1. Division of Medicinal Chemistry, Graduated School of Biomedical Science, Hiroshima University, Kasumi 1-2-3, Minamiku, Hiroshima 734-8551, Japan, Institute for Medical Research, Wakunaga Pharmaceutical Co., Ltd., 1624 Shimokotachi Kodacho, Takata-gun, Hiroshima 739-1195, Japan, and Rigaku Cooporation X-ray Research Laboratory, Matsubaracho 3-9-12, Akishima, Tokyo 196-8666, Japan