1. Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan, Graduate School of Pharmaceutical Sciences, University of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, and The Institute of Physical and Chemical Research, RIKEN, Wako-shi, Saitama 351-0198, Japan