Abstract
Irradiation of the 2-oxoindeno[1,7-bc]furan-6-carbonitrile 1 in protic solvents with light of 254 nm induces a 1,3-carbalkoxy shift. Supported by X-ray analyses of 2 and 3a it could be shown that the resulting 3,7-dioxo-3H-1,3a-ethano-isobenzofuran-9-carbonitriles 2 and 3 are distinguished by the sterical arrangement of the substituents at C-atom 9. In MeOH 9α-carbonitrile 2 and in t.-BuOH 9β-carbonitrile 3 is the main product. In contrast to t.-BuOH the primary reaction in MeOH is overlapped by typical ketone photochemistry of the newly formed 7-oxo group. Lack of any photoreaction of 1 in aprotic solvents indicates an ionic mechanism rather than the involvement of free radicals. All new compounds were fully characterized by 1H and 13C NMR and the assignments supported by NOE and 1H-detected one-bond and multiple bond 1H, 13C-COSY.
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