1. Department of Chemistry, Ludwig-Maximilians-Universität München, Butenandtstr. 5 – 13, 81377 München, Germany
2. Present address: Baerlocher GmbH, 85716 Unterschleißheim
3. Institute of Organic Chemistry, TU Braunschweig, Hagenring 30, 38106 Braunschweig, Germany