M. Shirahase et al. / Tetrahedron Letters 45 (2004) 4061–4063
4063
Table 1. Enantioselective nitrone cycloadditions to a,b-unsaturated
References and notes
1. Shirahase, M.; Kanemasa, S. Org. Lett. 2004, 6, 675–678.
aldehydes 5–8a
Aldehyde
Conditions and results
Productsb
€
2. (a) Viton, F.; Bernardinelli, G.; Kundig, E. P. J. Am.
BþNi(ClO4)2Æ6H2O
(10 mol %), rt, 22 h, 98%,
single, 99% ee
PhN
O
Me
Chem. Soc. 2002, 124, 4968–4969; (b) Mita, T.; Ohtsuki,
N.; Ikeno, T.; Yamada, T. Org. Lett. 2002, 4, 2457–2460;
(c) Ohtsuki, N.; Kezuka, S.; Kogami, Y.; Mita, T.;
Ashizawa, T.; Ikeno, T.; Yamada, T. Synthesis 2003, 9,
1462–1466; (d) Carmona, D.; Lamata, M. P.; Viguri, F.;
Me
CH2OH
Ph
CHO
5
9
BþNi(ClO4)2Æ6H2O
(2 mol %), 0 °C, 48 h, 91%,
single, 96% ee
ꢀ
Rodrıguez, R.; Oro, L. A.; Balana, A. I.; Lahoz, F. J.;
PhN
PhN
PhN
O
Et
Tejero, T.; Merino, P.; Franco, S.; Montesa, I. J. Am.
Chem. Soc. 2004, 126, 2716–2717.
3. The colorless suspension remained all the time until the
reaction mixture was quenched with saturated ammonium
chloride after the completion of reaction.
4. General procedure for the preparation of DBFOX/Ph
complexes is as follows: Equimolar amounts of DBFOX/
Ph and a metal salt are stirred in dichloromethane for a
few hours at room temperature, and a clear solution
including the resulting complex is evaporated in vacuo to
give a solid of the complex.
BþNi(ClO4)2Æ6H2O
(10 mol %), rt, 29 h, quant,
single, 92% ee
Et
CH2OH
Ph
Ph
Ph
CHO
6
10
O
Ph
BþNi(ClO4)2Æ6H2O
(10 mol %), rt, 24 h, 67%,
single, 84% ee
Ph
CH2OH
CHO
7
11
O
BþZn(OTf)2 (10 mol %),
rt, 14 h, SA, 74%, 99:1,
77% ee
Me
CHO
Me
8
CH2OH
12
5. We have confirmed, on the basis of the experiment by use
of a reference compound, that the DBFOX/Ph ligand used
for the complex preparation experiment was recovered
almost quantitatively without formation of complex.
6. A variety of the DBFOX/Ph complexes of other zinc(II)
salts could be isolated and they were easily characterized
a All the reactions were performed in dichloromethane in the presence
of the complexes of B and MS 4 A.
b Products 9–12 were obtained by reduction of the cycloadducts with
sodiumborohydride in ethanol.
1
by H NMR spectra.
7. (a) Kanemasa, S.; Oderaotoshi, Y.; Sakaguchi, S.;
Yamamoto, H.; Tanaka, J.; Wada, E.; Curran, D. P. J.
Am. Chem. Soc. 1998, 120, 3074–3088; (b) Iserloh, U.;
Curran, D. P.; Kanemasa, S. Tetrahedron: Asymmetry
1999, 10, 2417–2428.
8. Complexes of B–D (M ¼ Zn(OTf)2) were isolable and
could be stored in open air for months without loss of
catalytic activity.
by structural modification of the chiral ligand DBFOX/
Ph. These new chiral ligands, named as the second
generation of DBFOX/Ph series, worked well to
improve catalysis in the nitrone cycloadditions using a
variety of a,b-unsaturated aldehyde dipolarophiles.
Excellent enantioselectivities up to 99% ee have been
demonstrated with a catalytic loading of 2 mol %.
9. A solution of equimolar amounts of ligand B and
Zn(OTf)2 in dichloromethane was stirred at room tem-
perature in a few minutes and the solvent was evaporated
in vacuo. The residue was dissolved in deuteriochloro-
form, and the solution was submitted to 1H NMR analysis
showing quantitative formation of the complex.
10. See the footnote ‘‘a’’ in the table included in Scheme 2.
11. Complexes derived from D show much smaller solubility
in dichloromethane than the complexes of other ligands
A–C.
Acknowledgements
A financial support from the Grant-in-Aid for Scientific
Research on Priority Areas (A) (No 13029087) from the
Ministry of Education, Culture, Sports, Science, and
Technology of Japanese Government is acknowledged.