On the Aromatic Character of Electrocyclic and Pseudopericyclic Reactions: Thermal Cyclization of (2Z)-Hexa-2,4-5-trienals and Their Schiff Bases This work was supported by the Spanish Ministerio de Investigación (Grant no.: SAF98-0143), by the Xunta de Galicia (Grant no.: PGIDT99PXI30105B), by the Eusko Jaurlaritza (Grant nos.: EX1998-126 and PI1998-116), by Gipuzkoako Foru Aldundia, and by Fondo de Cooperación Aquitania-Euskadi. We thank Dr. J. Rodriguez-Otero and Dr. S. Lopez (Universidade de Santiago) for helpful discussions