1. School of Pharmaceutical Sciences and Chongqing Key Laboratory of Natural Drug Research, Chongqing University, Chongqing 401331, P. R. China
2. Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, P. R. China
3. Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China