1. Department of Chemistry, Dalhousie University, PO BOX 15000, Halifax, NS B3H 4R2, Canada.
2. Chemistry Department, Taibah University, Almadinah Almunawarah, P.O. Box 30002, Saudi Arabia.
3. Institute of Organic Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland.