1. Life Sciences Institute and Departments of Medicinal Chemistry, Microbiology and Immunology, and Chemistry, University of Michigan, Ann Arbor, MI 48109, USA.
2. Graduate School of Pharmaceutical Sciences, Kumamoto University, 5-1 Oe-honmachi, Kumamoto 862-0973, Japan.
3. Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA.
4. The University of Colorado Cancer Center, Aurora, CO 80045, USA.