1626
K. Takada et al. / Tetrahedron Letters 49 (2008) 1623–1626
´
Chem. Soc. 2006, 128, 11776; (j) Tur, F.; Saa, J. M. Org. Lett. 2007, 9,
5079.
catalytic diastereo- and enantioselective nitroaldol reac-
tions of a-ketoesters were also demonstrated. Further stud-
ies aimed at the improvement of the catalytic efficiency and
gaining insight into the mechanistic role of the subzero
temperature condition are in progress.
6. Guanidine–thiourea bifunctional organocatalyst, see: (a) Sohtome,
Y.; Hashimoto, Y.; Nagasawa, K. Adv. Synth. Catal. 2005, 347, 1643;
(b) Sohtome, Y.; Takemura, N.; Iguchi, T.; Hashimoto, Y.; Nagas-
awa, K. Synlett 2006, 144; (c) Sohtome, Y.; Hashimoto, Y.;
Nagasawa, K. Eur. J. Org. Chem. 2006, 2894; (d) Sohtome, Y.;
Takemura, N.; Takada, K.; Takagi, R.; Iguchi, T.; Nagasawa, K.
Chem. Asian. J. 2007, 2, 1150.
Acknowledgments
7. (a) Nagasawa, K.; Georgieva, A.; Takahashi, H.; Nakata, T.
Tetrahedron 2000, 56, 187; (b) Nagasawa, K.; Georgieva, A.;
Takahashi, H.; Nakata, T. Tetrahedron 2001, 57, 8959; (c) Kita, T.;
Georgieva, A.; Hashimoto, Y.; Nakata, T.; Nagasawa, K. Angew.
Chem., Int. Ed. 2002, 41, 2832; (d) Sohtome, Y.; Tanatani, A.;
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Sohtome, Y.; Tanatani, A.; Hashimoto, Y.; Nagasawa, K. Tetrahe-
dron Lett. 2004, 45, 5589.
We are grateful to Professor K. Noguchi for X-ray anal-
ysis. Y.S. gratefully acknowledges Grant-in-Aid for Young
Scientists (B). This research was supported in part by
grants from Mitubishi Chemical Corporation, the Uehara
Memorial Foundation, and the Nagase Science and Tech-
nology Foundation.
8. For reviews concerning asymmetric catalysis by chiral hydrogen-
bonding donors, see: (a) Connon, S. J. Chem. Eur. J. 2006, 12, 5418;
(b) Taylor, M. S.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2006, 45,
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Kumamoto, T. Synthesis 2006, 737; (b) Ishikawa, T.; Isobe, T. Chem.
Eur. J. 2002, 8, 552.
Supplementary data
Supplementary data associated with this article can be
References and notes
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17. No epimerization and retro-nitroaldol reaction were observed.
18. The determination of the absolute configuration of 7 is in progress.