1. Nanyang Technological University Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences Singapore 637371 Singapore
2. Present address: School of Chemistry Sun Yat-Sen University Guangzhou 510275 P. R. China
3. School of Pharmacy Guizhou University of Traditional Chinese Medicine, Huaxi District Guiyang Guizhou 550025 China
4. Key Laboratory of Green Pesticide and Agricultural Bioengineering Ministry of Education Guizhou University, Huaxi District Guiyang 550025 China