1. Key Laboratory of Bioorganic Phosphorous and Chemical Biology (Ministry of Education), Tsinghua University, 100084 Beijing, P. R. China
2. Laboratory of Synthesis and Natural Products, Institute of Chemical Sciences and Engineering, Ecole Poltechnique Fédérale de Lausanne EPFL-SB-ISIC-LSPN, BCH 5304, 1015 Lausanne, Switzerland