1. Faculty of Chemistry, University of Sofia, 1, J. Bourchier Blvd., 1164 Sofia, Bulgaria
2. Fakultät Chemie/Organische Chemie, Hochschule Aalen, Beethovenstraße 1, 73430 Aalen, Germany
3. Institute of Organic Chemistry I, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany