Stereoselective triplet-sensitised radical reactions of furanone derivatives

The stereo- and regioselectivity of triplet-sensitised radical reactions of furanone derivatives have been investigated. Furanones 7a,b were excited to the (3)pipi* state by triplet energy transfer from acetone. Intramolecular hydrogen abstraction then occurred such that hydrogen was transferred from the tetrahydropyran to the beta position of the furanone moiety. Radical combination of the tetrahydropyranyl and the oxoallyl radicals led to the final products 8a,b. In the intramolecular reaction, overall, a pyranyl group adds to the alpha position of the furanone. The effect of conformation was first investigated with compounds 9a,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 abstraction occurred at the anomeric centre and at the 5'-position of the glucosyl moiety. Computational studies of the hydrogen-abstraction step were carried out with model structures. The activation barriers of this step for different stereoisomers and the abstraction 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-abstraction step. Intramolecular hydrogen abstraction (almost simultaneous electron and proton transfer) in (3)pipi* 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.