1. Department of Chemistry; Massachusetts Institute of Technology; 77 Massachusetts Avenue Cambridge MA 02139 USA
2. Present address: Westfälische Wilhelms-Universität Münster, Organisch-Chemisches Institut; Corrensstr. 40 48149 Münster Germany
3. Present address: Department of Chemistry, Biochemistry & Physics; The University of Tampa; 401 W. Kennedy Blvd. Tampa FL 33606 USA