1. Contribution from the Department of Pharmaceutical Chemistry, University of California, San Francisco, San Francisco, California 94143, Laboratoire de Chimie Théorique, Unité de Recherche Associée au CNRS No. 510, Université Henri Poincaré-Nancy I, BP. 239, 54506 Vandaeuvre-lès-Nancy Cedex, France, and Vertex Pharmaceuticals Incorporated, 40 Allston Street, Cambridge, Massachusetts 02139-4211