1. Contribution from the Department of Pharmaceutical Chemistry, University of California San Francisco, QB3 Building Room 508D, 1700 4th Street, San Francisco, California 94143-2550, Department of Molecular Pharmacology & Biological Chemistry, Northwestern University, 303 East Chicago Avenue, Chicago, Illinois 60611-3008, and Dipartimento di Chimica, Universitá degli studi di Modena e Reggio Emilia, via Campi 183, Modena, Italy