1. Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen (Germany), Fax: (+49) 241–809–2127 http://www.oc.rwth‐aachen.de
2. Institute of Organic Chemistry and Chemical Biology, University of Frankfurt, Marie‐Curie‐Strasse 11, 60439 Frankfurt am Main (Germany)