1. Contribution from the Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, Department of Chemistry, Yale University, 225 Prospect Street, P.O. Box 6666, New Haven, Connecticut 06511, and Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02167-3860