1. G. Ronzoni Institute for Chemical and Biochemical Research, via G. Colombo 81, 20133 Milano, Italy, Department of Fine Chemicals, Advanced Synthesis and Catalysis, DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands, and Department of Organic Chemistry, University of Padova, Institute of Biomolecular Chemistry, CNR, 35131 Padova, Italy