1. Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan, Department of Chemistry, University of Cologne, Greinstr. 4, 50939 Cologne, Germany, and Institute of Microbial Chemistry, Tokyo, Kamiosaki 3-14-23, Shinagawa-ku, Tokyo 141-0021, Japan