1. Department of Chemistry, National Chung Hsing University, Taichung, Taiwan 402, Republic of China
2. School of Chinese Medicine, China Medical University, Taichung, Taiwan 404, Republic of China
3. Department of Chemistry and Institute for Chemical Biology, Sejong University, Seoul 143‐747, Korea
4. Department of Biochemistry, School of Medicine, Taipei Medical University, Taipei, Taiwan 101, Republic of China