1. College of Chemistry & Pharmacy, Shaanxi Key Laboratory of Natural Products & Chemical Biology, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China
2. Shanxi Normal University, Linfen, Shanxi 041004, P. R. China
3. Key Laboratory of Botanical Pesticide R&D in Shaanxi Province, Yangling, Shaanxi 712100, P. R. China
4. State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China