1. Institute of Organic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany, DSM Fine Chemicals, Advanced Synthesis and Catalysis, P.O. Box 18, 6160 MD Geleen, The Netherlands, Department of Organic Chemistry, Weizmann Institute of Science, Rehovet 76100, Israel, and Laboratory of Organic Chemistry, Eidgenössische Technische Hochschule, ETH Hönggerberg, HCI, Wolfgang Pauli Strasse 10, CH-8093 Zürich, Switzerland