1. Institute of Organic and Pharmaceutical Chemistry, The National Hellenic Research Foundation, 48 Vas. Constantinou Avenue, 11635 Athens, Greece, Department of Biology, Chemistry and Pharmacy, Freie Universität Berlin, Takustr. 3, 14195 Berlin, Germany, and Center for Drug Discovery, Northeastern University, 116 Mugar Hall, 360 Huntington Avenue, Boston, Massachusetts 02115