1. Contribution from the Department of Chemistry and Biochemistry and Department of Pharmacology, University of California at San Diego, La Jolla, California 92093-0365, Amgen, Inc., 1840 DeHavilland Drive, Thousand Oaks, California 91320-1789, Department of Chemistry, Southwestern University, Georgetown, Texas 78626, and Stereochimie et Interactions moleculaires, Ecole Normale Superieure de Lyon, 46, allee d'Italie, F-69364 Lyon Cedex 07, France