1. State Key Laboratory of Natural and Biomimetic Drugs, (Peking University), 38 Xueyuan Road, Haidian District, Beijing 100191, The People’s Republic of China, Division of Chemistry and Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, and Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543