Synthesis of Fluorinated Tricyclic Scaffolds by Intramolecular [2+2] Photocycloaddition Reactions

2016

Chemistry A European J

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β-(3-Iodopropoxy)-substituted α,β-unsaturated lactams, lactones, and cycloalkenones (eight examples) underwent reductive radical reactions in a dichloromethane solution of N,N-diisopropylethylamine (Hünig's base) upon irradiation with visible light (λ=419 nm). Apart from plain reduction reactions (hydro-de-iodination), a significant degree of cyclization was observed in three cases. In parallel to the conversion of the substrates, the formation of intensely colored by-products was observed. Based on mass spectrometric evidence and upon comparison with known compounds, the by-products were identified as cyanine dyes. Their formation supports the hypothesis that irradiation of dichloromethane solutions of Hünig's base leads to the formation of radicals, which in turn can either initiate a radical reaction or combine with cyanine precursors. It was shown by deuterium-labelling experiments, that one equivalent of dichloromethane is incorporated into the cyanine dyes and that the reductive quenching of radical intermediates is at least partially due to hydrogen abstraction from the solvent. As a consequence, a reductive cyclization of the starting materials is favored in CD Cl solutions as shown for two β-(3-iodopropoxy)-substituted tetronates, which underwent in dichloromethane almost exclusive reduction, but gave predominantly the cyclization products in CD Cl .

Section: Resultsmentioning

confidence: 99%

“…2016, 22,15921 -15928 www.chemeurj.org 2016 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 77.16 ppm). 2016, 22,15921 -15928 www.chemeurj.org 2016 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 77.16 ppm).…”

Section: Methodsunclassified

Radical Reactions Induced by Visible Light in Dichloromethane Solutions of Hünig's Base: Synthetic Applications and Mechanistic Observations

Böhm

2016

Chemistry A European J

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“…Under the reaction conditions of entry 9 in Table 1, photocycloaddition product 2 a could be isolated in 90 % yield and with 80 % ee (Table 2, entry 1). Further cycloalkenones 1 reacted equally efficiently under optimized reaction conditions [14] and in several cases (entries 2, 5-9) with significantly higher enantioselectivities. Only sterically demanding (entry 3) and electron-deficient olefins (entry 4) led to slightly lower selectivities.…”

Section: Methodsmentioning

confidence: 94%

[2+2] Photocycloaddition of 3‐Alkenyloxy‐2‐cycloalkenones: Enantioselective Lewis Acid Catalysis and Ring Expansion

Brimioulle

2014

Angew Chem Int Ed

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By application of substoichiometric amounts (50 mol %) of a chiral Lewis acid, the intramolecular [2+2] photocycloaddition of the title compounds was achieved with high enantioselectivity (up to 94 % ee). Upon cleavage of the cyclobutane ring the resulting tricyclic products underwent ring-expansion reactions under acidic conditions and formed anellated seven- or eight-membered-ring systems without racemization. The ring expansion could be combined with a diastereoselective reduction (triethylsilane) or allylation (allyltrimethylsilane) upon BF3 catalysis (48-87 % yield).

“…Weitere Cycloalkenone 1 reagierten unter den optimierten Bedingungen ähnlich effektiv [14] und mit teilweise sogar deutlich hçherer Enantioselektivität (Einträge 2, 5-9). Weitere Cycloalkenone 1 reagierten unter den optimierten Bedingungen ähnlich effektiv [14] und mit teilweise sogar deutlich hçherer Enantioselektivität (Einträge 2, 5-9).…”

Section: Methodsunclassified

[2+2]‐Photocycloaddition von 3‐Alkenyloxy‐2‐cycloalkenonen: enantioselektive Lewis‐Säure‐Katalyse und Ringerweiterung

Brimioulle

2014

Angewandte Chemie

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Durch Verwendung substçchiometrischer Mengen (50 Mol-%) einer chiralen Lewis-Säure ist es gelungen, die intramolekulare [2+2]-Photocycloaddition der Titelverbindungen mit hoher Enantioselektivität (bis zu 94 % ee) zu realisieren. Die resultierenden tricyclischen Produkte unterziehen sich im sauren Milieu einer Ringerweiterungsreaktion, in deren Verlauf der Cyclobutanring gespalten wird und racemisierungsfrei anellierte Sieben-oder Achtringe gebildet werden. Die Ringerweiterung kann unter BF 3 -Katalyse mit einer diastereoselektiven Reduktion (Triethylsilan) oder Allylübertragung (Allyltrimethylsilan) kombiniert werden (48-87 % Ausbeute).