Stereoselective Triplet-Sensitised Radical Reactions of Furanone Derivatives

The stereo‐ and regioselectivity of triplet‐sensitised radical reactions of furanone derivatives have been investigated. Furanones 7 a,b were excited to the 3ππ* state by triplet energy transfer from acetone. Intramolecular hydrogen ion then occurred such that hydrogen was transferred from the tetrahydropyran to the β position of the furanone moiety. Radical combination of the tetrahydropyranyl and the oxoallyl radicals led to the final products 8 a,b. In the intramolecular reaction, overall, a pyranyl group adds to the α position of the furanone. The effect of conformation was first investigated with compounds 9 a,b carrying an additional substituent on the tether between the furanone and pyranyl moiety. Further information on the effect of conformation and the relative configuration at the pyranyl anomeric centre and the furanone moiety was obtained from the transformations of the glucose derivatives 12, 14, 17 and 18. Radical ion occurred at the anomeric centre and at the 5′‐position of the glucosyl moiety. Computational studies of the hydrogen‐ion step were carried out with model structures. The activation barriers of this step for different stereoisomers and the ion at the anomeric centre and at the 6‘‐position of the tetrahydropyranyl moiety were calculated. The results of this investigation are in accordance with experimental observations. Furthermore, they reveal that the reactivity and regioselectivity are mainly determined in the hydrogen‐ion step. Intramolecular hydrogen ion (almost simultaneous electron and proton transfer) in 3ππ* excited furanones only takes place under restricted structural conditions in a limited number of conformations that are defined by the relative configuration of the substrates. It is observed that in the biradical intermediate, back‐hydrogen transfer occurs leading to the starting compound. In the case of glucose derivatives, this reaction led to epimerisation at the anomeric centre. Radical addition to the α‐position of furanones has been achieved. When the α,β‐unsaturated lactone is excited to the ππ* triplet state by sensitisation, intramolecular hydrogen ion occurs at the β‐position and radical combination leads to the final products (see scheme). A detailed insight into the stereochemical requirements has been obtained by DFT calculations.