1. Laboratory of Drug Discovery Research and Development, Developmental Therapeutics Program, Division of Cancer Treatment and Diagnosis, National Cancer Institute, Frederick Cancer Research and Development Center, Building 1052, Room 102, Frederick, Maryland 21702-1201, AIDS Vaccine Program, SAIC, Inc., NCI−FCRDC, Frederick, Maryland 21702-1201, and Laboratory of Bioorganic Chemistry, NIDDK, NIH, Bethesda, Maryland 20892