Inter- and intra-molecular photocycloaddition of enol ethers to naphthalene

The isomeric non-conjugated naphthyl enol ether bichromophores (6) and (7) display markedly different photoreactivities. The former undergoes [2_π + 2_π] cycloaddition from the S1 state whereas the latter is relatively photostable. The two cyano derivatives (4) and (5) both yield intramolecular cycloadducts but the orientation of the addition is the opposite of that predicted from the reported intermolecular cyanonaphthalene-enol ether photoreactions. 2,3-Dihydrofuran undergoes specific ‘head to head’ endo photoaddition to naphthalene but in contrast the major adducts from 2,3-dihydrofuran and this arene have 'head to head' exo and ‘head to tail’ endo structures. The adduct (18) undergoes a facile 1,3-shift to yield the (4_π + 2_π) endo adduct (19). The reactivities of the bichromophores and the regiochemistries of the additions are considered in relation to the relative charge densities at the 1- and 2-positions of the S₁ arene.