1. Graduate School of Pharmaceutical Sciences Kyoto University Sakyo-ku, Kyoto 606-8501 Japan
2. Current Address: Department of Medicinal Chemistry Kyoto Pharmaceutical University Yashima-ku, Kyoto 607-8412 Japan
3. Department of Chemistry Faculty of Science Hokkaido University Sapporo 060-0810 Japan
4. Current Address: Graduate School of Arts and Sciences The University of Tokyo Meguro-ku, Tokyo 153-8902 Japan
5. Organisch-Chemisches Institut Heidelberg University Im Neuenheimer Feld 270 69120 Heidelberg Germany