1. Contribution from the Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, Department of Chemistry, University of Utah, Salt Lake City, Utah 84112-0850, Department of Chemistry and Biochemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, and Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 16610 Prague 6, Czech Republic