1. Contribution from the Institut de recherches cliniques de Montréal (IRCM), Bio-organic Chemistry Laboratory, 110 avenue des pins ouest, Montréal, Québec, Canada H2W 1R7, Department of Biochemistry and Chemistry, Université de Montréal, Montréal, Québec, Canada H3C 3J7, and Bio-Méga/Boehringer Ingelheim Research, Inc., 2100 rue Cunard, Laval, Québec, Canada H7S 2G5