1. Contribution from the Department of Organic Chemistry, University of Göttingen, Tammannstrasse 2, D-37077 Göttingen, Germany, Department of Physical Chemistry, University of Rostock, Hermannstrasse 14, D-18051 Rostock, Germany, Department for Organic Chemistry and Biochemistry, University of Freiburg, Albertstrasse 21, D-79104 Freiburg i. Br., Germany, and Department of Chemistry, University of Durham, Durham, South Road, DH1 3LE, England