1. Institut für Organische Chemie und Makromolekulare Chemie der Universität Düsseldorf, Universitätsstr. 1, W‐4000 Düsseldorf 1, F.R.G.
2. Institute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan, Republic of China
3. Department of Chemistry, University of North Texas, P.O.Box 5068, Denton, Texas 76203, U.S.A.
4. Department of Organic Chemistry, University of New South Walesd, P.O.Box 1, Kensington, N.S.W. 2033, Australia