1. Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States
2. Department of Chemistry, Tsinghua University, Beijing 100084, China
3. Department of Chemistry, China Agricultural University, Beijing 100193, P. R. China
4. Departamento de Química Orgánica, Universidad de Valencia, 46100 Burjassot, Spain