1. Department of Chemistry, Swedish University of Agricultural Sciences, P. O. Box 7015, SE-750 07 Uppsala, Sweden, Department of Genetic Medicine, Women's and Children's Hospital, 72 King William Road, North Adelaide, S.A. 5006, Australia, Laboratorium voor Organische Synthese, K. U. Leuven, Celestijnenlaan 200F, 3001 Leuven-Heverlee, Belgium, and Laboratoire de Recherches sur les Macromolécules, UMR 7540 CNRS, Université Paris 13, avenue J.-B. Clément, 93430 Villetaneuse, France