Enantioselective radical cyclisation reactions of 4-substituted quinolones mediated by a chiral template

Bakowski, Aline; Dressel, Martina; Bauer, Andreas; Bach, Thorsten

doi:10.1039/c0ob01272f

Cited by 50 publications

(34 citation statements)

References 66 publications

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“…The respective alcohols, which were required to produce the substitution products 11 and 12 , are commercially available. The subsequent rearrangement of the N ‐oxides to the desired products was performed according to a previously reported method,18 which we have optimized in recent years 19. Two modifications are crucial.…”

Section: Resultsmentioning

confidence: 99%

Intramolecular [2+2] Photocycloaddition of 3‐ and 4‐(But‐3‐enyl)oxyquinolones: Influence of the Alkene Substitution Pattern, Photophysical Studies, and Enantioselective Catalysis by a Chiral Sensitizer

Maturi

Wenninger

Alonso

et al. 2013

Chemistry A European J

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The intramolecular [2+2] photocycloaddition of four 4-(but-3-enyl)oxyquinolones (substitution pattern at the terminal alkene carbon atom: CH2, Z-CHEt, E-CHEt, CMe2) and two 3-(but-3-enyl)oxyquinolones (substitution pattern: CH2, CMe2) was studied. Upon direct irradiation at λ=300 nm, the respective cyclobutane products were formed in high yields (83-95 %) and for symmetrically substituted substrates with complete diastereoselectivity. Substrates with a Z- or E-substituted terminal double bond showed a stereoconvergent reaction course leading to mixtures of regio- and diastereomers with almost identical composition. The mechanistic course of the photocycloaddition was elucidated by transient absorption spectroscopy. A triplet intermediate was detected for the title compounds, which-in contrast to simple alkoxyquinolones such as 3-butyloxyquinolone and 4-methoxyquinolone-decayed rapidly (τ≈1 ns) through cyclization to a triplet 1,4-diradical. The diradical can evolve through two reaction channels, one leading to the photoproduct and the other leading back to the starting material. When the photocycloaddition was performed in the presence of a chiral sensitizer (10 mol %) upon irradiation at λ=366 nm in trifluorotoluene as the solvent, moderate to high enantioselectivities were achieved. The two 3-(but-3-enyl)oxyquinolones gave enantiomeric excesses (ees) of 60 and 64 % at -25 °C, presumably because a significant racemic background reaction occurred. The 4-substituted quinolones showed higher enantioselectivities (92-96 % ee at -25 °C) and, for the terminally Z- and E-substituted substrates, an improved regio- and diastereoselectivity.

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Section: Resultsmentioning

confidence: 99%

Intramolecular [2+2] Photocycloaddition of 3‐ and 4‐(But‐3‐enyl)oxyquinolones: Influence of the Alkene Substitution Pattern, Photophysical Studies, and Enantioselective Catalysis by a Chiral Sensitizer

Maturi

Wenninger

Alonso

et al. 2013

Chemistry A European J

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“…The hydrogen-bonding properties of the 1,5,7-trimethyl-3azabicyclo[3.3.1]nonan-2-one skeleton, to which the thioxanthone part of 1 is connected, have been established [21] and it appears safe to assume that a precoordination of quinolone substrates occurs, as depicted in Figure 3, to give the putative complex 1·7. The thioxanthone acts as light-harvesting antenna, which transmits the energy of the absorbed photon to the quinolone, presumably by triplet-energy transfer.…”

Section: Methodsmentioning

confidence: 99%

A Chiral Thioxanthone as an Organocatalyst for Enantioselective [2+2] Photocycloaddition Reactions Induced by Visible Light

2014

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“…K a = association constant, determined by NMR titration [23] at 25 8 8Cin[D 6 ]benzene. Mechanistic picture for the preferred formation of enantiomer 6a in the photocatalytic deracemization mediated by chiral thioxanthone 4.…”

Section: Angewandte Chemiementioning

confidence: 99%

Enantioselective Visible‐Light‐Mediated Formation of 3‐Cyclopropylquinolones by Triplet‐Sensitized Deracemization

et al. 2019

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3-Allyl-substituted quinolones undergo at ripletsensitized di-p-methane rearrangement reaction to the corresponding 3-cyclopropylquinolones upon irradiation with visible light (l = 420 nm). Ac hiral hydrogen-bonding sensitizer (10 mol %) was shown to promote the reaction enantioselectively (88-96 %y ield, 32-55 %e e). Surprisingly,i tw as found that the enantiodifferentiation does not occur at the state of initial product formation but that it is the result of aderacemization event. The individual parameters that control the distribution of enantiomers in the photostationary state have been identified.Triplet sensitization represents an efficient and elegant way to promote amolecule from its ground state to atriplet state, in most cases to the lowest-lying triplet state T 1 .T he mechanism of the process has been elucidated, [1] and apart from an energetic driving force,itiscrucial that the sensitizer and the substrate are in close spatial proximity to allow for efficient sensitization. Thei dea to employ triplet energy transfer by ac hiral sensitizer and to perform photochemical reactions enantioselectively was first disclosed in the 1960s. Pioneering studies were directed to the formation of chiral trans-1,2-diphenylcyclopropane (1)from its racemate (rac-1). After initial work by Hammond with as ensitizer that was later shown to operate on the singlet hypersurface, [2] Ouanns and co-workers found in 1973 that chiral ketone 2 induced an otable but small enantioselectivity (3 % ee)i nt he formation of 1 (Scheme 1). [3] Thee fficiency of the reaction suffered from the simultaneous formation of the achiral cyclopropane 3.In the 1980s and 1990s,l ittle attention was paid to the topic of enantioselective triplet-sensitized reactions. [4,5] The few reported studies were discouraging,a nd enantioselectivities did not exceed 20 % ee. [6] With the advent of efficient hydrogen-bonding templates and catalysts, [7] it became evident, however,t hat the issue of insufficient enantioface differentiation could be overcome,a nd the first highly enantioselective triplet-sensitized reaction was disclosed in 2009. [8] Am odified version of the xanthone catalyst used in this and related studies [9] was presented in 2014 and was based on at hioxanthone as the sensitizing unit. [10] Thec ompound, which is available in both enantiomeric forms 4 ( Figure 1) [11] and ent-4,has the advantage to operate with visible light and holds promise to be av ersatile catalyst for several photochemical reactions. [12,13] In the current study,weattempted to use catalyst 4 in an enantioselective [14] di-p-methane rearrangement reaction [15] of 3-allyl-substituted quinolones.A lthough the enantioselectivities of the reaction remained moderate,the mode of action turned out to be remarkable.I tw as found that the enantioenriched cyclopropanes are formed in ad eracemization process,w hich resembles the seminal work on chiral cyclopropanes [2,3] (see Scheme 1), and which likely proceeds via a1 ,3-diradical intermediate.T he results of our preliminary exper...

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Enantioselective radical cyclisation reactions of 4-substituted quinolones mediated by a chiral template

Cited by 50 publications

References 66 publications

Intramolecular [2+2] Photocycloaddition of 3‐ and 4‐(But‐3‐enyl)oxyquinolones: Influence of the Alkene Substitution Pattern, Photophysical Studies, and Enantioselective Catalysis by a Chiral Sensitizer

Intramolecular [2+2] Photocycloaddition of 3‐ and 4‐(But‐3‐enyl)oxyquinolones: Influence of the Alkene Substitution Pattern, Photophysical Studies, and Enantioselective Catalysis by a Chiral Sensitizer

A Chiral Thioxanthone as an Organocatalyst for Enantioselective [2+2] Photocycloaddition Reactions Induced by Visible Light

Enantioselective Visible‐Light‐Mediated Formation of 3‐Cyclopropylquinolones by Triplet‐Sensitized Deracemization

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