1. a Department of Organic Chemistry, Arrhenius Laboratory , University of Stockholm , S-106 91, Stockholm , Sweden
2. c Department of Organic Pharmaceutical Chemistry , Uppsala Biomedical Center , Box 574, S-751 23 , Uppsala , Sweden
3. b Westfälische Wilhelms-Universität Münster, Organisch-Chemisches Institut , Orléans-Ring 23, D-4400 , Münster , Federal Republic of Germany