1. Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa, and Institut für Organische Chemie, Technische Universität Bergakademie Freiberg, Leipziger Strasse 29, D-09596 Freiberg/Sachs., Germany, and Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece