1. School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Chemistry and Biochemistry, University of California at Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095-1569, and Institut für Organische Chemie, Johannes Gutenberg-Universität, Becher Weg 18-20, D-55099 Mainz, Germany