1. Contribution from Laboratory for Bioorganic Chemistry, Preclinical Pharmacology Core Facility and Analytical Pharmacology Core Facility, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, New York 10021, Department of Chemistry, Columbia University, Havemeyer Hall, New York, New York 10027, and Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461