1. Department of Chemistry, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-701, Korea
2. Department of Chemistry, Sogang University, 1 Sinsu-dong, Mapo-gu, Seoul 121-742, Korea
3. Bio-Organic Science Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusung-gu, Daejeon 305-600, Korea
4. College of Pharmacy, Ewha Womans University, 11-1 Daehun-dong, Seodaemun-gu, Seoul 120-750, Korea