1. Contribution from the Universita' di Milano, Dipartimento di Chimica Organica e Industriale e Centro di Eccellenza CISI, via Venezian 21, 20133 Milano, Italy, Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Ramiro de Maeztu 9, 28040 Madrid, Spain, and Institute of Organic Chemistry, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland