1. Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States
2. Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, United States
3. Cancer Center, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, United States
4. Department of Chemistry, Syracuse University, Syracuse, New York 13244, United States
5. Department of Medicinal Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States
6. Department Chemistry, Tsinghua University, Beijing 100084, China