1. Institute of Chemical Physics, Department of Chemistry, University of Tartu, Jakobi 2, 51014 Tartu, Estonia, Institut für Organische Chemie und Biochemie, Universität Freiburg, Albertstrasse 21, D-79104 Freiburg, Germany, and Institute for Fundamental Research of Organic Chemistry, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581 Japan