1. Contribution from the Institute of Chemistry, University of Luebeck, Ratzeburger Allee 160, 23538 Luebeck, Germany, Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany, University of Alabama, Birmingham, Alabama 35294-2041, and Carlsberg Laboratory, Gamle Carlsberg Vej10, DK-2500 Valby, Denmark