1. Contribution from the Bio-organic Chemistry Laboratory, Institut de recherches cliniques de Montréal (IRCM), 110 avenue des Pins Ouest, Montréal, Québec, Canada H2W 1R7, Department of Chemistry and Department of Pharmacology, Université de Montréal, C.P. 6128, succursale Centre-ville, Montréal, Québec, Canada H3C 3J7, and Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montréal, Québec, Canada H3A 2K6