Fulvenes as Isomers of Benzenoid Compounds

With regard to the nature of their bonding and reactivity, fulvenes occupy a position intermediate between their benzenoid isomers and the olefins. The chemical and physical behavior of the fulvenes is determined either by the diene character of the cross-conjugated system or by the cylic conjugation in the five-membered ring, depending on the type of substituent at the exocyclic carbon atom. In addtion to several new substitution reactions, a description is given of the syntheses and reactions of 6-amino- and 6-hydroxyfulvenes, isomeric with anilines and phenols, the derivatives of which can be used for the preparation of new types of nonbenzenoid cyclic conjugated systems such as carbocyclic and heterocyclic azulenes, pseudoazulenes, thiepines, dihydropyridazines, and s-indacene.