1. Department of Chemistry, Shanghai University, 99 Shangda Rd, Shanghai 200444 (P.R. China), Fax: (+86) 21‐66134856
2. Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032 (P.R. China), Fax: (+86) 21‐64166128