1. Department of Organic and Nuclear Chemistry and Centre for New Antivirals and Antineoplastics, Faculty of Science, Charles University, Hlavova 8, 128 43 Praha 2, Czech Republic, Fax: +420‐221‐951‐326
2. Summer research participant. Current address: Ridley College, P. O. Box 3013, 2 Ridley Rd., St. Catharines ON L2R 7C3, Canada
3. Department of Inorganic Chemistry, Faculty of Science, Charles University, Hlavova 8, 128 43 Praha 2, Czech Republic
4. Institute of Organic Chemistry and Biochemistry, v.v.i., Academy of Sciences of the Czech Republic, Flemingovo nám. 2, 166 10 Praha 6, Czech Republic