1. Natural Products Research Institute, College of Pharmacy, Seoul National University, #28 Yungun, Jongro, Seoul 110-460, Korea, Department of Applied Chemistry, Hanbat National University, 16-1 Dukmyung, Yusung, Taejeon, 305-719, Korea, and Marine Natural Products Laboratory, Korea Ocean Research & Development Institute, Ansan P.O. Box 29, Seoul 425-600, Korea