Self-Terminating, Oxidative Radical Cyclizations

Dreessen, Tim; Jargstorff, Christian; Lietzau, Lars; Plath, Christian; Stademann, Arne; Wille, Uta

doi:10.3390/90600480

Cited by 14 publications

(8 citation statements)

References 76 publications

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“…Alkoxyl radicals can be generated from peroxyl radicals via tetroxides or one-electron reduction of alkyl hydroperoxides [36]. Alkoxyl radicals undergo rapid addition and hydrogen abstraction reactions, as well as unimolecular fragmentation and rearrangements [37]. Most primary and secondary alkoxyl radicals undergo a rapid 1,2-hydrogen shift, which results in the formation of α-hydroxyalkyl radicals.…”

Section: Resultsmentioning

confidence: 99%

Collagen structural hierarchy and susceptibility to degradation by ultraviolet radiation

Rabotyagova

Cebe

Kaplan

2008

Materials Science and Engineering: C

188

159

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Collagen type I is the most abundant extracellular matrix protein in the human body, providing the basis for tissue structure and directing cellular functions. Collagen has complex structural hierarchy, organized at different length scales, including the characteristic triple helical feature. In the present study, the relationship between collagen structure (native vs. denatured) and sensitivity to UV radiation was assessed, with a focus on changes in primary structure, changes in conformation, microstructure and material properties. A brief review of free radical reactions involved in collagen degradation is also provided as a mechanistic basis for the changes observed in the study. Structural and functional changes in the collagens were related to the initial conformation (native vs. denatured) and the energy of irradiation. These changes were tracked using SDS-PAGE to assess molecular weight, Fourier transform infrared (FTIR) spectroscopy to study changes in the secondary structure, and atomic force microscopy (AFM) to characterize changes in mechanical properties. The results correlate differences in sensitivity to irradiation with initial collagen structural state: collagen in native conformation vs. heat-treated (denatured) collagen. Changes in collagen were found at all levels of the hierarchical structural organization. In general, the native collagen triple helix is most sensitive to UV-254nm radiation. The triple helix delays single chain degradation. The loss of the triple helix in collagen is accompanied by hydrogen abstraction through free radical mechanisms. The results received suggest that the effects of electromagnetic radiation on biologically relevant extracellular matrices (collagen in the present study) are important to assess in the context of the state of collagen structure. The results have implications in tissue remodeling, wound repair and disease progression.

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Section: Resultsmentioning

confidence: 99%

Collagen structural hierarchy and susceptibility to degradation by ultraviolet radiation

Rabotyagova

Cebe

Kaplan

2008

Materials Science and Engineering: C

188

159

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“…Future work will also focus on using computational methods to gain additional theoretical insights into the driving forces of the diastereoselection process in 5-exo cyclizations of such highly substituted radicals, the role of the geometry at the radical centre and the α nitrogen (e.g., planar or sp 3hybridized), and the possible influence of complexation to Ce III/IV in the transition state. [7,29] The nearly entirely different behaviour of the triflatesubstituted alkyne 15 compared to the tosylated derivatives 14 shows that, besides the substitution pattern at nitrogen, the conformation of the starting materials must also be considered. Compound 15 exists at room temperature as an equilibrium of two conformers (the reason for this is not clear), whereas 14 is conformationally pure.…”

Section: Discussionmentioning

confidence: 99%

“…[5] We have meanwhile demonstrated that this type of self-termination is a very general concept in radical chemistry, and many different inorganic, organic, uncharged, and even charged O-centred radicals can act, similar to NO • 3 , as O-atom donors in their reactions with alkynes. [6] Both experimental [4] and theoretical investigations [7] have revealed that the 1,5-HAT is the rate-limiting step in this sequence, whereas the diastereoselectivity is likely determined in the subsequent 5-exo cyclization of radical intermediate 4. Besides the size and stereochemistry (cis or trans) of the fused cycloalkane ring, it was found that both absence or presence of the heteroatom (X = O) as well as the nature of the substituent R influenced the diastereoselectivity of this cyclization step.…”

Section: Introductionmentioning

confidence: 99%

NO3• Induced Self-Terminating Radical Oxygenations: Diastereoselective Synthesis of Anellated Pyrrolidines

Stademann

Wille

2004

Aust. J. Chem.

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Anellated pyrrolidines 19-22 were obtained through a diastereoselective self-terminating, oxidative radical cyclization cascade by treating the cis-cyclopentyl substituted alkynyl amines 14-18 with photochemically generated nitrate radicals, NO • 3 . A fast and modular access to the starting materials 14-18 was developed, which readily enables variation of the substitution pattern at the pyrrolidine ring formed upon radical cyclization. The diastereoselectivity of this reaction sequence was found to be strongly influenced by the nature of the substituents at the nitrogen atom. This shows that a complex interplay of both steric and stereoelectronic effects orchestrates the stereoselectivity of 5-exo radical cyclizations of highly substituted radicals.

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“…Recently, Wille and coworkers have described a collection of innovative new transformations that they have classed as "self-terminating radical reactions" [ 5 - 10 ]. For example, addition of broad assortment of oxygen-centered radicals to cyclodecyne 1 provides isomeric ketones 2 (major) and 3 (minor) in variable yields, depending on the specific radical involved and the reaction conditions.…”

Section: Introductionmentioning

confidence: 99%

Do α-acyloxy and α-alkoxycarbonyloxy radicals fragment to form acyl and alkoxycarbonyl radicals?

Curran¹,

Turner²

2006

Beilstein J. Org. Chem.

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The generation of α-acyloxy and α-alkoxycarbonyloxy radicals under reductive conditions in fragmentable probe experiments does not provide unequivocal evidence for the fragmentation of such radicals to give ketones and acyl or alkoxycarbonyl radicals. Instead, standard reduction predominates, even at low tin hydride concentrations. Some ketone product is formed in the α-acyloxy substrate at low concentrations, but it is unclear whether this product arises through a slow radical fragmentation process or an inefficient, chain-breaking oxidative process.

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Self-Terminating, Oxidative Radical Cyclizations

Abstract: The recently discovered novel concept of self-terminating, oxidative radical cyclizations, through which alkynes can be converted into carbonyl compounds under very mild reaction conditions using O-centered inorganic and organic radicals as oxidants, is described

Cited by 14 publications

References 76 publications

Collagen structural hierarchy and susceptibility to degradation by ultraviolet radiation

Collagen structural hierarchy and susceptibility to degradation by ultraviolet radiation

NO3• Induced Self-Terminating Radical Oxygenations: Diastereoselective Synthesis of Anellated Pyrrolidines

Do α-acyloxy and α-alkoxycarbonyloxy radicals fragment to form acyl and alkoxycarbonyl radicals?

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