1. Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, Department of Molecular Structure and Design, Amgen, Inc., M/S 29-M-B, One Amgen Center Drive, Thousand Oaks, California 91320, and Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan