1. Department of Chemistry, Ahmadu Bello University, 15 Sokoto Road Samaru, 810107 Zaria, Nigeria
2. Institute of Organic Chemistry, University of Würzburg, Am Hubland, 97074 Würzburg, Germany
3. Department of Chemistry and Biology, University of Siegen, Adolf-Reichwein-Str. 2, 57068 Siegen, Germany