1. College of Chemistry and Chemical Engineering Hainan Normal University, Haikou Hainan 571158 P. R. China
2. Key Laboratory of Pollution Control of Hainan Province Hainan Normal University, Haikou Hainan 571158 P. R. China
3. Key Laboratory of Tropical Medicinal Resource Chemistry of Ministry of Education Hainan Normal University, Haikou Hainan 571158 P. R. China