1. Key Laboratory of Biology and Genetic Resources of Tropical Crops, Ministry of Agriculture, Institute of Tropical Bioscience and Biotechnology, Chinese Academy of Tropical Agricultural Sciences, Haikou 571101, P. R. China
2. Hainan Key Laboratory for Research and Development of Natural Product from Li Folk Medicine, Haikou 571101, P. R. China
3. College of Pharmacy and Chemistry, Dali University, Dali 671000, P. R. China