1. Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba-yama, Aoba-ku, Sendai 980-8578, Japan, Graduate Institute of Natural Products, College of Pharmacy, Kaohsiung Medical University, Kaohsiung 807, Taiwan, Department of Chemistry and Materials Science, Tokyo Institute of Technology, 2-12-1-H-63 O-okayama, Meguro-ku, Tokyo 152-8551, Japan, and Tohoku Pharmaceutical University, 4-4-1 Komatsushima, Aoba-ku, Sendai 981-8558, Japan