1. For example, chiral Lewis acids have been used to promote nitrone cycloadditions to oxazolidinone-substituted olefins (Jensen, K. B.; Gothelf, K. V.; Hazell, R. G.; Jørgensen, K. A. J. Org. Chem. 1997, 62, 2471–2477 and references cited therein) or enol ethers (Simonsen, K. B.; Bayón, P.; Hazell, R. G.; Gothelf, K. V.; Jørgensen, K. A. J. Am. Chem. Soc. 1999, 121, 3845–3853 and references cited therein), while chiral rhodium catalysts have been used to promote asymmetric cycloadditions of oxonium ylides. (Hodgson, D. M.; Stupple, P. A.; Johnstone, C. Chem. Commun. 1999, 2185–2186 and references cited therein).
2. (a) Yoshida, Y.; Ukaji, Y.; Fujinami, S.; Inomata, K. Chem. Lett. 1998, 1023–1024;
3. (b) Shimizu, M.; Ukaji, Y.; Inomata, K. Chem. Lett. 1996, 455–456; Ukaji, Y.; Sada, K.; Inomata, K. Chem. Lett. 1993, 1847–1850.
4. Gothelf, K. V.; Jørgensen, K. A. Chem. Rev. 1998, 98, 863–909.
5. Davies, C. D.; Marsden, S. P.; Stokes, E. S. E. Tetrahedron Lett. 1998, 39, 8513–8516.