1. Department of Chemistry National University of Singapore Singapore 117543 Singapore
2. School of Pharmaceutical Sciences (Shenzhen) Sun Yat-sen University Shenzhen 518107 China
3. School of Pharmacy Jinan University 601 Huangpu Avenue West Guangzhou 510632 China
4. State Key Laboratory of Bioactive Substance and Function of Natural Medicines Institute of Materia Medica Chi-nese Academy of Medical Sciences and Peking Union Medical College Beijing 100050 China