1. Contribution from the Institute of Organic Chemistry, Eötvös University, 112 Budapest P.O.B. 32, H-1117, Hungary, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, England, Department of Chemistry, Wayne State University, Detroit, Michigan 48202, and Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1A1, Canada