1. Laboratory of Bio-organic Chemistry, Department of Organic Chemistry, CP 160/07, Free University of Brussels, Avenue F. D. Roosevelt, 50-1050 Brussels, Belgium, Laboratory of Cellular and Animal Biology, CP 160/12, Free University of Brussels, Avenue F. D. Roosevelt, 50-1050 Brussels, Belgium, and Smithsonian Tropical Research Institute, Apartado 2072, Balboa-Ancon, Panama