1. Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, and Institute of Chemical and Engineering Sciences, Chemical Synthesis Laboratory, Biopolis, 11 Biopolis Way, Helios Block #03-08, Singapore 138667