| Abstract
| - Intramolecular excited triplet state interactions in diastereomeric compounds composed of abenzophenone chromophore (ketoprofen) and various hydrogen donor moieties (tetrahydrofuran, isopropylbenzene) have been investigated by laser flash photolysis. The rate constants for hydrogen abstractionby excited triplet benzophenone are in the order of 104−105 s-1, with the highest reactivity for thetetrahydrofuran residue. A remarkable diastereodifferentiation, expressed in the triplet lifetimes of thecarbonyl chromophore (e.g., 1.6 versus 2.7 μs), has been found for these compounds. With an alkylaromaticmoiety as donor, related effects have been observed, albeit strongly dependent on the length of the spacer.The reactivity trend for the initial hydrogen transfer step is paralleled by the quantum yields of the overallphotoreaction. The biradicals, formed via remote hydrogen abstraction, undergo intramolecular recombinationto macrocyclic ring systems. The new photoproducts have been isolated and characterized by NMRspectroscopy. The stereochemistry of the macrocycles, which contain up to four asymmetric carbons, hasbeen unambiguously assigned on the basis of single-crystal structures and/or NOE effects. Interestingly,a highly regio- and stereoselective macrocyclization has been found for the ketoprofen−tetrahydrofuranconjugates, where hydrogen abstraction from the less substituted carbon is exclusive; cisoid ring junctionis always preferred over the transoid junction. The photoreaction is less regioselective for compounds withan isopropylbenzene residue. The reactivity and selectivity trends have been rationalized by DFT (B3LYP/6-31G*) calculations.
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