1. State Key Laboratory and Institute of Elemento-Organic Chemistry College of Chemistry Nankai University Tianjin 300071 P. R. China
2. State Key Laboratory of Natural and Biomimetic Drugs Chemical Biology Center State Key Laboratory of Natural and Biomimetic Drugs Chemical Biology Center, and School of Pharmaceutical Sciences Peking University Beijing 100191 P. R. China
3. School of Life Sciences Tianjin University Tianjin 300072 P. R. China