The authors are grateful to CICYT for financial support (contract MAT2005-00374 and contract MAT2001-1694) and M. C. Mohedano for her technical support. We thank SCAI (UCO Central Service for Research Support) and J.I. Corredor for the recording of the NMR spectra. R.A. is indebted to MCYT (Programa Ramón y Cajal). A. Concheso is indebted to MCYT for his predoctoral grant.