1. Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80302-0215, Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan, School of Medicine, Keio University, Hiyoshi-4, Kohoku, Yokohama 223-8521, Japan, and Department of Organic Chemistry, Institute of Chemistry, Box 531, Uppsala University, 751 21 Uppsala, Sweden