1. Organisch-Chemisches Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
2. Centro de Quı́mica, Instituto Venezolano de Investigaciones Científicas, Altos de Pipe, Venezuela
3. Departamento de Quı́mica, Universidad Simón Bolívar, Valle de Sartenejas, Caracas 1080A, Venezuela
4. School of Chemistry and Biochemistry, University of Western Australia, 35 Stirling Highway, Crawley 6009, Australia