1. Departments of Chemistry and Medicinal Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, Department of Biology, Molecular Basis of Disease, Georgia State University, Atlanta, Georgia 30303, Kumamoto University School of Medicine, Kumamoto 860-8556, Japan, Department of Hematology and Infectious Diseases, Experimental Retrovirology Section, HIV and AIDS Malignancy Branch, National Cancer Institute, Bethesda, Maryland 20892