Photochemistry of cyclopentenones. Hydrogen abstraction by the .beta.-carbon atom

Wolff, Steven; Schreiber, William L.; Smith, Amos B.; Agosta, William C.

doi:10.1021/ja00777a023

Cited by 59 publications

(19 citation statements)

References 3 publications

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“…The resulting biradical intermediate V undergoes radical combination to give the final product 6. [25,26] The spin density distribution in the 3 pp* excited a,b-unsaturated carbonyl compound also induces characteristic regio-and stereoselectivity in other reactions, for example, in photoreduction and [2 + 2] photocycloaddition reactions with alkenes. [24,27] As has already been mentioned, in the first case, in addition to a-aminoalkyl radicals, hydroxyalkyl, alkoxyalkyl, acyl, or alkyl radicals can also be added to alkenes using photochemical sensitisation.…”

Section: Introductionmentioning

confidence: 99%

Stereoselective Triplet‐Sensitised Radical Reactions of Furanone Derivatives

Jahjah

Gassama

Bulach

et al. 2010

Chemistry A European J

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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.

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Section: Introductionmentioning

confidence: 99%

Stereoselective Triplet‐Sensitised Radical Reactions of Furanone Derivatives

Jahjah

Gassama

Bulach

et al. 2010

Chemistry A European J

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“…Similar reactions with α,β-unsaturated carbonyl compounds involving hydrogen abstraction and radical combination have been previously reported, often without considering them in a broader context of organic synthesis [79,80,[101][102][103][104][105][106][107][108][109][110][111][112][113]. Acetone is frequently used as a sensitizer in photochemical reactions of α,β-unsaturated lactones.…”

Section: Intramolecular Radical Addition With Electronically Excited mentioning

confidence: 63%

Photochemically induced radical reactions with furanones

Oelgemöller

Hoffmann

2015

Pure and Applied Chemistry

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Radicals are easily generated via hydrogen transfer form secondary alcohols or tertiary amines using photochemical sensitization with ketones. They can subsequently add to the electron deficient double bond of furanones. The addition of the alcohols is particularly efficient. Therefore, this reaction was used to characterize and to compare the efficiency of different photochemical continuous flow microreactors. A range of micro-structured reactors were tested and their performances evaluated. The enclosed microchip enabled high space-time-yields but its microscopic dimensions limited its productivity. In contrast, the open microcapillary model showed a greater potential for scale-up and reactor optimization. A 10-microcapillary reactor was therefore constructed and utilized for typical R&D applications. Compared to the corresponding batch processes, the microreactor systems gave faster conversions, improved product qualities and higher yields. Similar reactions have also been carried out with electronically excited furanones and other α,β-unsaturated ketones. In this case, hydrogen is transferred directly to the excited olefin. This reaction part may occur either in one step, i.e., electron and proton are transferred simultaneously, or it may occur in two steps, i.e., the electron is transferred first and the proton follows. In the first case, a C-C bond is formed in the α position of the α,β-unsaturated carbonyl compound and in the second case this bond is formed in the β position. For the first reaction, the influence of stereochemical elements of the substrate on the regioselectivity of the hydrogen abstraction on the side chain has been studied.

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“…The dimer of phenethanoll2 emerged as a significant side product in the phenethanol reaction, a phenomenon observed in a previous study. 13 This was apparent as a mixture of (RR and SS:meso) diastereoisomers in a 1 : 1 ratio. However, chromatography allowed the separation of this side product and the isolation of each of the product diastereoisomers 10a and lob.…”

Section: Resultsmentioning

confidence: 99%