Visible-Light-Mediated Enantioselective Photoreactions of 3-Alkylquinolones with 4-<i>O</i>-Tethered Alkenes and Allenes

Li, Xinyao; Jandl, Christian; Bach, Thorsten

doi:10.1021/acs.orglett.0c01065

Cited by 38 publications

(21 citation statements)

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“…8b In recent works on visible light-mediated reactions, we have also seen that catalyst loadings can be as low as 1 mol % for a thioxanthone sensitizer. 13 Experiments with a selection of potential sensitizers (see the Supporting Information for details) confirmed the suitability of thioxanthen-9-one ( 5 ) as the catalyst in the planned reaction ( E T = 268 kJ mol –1 ). 14 Dichloromethane was established as the preferred solvent and there was no improvement in selectivity if the reaction was performed at low temperatures.…”

Section: Resultsmentioning

confidence: 77%

Visible Light-Mediated Photochemical Reactions of 2-(2′-Alkenyloxy)cycloalk-2-enones

2020

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The title compounds were prepared, and their reactivity was studied upon sensitized irradiation at λ = 420 nm. Thioxanthen-9-one was employed as the sensitizer at a loading of 10 mol % in small-scale reactions and of 2.5 mol % on a larger scale. Cyclohex-2-enones substituted by a 2′-propenyloxy, 2′-butenyloxy, 2′-pentenyloxy, or 2′-methyl-2′-propenyloxy group in the 2-position gave the products of an intramolecular [2 + 2] photocycloadditon. The reaction proceeded with high regioselectivity (crossed product) and perfect diastereoselectivity (nine examples, 34–99% yield). If the olefin in the tether was trisubstituted (3′-methyl-2′-butenyloxy), no cycloaddition was observed. Rather, a cyclization with subsequent hydrogen abstraction occurred (three examples, 65–86% yield). The results are consistent with a reaction course via a triplet enone intermediate and the formation of a 1,4-diradical by an initial cyclization. The analogous cyclopent-2-enones were less prone to an intramolecular reaction. Instead, decomposition or intermolecular [2 + 2] photocycloaddition reactions prevailed. In the latter event, two main products were identified (three examples, 30–43% yield), resulting either from a head-to-head [2 + 2]-photodimerization or from a twofold [2 + 2] photocycloaddition of the enone to the olefin. The latter reaction sequence generated pentacyclic products with a central [1,5]dioxocane ring. The structure assignment of the two product types was corroborated by a single-crystal X-ray analysis.

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Section: Resultsmentioning

confidence: 77%

Visible Light-Mediated Photochemical Reactions of 2-(2′-Alkenyloxy)cycloalk-2-enones

2020

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“…The reaction afforded the [2+2] cycloadduct rac-3b which underwent ring opening to form the final azocane-annulated furan rac-4b in 97% yield upon treatment with acid. The structure of the final product, which had been previously assigned based on NMR data only, 7 was confirmed by single crystal X-ray crystallography (Scheme 2). 15…”

Section: Paper Synthesismentioning

confidence: 60%

“…6 In very recent work, the visible-light-mediated intramolecular [2+2] photocycloaddition of 3-alkylquinolones with 4-Otethered alkenes and allenes was found to proceed with high enantioselectivity. 7 The use of allenes as olefinic partners in the intramolecular quinolone-olefin [2+2] photocycloaddition is of special interest, 8 because allenes have demonstrated high synthetic utility and product diversity in photochemistry. 9,10 Scheme 1 Enantioselective visible-light-mediated inter-and intramolecular [2+2] photocycloaddition reactions of quinolones I (top), and structure of substrate 2a, product 3a, and ring-opening product rac-4b…”

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confidence: 99%

“…In this context, we noted that 4-(buta-2,3-dienyloxy)-3methylquinolones in which an unsubstituted allene was linked only by a methylene linker to the oxygen atom yielded an unstable intermediate upon irradiation which was prone to a consecutive reaction. While product 3a could be obtained from 6-chloroquinolone 2a (85%, 88% ee) in an enantioselective [2+2] reaction, 7 photocycloaddition products from other quinolones could not be obtained in pure form. Eventually, we managed to isolate the azocane-annulated…”

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confidence: 99%

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Synthesis of Azocane- and Oxocane-Annulated Furans by a [2+2] Photocycloaddition–Ring-Opening Cascade

Jandl²,

Bach³

2020

Synthesis

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The title compounds were synthesized from readily available quinolone and coumarin derivatives by a cascade reaction (12 examples, 90–98% yield). The cascade comprised a [2+2] photocycloaddition which occurred upon sensitized irradiation at λ = 420 nm (or direct UV irradiation at λ = 366 nm) and a subsequent acid-catalyzed ring-opening reaction. A variety of substituents are compatible with the conditions and a 3-alkyl group in the coumarin (or quinolone) is crucial to achieve a high chemoselectivity. Key to the success of the ring opening is the formation of a 4,5,5a-trihydrocyclobuta-2H-furan containing a strained bridgehead double bond which stems from the allenyl group tethered to the 4-position of the starting materials.

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“…Another intermolecular reaction was later developed, this time using catalyst 185 for the [2 + 2] photocycloaddition of quinolones 189 and electron-deficient alkenes 190 to synthesise cyclobutanes 191 ( Scheme 28 ) [ 83 ]. Recently, Bach et al also employed this methodology for the intramolecular [2 + 2] cycloaddition of quinolones 192 containing either a pendant alkene or allene to obtain cyclobutanes 193 ( Scheme 28 ) [ 84 ].…”

Section: Reviewmentioning

confidence: 99%

Recent developments in enantioselective photocatalysis

Prentice¹,

Morrisson²,

Smith³

et al. 2020

Beilstein J. Org. Chem.

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Enantioselective photocatalysis has rapidly grown into a powerful tool for synthetic chemists. This review describes the various strategies for creating enantioenriched products through merging enantioselective catalysis and photocatalysis, with a focus on the most recent developments and a particular interest in the proposed mechanisms for each. With the aim of understanding the scope of each strategy, to help guide and inspire further innovation in this field.

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Visible-Light-Mediated Enantioselective Photoreactions of 3-Alkylquinolones with 4-O-Tethered Alkenes and Allenes

Cited by 38 publications

References 50 publications

Visible Light-Mediated Photochemical Reactions of 2-(2′-Alkenyloxy)cycloalk-2-enones

Visible Light-Mediated Photochemical Reactions of 2-(2′-Alkenyloxy)cycloalk-2-enones

Synthesis of Azocane- and Oxocane-Annulated Furans by a [2+2] Photocycloaddition–Ring-Opening Cascade

Recent developments in enantioselective photocatalysis

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