1. Institute of Organic Chemistry and Biochemistry, Flemingovo nám. 2, 16610 Praha 6, Czech Republic, Department of Organic Chemistry, Charles University in Prague, Faculty of Science, Hlavova 8, 12843 Praha 2, Czech Republic, Institut für Chemie der Technischen Universität Berlin, Strasse des 17 Juni 135, 10623 Berlin, Germany, and Department Chemie und Biochemie der Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (Haus F), 81377 München, Germany