1. State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130022 P. R. China
2. University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Key Laboratory of Medicinal Chemistry for Natural Resource Ministry of Education Yunnan Provincial Center for Research & Development of Natural Products School of Chemical Science and Technology Yunnan University Kunming 650091 P. R. China
4. University of Science and Technology of China Hefei 230026 P. R. China