1. Laboratory of Aquatic Natural Products Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan, Misaki Marine Biological Station, The University of Tokyo, Miura, Kanagawa 238-0225, Japan, Takehara Marine Science Station, Hiroshima University, Takehara, Hiroshima 725-0024, Japan, and Division of Molecular Pharmacology, Cancer Chemotherapy Center, Japanese Foundation for Cancer Research, Koto-ku, Ariake, Tokyo 135-8550, Japan