1. Departamento de Química Orgánica y Unidad Asociada al C.S.I.C., Facultad de Química, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain, and Institute of Cell and Molecular Biosciences, Catherine Cookson Building, Medical School, University, Newcastle upon Tyne, Framlington Place, Newcastle upon Tyne NE2 4HH, U.K.