We are grateful to Isao Tanaka, Masayuki Okada, Mitsuhiro Ino, Norimasa Miyamoto, Kappei Tsukahara, Hiroyuki Kato, Hiroo Ogura, Tatsuto Fukushima, Yoshiaki Furuya, Jiro Sonoda (Eisai Co., Ltd.), Paul Frankland (University of Toronto), and Hachiro Sugimoto (Kyoto University) for fruitful discussions and advice.