1. Division of Medicinal Chemistry, Graduate School of Biomedical Sciences, Hiroshima University, Kasumi 1-2-3, Minami-ku, Hiroshima 734-8551, Japan, Faculty of Pharmacy, Fukuyama University, Gakuen-cho 1, Fukuyama 729-0292, Japan, and Faculty of Integrated Arts and Sciences, Hiroshima University, Kagamiyama 1-7-1, Higashi-Hiroshima 739-8521, Japan