1. Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Drive, Los Angeles, California 90095-1569
2. Department of Chemistry, Columbia University, Havemeyer Hall, New York, New York 10027
3. Laboratory for Bioorganic Chemistry, The Sloan-Kettering Institute for Cancer Research Center, 1275 York Avenue, Room RRL 1301, New York, New York 10021