1. Laboratory of Medicinal Chemistry, Center for Cancer Research, National Cancer Institute−Frederick, National Institutes of Health, Frederick, Maryland 21702, Laboratory of Cellular Carcinogenesis & Tumor Promotion, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, and Laboratory of Medicinal Chemistry, College of Pharmacy, Seoul National University, Shinlin Dong, Kwanak-ku, Seoul 151-742, South Korea