1. Molecular Targets Laboratory, Center for Cancer Research, National Cancer Institute, NCI-Frederick, Building 1052, Room 121, Frederick, Maryland 21701-1201, Chemical Biology Laboratory, SAIC-Frederick, Inc., NCI-Frederick, Building 376, Frederick, Maryland, 21702, and Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, Maryland, 20892