ABSTRACT
While thermal reactions of aromatic compounds are mostly characterized by the retention of aromaticity in the final products, chemical reactions of the electronically excited states most frequently lead to a large variety of nonaromatic products. Photochemical reactions enrich considerably the chemistry of aromatic compounds and thereby offer many unusual applications to organic synthesis. This context was immediately recognized when the first ortho or [2þ2] photocycloaddition was described [1]. The presence of reactive functional groups as well as ring constrain of the polycyclic compounds make the products very attractive as intermediates for organic synthesis. The meta or [2þ3] photocycloaddition was discovered almost at the same time [2]. These reaction were frequently applied to the synthesis of natural products. For instance, many polycyclic terpene derived products are easily available when an intramolecular meta photocycloaddition is used as key step. Several reviews dealing with photocycloaddition were published before [3-7]. The aim of this chapter is therefore to summarize the principle items of reactivity and selectivity of these reactions and to encourage to apply them to organic synthesis.
