1. Contribution from the Department of Chemistry, Columbia University, New York, New York 10027, Laboratory for Bio-Organic Chemistry, and Program for Molecular Pharmacology, Sloan-Kettering Institute for Cancer Research, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, New York 10021, Department of Chemistry, Yale University, New Haven, Connecticut 06511, and Sphinx Pharmaceuticals, A Division of Eli Lilly and Co., Durham, North Carolina 27707