1. Gustaf H. Carlson School of Chemistry, Clark University, 950 Main Street, Worcester, Massachusetts 01610, Department of Organic Chemistry, University of Padova, Biopolymer Research Center, Via Marzolo 1, Padova, Italy I-35131, and Department of Pharmacology and Molecular Toxicology, University of Massachusetts, Medical Center, 55 Lake Avenue North, Worcester, Massachusetts 01655