Synthesis of novel indenoquinolines and indenopyridazines via photoisomerization of benzotropolone derivatives

We describe the photoisomerization of hetero Diels–Alder adducts of tetramethylpurpurogallin bearing β,γ-unsaturated ketone chromophores and endocyclic –N–O– or –N–N– groups. Based on spectroscopic and crystallographic data, our results indicate that the outcome of the reaction for the two types of substrates can be correlated with the difference in the stabilities of the radical intermediates generated in each case. For oxazines, the photoisomerization involves both the β,γ-unsaturated ketone and –N–O– moieties, and proceeds through the formation of allyl, benzoyl, phenyl, and α-carbonyl radical intermediates. While the same allylic and benzoyl radicals are formed in adducts with an –N–N– group, the hydrazine does not participate in the reaction and the product is a simple rearrangement of the initial intermediate.