1. Medicinal Chemistry Research Center, Korea Research Institute of Chemical Technology, Daejeon 305‐345, Republic of Korea, Fax: (+82)‐42‐860‐7160; phone: (+82)‐42‐860‐7177
2. Department of Chemistry, Hanyang University, Seoul 133‐791, Republic of Korea
3. Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18thAvenue, Columbus, Ohio 43210, USA