1. Research and Development Division, Pharmaceuticals Group, Nippon Kayaku Co. Ltd., 31-12, Shimo 3-chome, Kita-ku, Tokyo 115-8588, Japan, Microbial Chemistry Research Center, 14-23, Kamiosaki 3-chome, Shinagawa-ku, Tokyo 141-0021, Japan, and Synthetic Group, NAC Co. Ltd., 31-12, Shimo 3-chome, Kita-ku, Tokyo 115-0042, Japan