ORGANIC
LETTERS
2001
Vol. 3, No. 13
2125-2128
C -Symmetric Cu(II) Complexes as
2
Chiral Lewis Acids. Catalytic,
Enantioselective Cycloadditions of Silyl
Ketenes
David A. Evans* and Jacob M. Janey
Department of Chemistry & Chemical Biology, HarVard UniVersity,
Cambridge, Massachusetts 02138
Received May 10, 2001
ABSTRACT
C -Symmetric bis(oxazoline)−Cu(II) complexes (4a−g) catalyze the enantioselective [2 + 2] cycloaddition between (silyl)ketenes and chelating
2
carbonyl substrates. A range of substituted â-lactones can be produced in excellent yields and selectivities. It was also found that (trimethylsilyl)-
ketene (1) may also undergo a highly selective hetero Diels−Alder reaction with â,γ-unsaturated r-keto esters.
The [2 + 2] cycloaddition between ketenes and carbonyl
compounds serves as an expedient route to substituted
â-lactones,1 useful acetate enolate aldol equivalents.2,3 Al-
though there are numerous examples of Lewis acid catalyzed
[2 + 2] reactions between ketenes and carbonyl substrates,1,4
there are few general routes to enantiopure â-lactones.5,6 We
have recently demonstrated that C2-symmetric Cu(II)-bis-
(oxazoline) complexes7 serve as efficient catalysts8 for
enantioselective Diels-Alder,9 hetero Diels-Alder,10 Mu-
(5) (a) Romo, D.; Yang, H. Tetrahedron: Asymmetry 1997, 55, 6403-
6434. (b) Cammas, S.; Renard, I.; Boutault, K.; Guerin, P. Tetrahedron:
Asymmetry 1993, 4, 1925-1930.
(6) Nelson, S. G.; Peelen, T. J.; Wan, Z. J. Am. Chem. Soc. 1999, 121,
9742-9743.
(7) Evans, D. A.; Peterson, G. S.; Johnson, J. S.; Barnes, D. M.; Campos,
K. R.; Woerpel, K. A. J. Org. Chem. 1998, 63, 4541-4544.
(8) (a) Evans, D. A.; Rovis, T.; Johnson, J. S. Pure Appl. Chem. 1999,
71, 1407-1415. (b) Evans, D. A.; Johnson, J. S. Acc. Chem. Res. 2000,
33, 325-335.
(9) (a) Evans, D. A.; Miller, S. J.; Lectka, T. J. Am. Chem. Soc. 1993,
115, 6460-6461. (b) Evans, D. A.; Murry, J. A.; von Matt, P.; Norcross,
R. D.; Miller, S. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 798-800. (c)
Evans, D. A.; Kozlowski, M. C.; Tedrow, J. S. Tetrahedron Lett. 1996, 37,
7481-7484. (d) Evans, D. A.; Barnes, D. M. Tetrahedron Lett. 1997, 38,
57-58. (e) Evans, D. A.; Johnson, J. S. J. Org. Chem. 1997, 62, 786-787.
(f) Evans, D. A.; Shaughnessy, E. A.; Barnes, D. M. Tetrahedron Lett.
1997, 38, 3193-3194. (g) Davies, I. W.; Senanayake, C. H.; Larsen, R.
D.; Verhoeven, T. R.; Reider, P. J. Tetrahedron Lett. 1996, 37, 1725-
1729. (h) Ghosh, A. K.; Mathivanan, P.; Cappiello, J. Tetrahedron Lett.
1996, 37, 3815-3819.
(1) (a) For a seminal reference, see; Staudinger, H.; Bereza, S. Ann. 1911,
380, 243-277. (b) For an excellent recent review concerning all aspects of
ketene chemistry, see: Tidwell, T. T. Ketenes; John Wiley and Sons:
Canada, 1995.
(2) Pommier, A.; Pons, J.-M. Synthesis 1993, 441-449.
(3) (a) Vederas, J. C.; Ratemi, E. S. Tetrahedron Lett. 1994, 35, 7605-
7608. (b) Nelson, S. G.; Spencer, K. L. Angew. Chem., Int. Ed. 2000, 39,
1323-1325. (c) Fujisawa, T.; Ito, T.; Fujimoto, K.; Shimizu, M.; Wynberg,
H.; Staring, E. G. J. Tetrahedron Lett. 1997, 38, 1593-1596. (d) Romo,
D.; Yang, H. W. J. Org. Chem. 1998, 63, 1344-1347. (e) Fujisawa, T.;
Ito, T.; Nishiura, S.; Shimizu, M. Tetrahedron Lett. 1998, 39, 9735-9738.
(f) Vederas, J. C.; Arnold, L. D.; May, R. G. J. Am. Chem. Soc. 1988, 110,
2237-2241. (g) Vederas, J. C.; Arnold, L. D.; Drover, J. C. G. J. Am.
Chem. Soc. 1987, 109, 4649-4659.
(4) (a) Hyatt, J. A.; Raynolds, P. W. Org. React. 1994, 45, 159-246.
(b) Nelson, S. G.; Wan, Z.; Peelen, T. J.; Spencer, K. L. Tetrahedron Lett.
1999, 40, 6535-6539. (c) Nelson, S. G.; Peelen, T. J.; Wan, Z. Tetrahedron
Lett. 1999, 40, 6541-6543. (d) Yamamoto, H.; Maruoka, K.; Concepcion,
A. B. Tetrahedron 1995, 51, 4011-4020. (e) Nelson, S. G.; Spencer, K. L.
J. Org. Chem. 2000, 65, 1227-1230.
(10) (a) Evans, D. A.; Johnson, J. S.; Olhava, E. J. J. Am. Chem. Soc.
2000, 122, 1635-1649. (b) Evans, D. A.; Olhava, E. J.; Johnson, J. S.;
Janey, J. M. Angew. Chem., Int. Ed. 1998, 37, 3372-3375. (c) Evans, D.
A.; Johnson, J. S.; Burgey, C. S.; Campos, K. R. Tetrahedron Lett. 1999,
40, 2879. (d) Thorhauge, J.; Johannsen, M.; Jørgensen, K. A. Angew. Chem.,
Int. Ed. 1998, 37, 2404-2406. (e) Thorhauge, J.; Johannsen, M.; Jørgensen,
10.1021/ol016096z CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/07/2001