1. Department of Organic Chemistry, Escuela Nacional de Ciencias Biológicas, I.P.N. Prol. Carpio y Plan de Ayala, 11340 México, D.F., Mexico, Department of Chemistry, Universidad Autónoma Metropolitana-Iztapalapa, P.O. 55-534, 09340 México, D.F., Mexico, and Department of Chemistry, CINVESTAV, I.P.N., P.O. 14-740, 07000 México, D.F., Mexico