1. Saarland University Organic Chemistry P. O. Box 151150 66041 Saarbrücken Germany
2. Current address Helmholtz Institute for Pharmaceutical Research Saarland (HIPS) University Campus E8.1, room 2.29 66123 Saarbrücken Germany
3. Current address: Endotherm GmbH Science Park 2 66123 Saarbrücken Germany
4. Rostock University Institute for Chemistry Albert-Einstein-Str. 3A 18059 Rostock Germany