1. Vrije Universiteit Brussel, High Resolution NMR Centre (HNMR), Department of Materials and Chemistry (MACH), Pleinlaan 2, B-1050 Brussels, Belgium, and NMR Laboratory, Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Acad. G. Bontchev Str., Bl.9, 1113 Sofia, Bulgaria