Anja Spiegel

Real time observation of the photo-Fries rearrangement

The photo-Fries rearrangement of 4-tert-butylphenyl acetate dissolved in cyclohexane is investigated by two-color femtosecond pump probe spectroscopy. The spectral transmission changes are characterized in the visible and ultraviolet spectral region and allow for the first time to temporally resolve the primary reaction steps. We find that the photoinduced homolytic cleavage of the CO bond occurs within 2 ps and that the geminate recombination of the generated radical pair to the intermediate substituted cyclohexadienone takes 13 ps. The experimental results support a model in which the initial reaction proceeds from the originally excited pipi(*) state via a barrier to a dissociative pisigma(*) state.

Stereoselective Photochemical Synthesis and Structure Elucidation of 1-Methyl-Substituted Tricyclo[6.2.0.02,6]decanes and Tricyclo[7.2.0.02,7]undecanes

The trans- and cis-substituted 2-allyl-1-(2-propenyl)cyclopentanes 5a (47% yield) and 5b (33% yield) as well as the trans- and cis-substituted 2-allyl-1-(2-propenyl)cyclohexanes 6a (48% yield) and 6b (64% yield) were synthesized from the corresponding 1-acetyl-1-cycloalkenes. Their intramolecular, Cu-catalyzed [2+2] photocycloaddition reaction was studied in ether as the solvent. The reaction proceeded with excellent facial diastereoselectivity. The trans-cycloalkanes 5a and 6a exclusively yielded the trans-anti-cis products 11a (80%) and 12a (80%), whereas the cis-cycloalkanes 5b and 6b yielded the cis-syn-cis products 11b (77%) and 12b (88%). The structures of the products 11a and 12a were elucidated by NMR spectroscopy. The configuration assignments of compounds 11b and 12b were confirmed by independent syntheses of these compounds. Stereoselective hydrogenation of the unsaturated tricycloalkenes 13 and 14 gave access to the tricycloalkanes 11b (69%) and 12b (79%).