1. Centro de Investigación y Desarrollo, C.S.I.C. Jordi Girona 18-26, E-08034 Barcelona, Spain, Department of Organic Chemistry, Universitat de Barcelona, Martí i Franquès 1-11, E-08028 Barcelona, Spain, Museum National d'Histoire Naturelle, 43 rue Cuvier Paris Cedex, France, and European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany