1. Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109,
United States
2. Department
of Chemistry, University of Pittsburgh,
Pittsburgh, Pennsylvania
15260, United States
3. Institut
de Chimie des Substances
Naturelles (ICSN), Centre National de la Recherche Scientifique (CNRS), Avenue de la Terrasse, 91198 Gif-sur-Yvette
Cedex, France