1. Contribution from the Department of Chemistry, University of Illinois at Chicago, M/C 111, Chicago, Illinois 60607-7061, Department of Organic Chemistry, Biopolymer Research Center, CNR, University of Padova, 35131 Padova, Italy, Organic Chemistry and Biotechnology Section, DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands, and DSM Fine Chemicals, 6401 JH Heerlen, The Netherlands