1. a Institute of Organic Chemistry , Bulgarian Academy of Sciences , Acad. G. Bonchev Str. build. 9, 1113 Sofia, Bulgaria
2. b Department of Organic Chemistry , Plovdiv University , Tzar Assen Str. 24, 4000 Plovdiv, Bulgaria
3. c Faculty of Chemistry , Sofia University “St. Kl. Ohridski” , 1, J. Bourchier Blvd., 1164 Sofia, Bulgaria