e.max
was observed for epoxidation of phenylstilbene (98%,
entry 6). Trans- or trisubstituted aromatic olefins gener-
ally afforded products with good e.e.max (entries 3–6).
Since there are few examples of the asymmetric epoxi-
dation of electron-poor alkenes with chiral dioxiranes,
we tested methyl (E)-cinnamate. Although good e.e.max
was observed, the conversion was very low (entry 7).
2-Cyclohexenone proved more reactive but the enan-
tioselectivity was very poor (entry 8).
We gratefully acknowledge generous unrestricted sup-
port from Pfizer, Merck and Bristol-Myers Squibb.
References
1. Review: Frohn, M.; Shi, Y. Synthesis 2000, 1979–2000.
2. (a) Armstrong, A.; Hayter, B. R. Tetrahedron 1999, 55,
11119–11126; (b) Tian, H.; She, X.; Shi, Y. Org. Lett.
2001, 3, 715–718.
3. (a) Armstrong, A.; Hayter, B. R. Chem. Commun. 1998,
621–622; (b) Armstrong, A.; Hayter, B. R; Moss, W. O.;
Reeves, J. R.; Wailes, J. S. Tetrahedron: Asymmetry
2000, 11, 2057–2061.
4. Yang, D.; Wong, M.-K.; Yip, Y.-C. J. Org. Chem. 1995,
60, 3887–3889.
5. Esters were prepared under standard conditions
(RCOCl, pyridine). All gave satisfactory spectroscopic
data.
6. Bunn, B. J.; Cox, P. J.; Simpkins, N. S. Tetrahedron
1993, 49, 207–218.
7. Tian, H.; She, X.; Xu, J.; Shi, Y. Org. Lett. 2001, 3,
1929–1931.
8. (a) Hayter, B. R. Ph.D. Thesis, University of Notting-
ham, 1998; (b) a graph of catalyst e.e. against product
e.e. (9 data points) was linear with R2=0.9991. Reeves,
J. R. Unpublished results.
In summary, we have further investigated the use of
esters of a-hydroxy-bicyclo[3.2.1]octan-3-one as cata-
lysts in the Oxone® epoxidation of alkenes. The highest
e.e.max was obtained using the a-acetoxy-oxabicycle 4a
and the scope of this catalyst was explored. We are
continuing to examine further derivatives of this ring
system as well as addressing the efficient preparation of
4a in enantiomerically pure form.
9. Tu, Y.; Wang, Z.-X.; Shi, Y. J. Am. Chem. Soc. 1996,
118, 9806–9807.
Acknowledgements
10. Sato, T.; Watanabe, M.; Honda, N.; Fujisawa, T. Chem.
Lett. 1984, 1175–1176.
We thank the EPSRC and AstraZeneca (CASE award
to J.R.R.) and the EPRSC (GR/M84534) for funding.