1. Department of Organic Chemistry, St. Petersburg State University, University prosp. 26, St. Petersburg, 198504, Russia
2. Current address: Institute of Chemistry, University of Osnabrück, Barbarastr. 7, 49076 Osnabrück, Germany, Fax: +49‐541‐969‐2370, E‐mail: skorneev@uni‐osnabrueck.de