1. Lavielle, G. et al. J. Med. Chem. 34, 1998, (1991) 2. Camp, N.P. et al. Bioorg. Med. Chem. Lett. 2, 1047, (1992) 3. Hirschmann, R. et al. Science 265, 234, (1994) 4. Allen, M.C. et al. J. Med. Chem. 32, 1652, (1989) 5. Bird, J. et al. J. Med. Chem. 37, 158, (1994) 6. Atherton, F.R.et al. J. Med. Chem. 29, 29, (1986)