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S. J. Oxenford et al. / Tetrahedron Letters 46 (2005) 8315–8318
´
hedron 2002, 58, 4669; (e) Pettersen, D.; Diner, P.;
imidazoles with imidazole 6 and racemic rearrangement
as a background reaction for cyclohexene oxide trans-8
with imidazoles 4, 6 and 16. By modifying the imidazole
structure, we have shown that epoxide ring opening for
both epoxides trans-8 and trans-9 can be minimised
whilst maintaining satisfactory enantioselectivity. Thus,
using imidazole 16, trans-8 gave allylic alcohol 10 in 86%
yield and 73% ee whereas trans-9 gave allylic alcohol 13
in 43% yield and 68% ee. For the catalytic rearrange-
ment of epoxide trans-8, the use of imidazole 4 was opti-
mal: allylic alcohol 10 was obtained in 95% yield and
82% ee in a relatively short reaction time (18 h). Unfor-
tunately, we were unable to extend the use of this imid-
azole to other epoxides. Our results indicate the
importance of imidazole structure on the yield and
enantioselectivity of epoxide rearrangement under Ahl-
bergÕs catalytic conditions.
Amedjkouh, M.; Ahlberg, P. Tetrahedron: Asymmetry
2004, 15, 1607.
9. Malhotra, S. V. Tetrahedron: Asymmetry 2003, 14, 645.
10. (a) Oxenford, S. J.; OÕBrien, P.; Shipton, M. R. Tetra-
hedron Lett. 2004, 45, 9053; (b) OÕBrien, P.; Pilgram, C. D.
Org. Biomol. Chem. 2003, 1, 523; (c) de Sousa, S. E.;
OÕBrien, P.; Pilgram, C. D. Tetrahedron 2002, 58, 4643; (d)
Barrett, S.; OÕBrien, P.; Steffens, H. C.; Towers, T. D.;
Voith, M. Tetrahedron 2000, 56, 9633.
11. de Sousa, S. E.; OÕBrien, P.; Steffens, H. C. Tetrahedron
Lett. 1999, 40, 8423.
12. Colman, B.; de Sousa, S. E.; OÕBrien, P.; Towers, T. D.;
Watson, W. Tetrahedron: Asymmetry 1999, 10, 4175.
13. Asami, M. Bull. Chem. Soc. Jpn. 1990, 63, 1402.
14. The enantioselectivity of formation of hydroxy imidazoles
11, 12, 14 and 15 was low (610% ee) and the absolute
stereochemistry of the major enantiomer has not been
determined.
15. Representative procedure for catalytic asymmetric rear-
rangement of epoxides: n-Butyllithium (0.66 mL of a
1.88 M solution in hexanes, 1.24 mmol, 2 equiv) was
added dropwise to a stirred solution of diamine (1R,2S)-
3 (26 mg, 0.12 mmol, 0.2 equiv) and imidazole 6 (108 mg,
1.12 mmol, 1.8 equiv) in THF (4 mL) at 0 ꢁC under N2.
The resulting solution was stirred at 0 ꢁC for 30 min and
then a solution of epoxide trans-8 (222 mg, 0.62 mmol) in
THF (1 mL) was added via a cannula. The reaction
mixture was then allowed to warm to room temperature
over 4 h and stirred at room temperature for 14 h. Then,
saturated NH4Cl(aq) (5 mL) was added and the two layers
were separated. The aqueous layer was extracted with
Et2O (2 · 10 mL). The combined organic extracts were
washed with saturated NaHCO3(aq) (15 mL), brine
(15 mL) and water (15 mL), dried (Na2SO4) and evapo-
rated under reduced pressure to give the crude product.
Purification by column chromatography on silica with 4:1
petrol–EtOAc and then 9:1 EtOAc–MeOH as eluent gave
known10c allylic alcohol 10 (171 mg, 77%, 66% ee by
MosherÕs ester formation) as a colourless oil, [a]D À84.8 (c
1.0 CHCl3) {lit.,10c [a]D À100.8 (c 1.0, CHCl3) for >95%
ee}, recovered epoxide trans-8 (11 mg, 5%) as a colourless
oil and hydroxy imidazole 11 (28 mg, 10%, ꢀ10% ee by
MosherÕs ester formation, absolute stereochemistry not
assigned) as a pale yellow solid, [a]D À7.5 (c 1.0, CHCl3);
mp 200–205 ꢁC; IR (CHCl3) 3269, 2916, 1655, 1537, 1363,
1217 cmÀ1; 1H NMR (400 MHz, CDCl3): 6.89 (d, J = 1.0,
1H), 6.78 (d, J = 1.0, 1H), 3.93–3.90 (m, 1H), 3.78 (app.
td, J = 11.0, 4.5, 1H), 3.62 (ddd, J = 13.5, 4.5, 2.5, 1H),
3.59 (s, 3H), 2.81 (dd, J = 16.0, 8.0, 1H), 2.72 (dd,
J = 16.0, 3.5, 1H), 2.07 (app. dt, J = 13.5, 4.5, 1H), 1.98–
1.91 (m, 1H), 1.83(app. q, J = 11.0, 1H), 1.50–1.41 (m,
2H), 0.89 (s, 9H), 0.88 (s, 9H), 0.06 (s, 3H), 0.05 (s, 3H),
0.044 (s, 3H), 0.041 (s, 3H); 13C NMR (100.6 MHz,
CDCl3) 148.0, 126.2, 120.5, 73.0, 71.4, 70.3, 41.4, 41.1,
34.9, 32.8, 32.2, 26.0, 25.8, 18.3, 18.1, À4.4, À4.5, À4.7,
À4.9; MS (CI, NH3) m/z 455 [(M+H)+]; HRMS (CI,
NH3) m/z calcd for C23H46N2O3Si2 (M+H)+ 455.3125,
found 455.3122.
Acknowledgements
We thank the EPSRC and GlaxoSmithKline for a
CASE award (to S.J.O.) and The University of York
and Hoffmann-La Roche (Basel) for a university
studentship (to J.M.W.).
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