1. Friedrich Miescher-Institut, P.O. Box 2543, CH-4002 Basel, Switzerland, Carlsberg Laboratory, Department of Chemistry, Gamle Carlsberg Vej 10, DK-2500 Valby, Denmark, Central Research Services, Ciba-Geigy Ltd., CH-4002 Basel, Switzerland, and Bijvoet Center, Department of Bio-organic Chemistry, Utrecht University, Padualaan 8, NL-3584 CH Utrecht, The Netherlands