5928
M. P. Sibi, H. Matsunaga / Tetrahedron Letters 45 (2004) 5925–5929
salts also adapt a slightly distorted square-planar
(R)
O
R
geometry.9b However, the tilt of the substrate is in the
opposite direction to that observed with t-butyl or
i-propyl bisoxazolines.9b The reversal of face selectivity
using ligand 11 can be accounted for by structure B,
Figure 1. In this model, reaction occurs with diene
approaching the si-face of the dienophile to give the
adduct 5d with opposite stereochemistry.
Sm(OTf)3
(0.1 eq.)
O
R
O
(R)
O
O
N
O
MeOH
r.t., 0.5 h
CO2Me
5d
20
[R = 4-CF3-C6H4]
(90% ee)
DBU
[α]D + 36.9˚
(Lit. [α]D +40.8˚)
MeCN
80 ˚C, 24 h
CO2Me
21
In conclusion, we have demonstrated the first successful
examples of highly enantioselective Diels–Alder cy-
cloadditions with b-acyloxy acrylates. This methodology
allows for the construction of chiral hydroxy-function-
alized [4 + 2] cycloadducts in a single step. Studies are
underway to broaden the utility of the new methodology
in the preparation of more complex adducts.
Scheme 1.
The absolute configuration of the cycloadduct 5d was
determined as follows (Scheme 1). Selective exocyclic
cleavage of pyrrolidinone template was carried out by
treatment with catalytic amount of Sm(OTf)3 in meth-
anol to afford methyl ester 20 in quantitative yield,11
which was converted to 2-(methoxycarbonyl)norborna-
diene 21 by heating in the presence of DBU. Compari-
son of the optical rotation of 21 with that in the
literature12 showed the absolute configuration of 5d to
be (2R,3R). This result was consistent with the predic-
tion that cyclopentadiene preferentially attacks the di-
enophile from re-face when ligand 8 was used.
Acknowledgements
We thank the NIH and NSF for generous support of
our research programs. We thank Prof. Craig Jasperse
for helpful discussions.
We propose two models for the observed stereochem-
istry with ligands 8 and 11. This is based on observations
in the literature on stereochemical reversal using bisox-
azoline ligands that have identical configuration at C4.13
We have also noted similar reversal in stereochemistry in
radical reactions.14 In model A, the substrate–CuX2–
ligand 8 complex adopts a distorted square-planar
geometry (Fig. 1, structure A).15 The tilt of the substrate
is similar to the one proposed by Evans et al. with
t-butyl bisoxazoline and copper triflate.9b The observed
stereochemistry for 5d is consistent with cyclopentadiene
preferentially approaching the s-cis conformer of the
dienophile from the less hindered re-face.
References and notes
1. Reviews: (a) Evans, D. A.; Johnson, J. S. In Comprehen-
sive Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A.,
Yamamoto, H., Eds.; Springer: New York, 1999; Vol. III,
p 1177; (b) Corey, E. J. Angew. Chem., Int. Ed. 2002, 41,
1650; (c) Nakano, H.; Suzuki, Y.; Kabuto, C.; Fujita, R.;
Hongo, H. J. Org. Chem. 2002, 67, 5011; (d) Desimoni, G.;
Faita, G.; Guala, M.; Pratelli, C. J. Org. Chem. 2003, 68,
7862; (e) Huang, Y.; Iwama, T.; Rawal, V. H. J. Am.
Chem. Soc. 2002, 124, 5950; (f) Palomo, C.; Oiarbide, M.;
ꢁ
Garcia, J. M.; Gonzalez, A.; Arceo, E. J. Am. Chem. Soc.
2003, 125, 13942; (g) Bolm, C.; Simic, O. J. Am. Chem.
Soc. 2001, 123, 3830; (h) Sprott, K. T.; Corey, E. J. Org.
Lett. 2003, 5, 2465; (i) Quaranta, L.; Corminboeuf, O.;
Renaud, P. Org. Lett. 2002, 4, 39; (j) Takenaka, N.;
Huang, Y.; Rawal, V. H. Tetrahedron 2002, 58, 8299; (k)
Sibi, M. P.; Venkatraman, L.; Liu, M.; Jasperse, C. P. J.
Am. Chem. Soc. 2001, 123, 8444; (l) Sibi, M. P.; Zhang, R.;
Manyem, S. J. Am. Chem. Soc. 2003, 125, 9306; (m) Ryu,
D. H.; Zhou, G.; Corey, E. J. J. Am. Chem. Soc. 2004, 126,
4800.
Reaction using ligand 11, in contrast to reaction with
ligand 8, led to an enantiomeric product. It has been
established in the literature that ligand 11 and copper(II)
2. Johnson, J. S.; Evans, D. A. Acc. Chem. Res. 2000, 33,
325.
O
O
H
O
O
H
H
N
N
O
N
H
N
Cu
Cu
ROCO
H
3. For racemic reactions using b-acyloxy acrylates or equiv-
alents see: (a) Martinez-Fresneda, P.; Vaultier, M. Tetra-
hedron Lett. 1989, 30, 2929; (b) Lorvelec, G.; Vaultier, M.
Tetrahedron Lett. 1998, 39, 5185; (c) Rasset, C.; Vaultier,
M. Tetrahedron 1994, 50, 3397; (d) Ranganathan, S.;
Ranganathan, D.; Mehrotra, A. K. Synthesis 1976, 620;
(e) Minato, H.; Horibe, I. J. Chem. Soc. Sect. C 1968,
2131; (f) Wang, J.; Morral, J.; Hendrix, C.; Herdewijn, P.
J. Org. Chem. 2001, 68, 8478; (g) Lin, J.; Nikaido, M. M.;
Clark, G. J. Org. Chem. 1987, 52, 3745; For a recent
example of radical reactions, see: (h) Sibi, M. P.; Zimm-
erman, J.; Rheault, T. R. Angew. Chem., Int. Ed. 2003, 42,
4521.
H
O
O
O
N
N
ROCO
R
A
B
R
Reaction
M
M
R
R
Reaction
4. (a) Narasaka, K.; Yamamoto, I. Tetrahedron 1992, 48,
5743; (b) Yamamoto, I.; Narasaka, K. Bull. Chem. Soc.
Jpn. 1994, 67, 3327.
Figure 1. Plausible coordination model of the substrate (R ¼ 4-CF3–
C6H4), Cu(OTf)2, and ligand 8 (structure A) or 11 (structure B).