1. Contribution from the Laboratory for Bioorganic Chemistry, Sloan Kettering Institute for Cancer Research, 1275 York Avenue, New York, New York 10021, Department of Chemistry, Columbia University, Havemayer Hall, 3000 Broadway, New York, New York 10027, and Department of Molecular, Cellular and Developmental Biology, Pharmacology, and Chemistry, Yale University, 219 Prospect Street, New Haven, Connecticut 06520-8103