1. Contribution from the Institut de recherches cliniques de Montréal (IRCM), Bioorganic Chemistry Laboratory, 110 avenue des Pins Ouest, Montréal, Québec, Canada H2W 1R7, and Department of Chemistry and Department of Pharmacology, Université de Montréal, C.P. 6128, succursale Centreville, Montréal, Québec, Canada H3C 3J7