1. Contribution from the School of Fisheries Sciences, Kitasato University, Sanriku-cho, Iwate 022-01, Japan, Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan, and Suntory Institute for Bioorganic Research, Wakayamadai, Shimamoto-cho Mishima-gun, Osaka 618, Japan