1. Contribution from the Institute of Organic Chemistry and Biochemistry, Flemingovo nám. 2, 16610 Prague 6, Czech Republic, Department of Organic Chemistry, Charles University, Hlavova 8, 12843 Prague 2, Czech Republic, and Laboratoire Chimie-Physique, UMR8000, Université Paris-Sud, 91405 Orsay Cedex, France