1. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, Research Center for Materials Science, Nagoya University, Chikusa, Nagoya, Japan, Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya, Japan, Tsukuba Laboratories, Eisai Pharmaceutical Company, Tsukuba, Japan, and Suntory Institute for Bioorganic Research, Mishima, Osaka, Japan