1. Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA, Fax: +1‐858‐784‐8850
2. Present address: Departamento de Química Orgánica, Universidad Autónoma de Madrid, 28049 Madrid, Spain
3. Department of Molecular Physiology and Biological Physics, University of Virginia, McCormick Rd., Charlottesville, VA 22901, USA
4. Department of Chemistry, University of Virginia, McCormick Rd., Charlottesville, VA 22901, USA