1. Marshall, J.A. Xie, S. J. Org. Chem. 60, 7230, (1995) 2. Stolle, A. et al. Tetrahedron Lett. 35, 3521, (1994) 3. Burk, M.J. et al. J. Am. Chem. Soc. 115, 10125, (1993) 4. Burk, M.J. et al. Tet. Asym. 2, 569, (1991) 5. Pichon, M. Figadere, B. Tet. Asym.