1. Contribution from the Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, Institut für Organische Chemie, Johannes-Gutenberg-Universität, J.-J.-Becher-Weg 34, SB1, D-55099 Mainz, Germany, Department of Chemistry and Biochemistry, University of Guelph, Ontario, Canada N1G 2W1, Hoechst AG, D-65926 Frankfurt/Main, Germany, and Fachbereich Biochemie/Biotechnologie, Martin-Luther-Universität, D-06099 Halle, Germany