Photocycloadditions of Arenes Derived from Lignin.

Intramolecular photocycloaddition reactions of 3,4-alkoxybenzonitriles derived from vanillin with alkenes have been investigated. In contrast to previous reports on photochemical reactions with these compounds, mainly [2 + 3] cycloaddition has been observed. A competing [2 + 2] photocycloaddition plays a minor role. Most probably, these additions occur at the singlet state S1. In the case of a triplet reaction, a different regioselectivity of the [2 + 2] cycloaddition would be observed. Linear and angular [2 + 3] cycloadducts are formed as major products. The first isomer is transformed in the second one by a photochemical vinyl-cyclopropane rearrangement, which increases the selectivity of the reaction. The influence of the substitution pattern on the reactivity and the selectivity has also been investigated.