@article{FudickarBauchIhmelsetal.2021, author = {Fudickar, Werner and Bauch, Marcel and Ihmels, Heiko and Linker, Torsten}, title = {DNA-triggered enhancement of singlet oxygen production by pyridinium alkynylanthracenes}, series = {Chemistry - a European journal}, volume = {27}, journal = {Chemistry - a European journal}, number = {54}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1521-3765}, doi = {10.1002/chem.202101918}, pages = {13591 -- 13604}, year = {2021}, abstract = {There is an ongoing interest in O-1(2) sensitizers, whose activity is selectively controlled by their interaction with DNA. To this end, we synthesized three isomeric pyridinium alkynylanthracenes 2 o-p and a water-soluble trapping reagent for O-1(2). In water and in the absence of DNA, these dyes show a poor efficiency to sensitize the photooxygenation of the trapping reagent as they decompose due to electron transfer processes. In contrast, in the presence of DNA O-1(2) is generated from the excited DNA-bound ligand. The interactions of 2 o-p with DNA were investigated by thermal DNA melting studies, UV/vis and fluorescence spectroscopy, and linear and circular dichroism spectroscopy. Our studies revealed an intercalative binding with an orientation of the long pyridyl-alkynyl axis parallel to the main axis of the DNA base pairs. In the presence of poly(dA : dT), all three isomers show an enhanced formation of singlet oxygen, as indicated by the reaction of the latter with the trapping reagent. With green light irradiation of isomer 2 o in poly(dA : dT), the conversion rate of the trapping reagent is enhanced by a factor >10. The formation of O-1(2) was confirmed by control experiments under anaerobic conditions, in deuterated solvents, or by addition of O-1(2) quenchers. When bound to poly(dG : dC), the opposite effect was observed only for isomers 2 o and 2 m, namely the trapping reagent reacted significantly slower. Overall, we showed that pyridinium alkynylanthracenes are very useful intercalators, that exhibit an enhanced photochemical O-1(2) generation in the DNA-bound state.}, language = {en} } @article{KruegerLinker2021, author = {Kr{\"u}ger, Tobias and Linker, Torsten}, title = {Synthesis of gamma-spirolactams by Birch reduction of arenes}, series = {European journal of organic chemistry}, volume = {2021}, journal = {European journal of organic chemistry}, number = {10}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1099-0690}, doi = {10.1002/ejoc.202100056}, pages = {1585 -- 1591}, year = {2021}, abstract = {A convenient method for the synthesis of gamma-spirolactams in only three steps is described. Birch reduction of inexpensive and commercially available aromatic carboxylic acids in the presence of chloroacetonitrile affords nitriles in moderate to good yields. Suitable precursors are methyl-substituted benzoic acids, naphthoic, and anthroic acid. Subsequent catalytic hydrogenation proceeds smoothly with PtO2 or Raney Ni as catalysts and lactams are isolated in excellent yields and stereoselectivities. Thus, up to 3 new stereogenic centers can be constructed as sole diastereomers from achiral benzoic acids. Furthermore, it is possible to control the degree of saturation at different pressures, affording products with 0, 1, or 2 double bonds. Overall, more than 15 new gamma-spirolactams have been synthesized in analytically pure form.}, language = {en} } @article{FudickarLinker2020, author = {Fudickar, Werner and Linker, Torsten}, title = {Structural motives controlling the binding affinity of 9,10-bis(methylpyridinium)anthracenes towards DNA}, series = {Bioorganic \& medicinal chemistry : a Tetrahedron publication for the rapid dissemination of full original research papers and critical reviews on biomolecular chemistry, medicinal chemistry and related disciplines}, volume = {28}, journal = {Bioorganic \& medicinal chemistry : a Tetrahedron publication for the rapid dissemination of full original research papers and critical reviews on biomolecular chemistry, medicinal chemistry and related disciplines}, number = {8}, publisher = {Elsevier}, address = {Oxford}, issn = {0968-0896}, doi = {10.1016/j.bmc.2020.115432}, pages = {7}, year = {2020}, abstract = {In the search of new DNA groove binding agents a series of substituted 9,10-methylpyridiniumanthracenes have been synthesized and their interactions with DNA have been studied by UV/vis absorption, CD and fluorescence spectroscopy. A minor groove binding mode is confirmed by DNA melting studies, strong CD effects, the dependence of the binding affinity on ionic strength, and the differentiation between AT and GC base pairs. No binding occurs to GC sequences. Binding constants to calf thymus DNA (ct-DNA) and poly(dA:dT) in the range between 1 x 10(4) and 3 x 10(5) M-1 have been determined. The binding strength decreases with the size of substituents attached at the anthracene site. Variation of the substitution pattern of the charged groups shows that methyl groups in meta position cause slightly stronger binding than methyl groups in para position. In contrast, with these groups in ortho position, no binding interaction has been observed. The strongest binding is achieved with an expansion of the peripheral heterocycle from pyridine to quinoline. Molecular modeling reveals the pivotal role of the substitution pattern: Anthracenes with para and meta pyridines align along the minor grooves. On the other hand, the ortho derivative adopts no groove-alignment.}, language = {en} } @article{FudickarMetzMaiLindeetal.2021, author = {Fudickar, Werner and Metz, Melanie and Mai-Linde, Yasemin and Kr{\"u}ger, Tobias and Kelling, Alexandra and Sperlich, Eric and Linker, Torsten}, title = {Influence of functional groups on the ene reaction of singlet oxygen with 1,4-cyclohexadienes}, series = {Photochemistry and photobiology : the official journal of the American Society for Photobiology}, volume = {97}, journal = {Photochemistry and photobiology : the official journal of the American Society for Photobiology}, number = {6}, publisher = {Wiley}, address = {Malden, Mass.}, issn = {0031-8655}, doi = {10.1111/php.13422}, pages = {1289 -- 1297}, year = {2021}, abstract = {The photooxygenation of 1,4-cyclohexadienes has been studied with a special focus on regio- and stereoselectivities. In all examples, only the methyl-substituted double bond undergoes an ene reaction with singlet oxygen, to afford hydroperoxides in moderate to good yields. We explain the high regioselectivities by a "large-group effect" of the adjacent quaternary stereocenter. Nitriles decrease the reactivity of singlet oxygen, presumably by quenching, but can stabilize proposed per-epoxide intermediates by polar interactions resulting in different stereoselectivities. Spiro lactams and lactones show an interesting effect on regio- and stereoselectivities of the ene reactions. Thus, singlet oxygen attacks the double bond preferentially anti to the carbonyl group, affording only one regioisomeric hydroperoxide. If the reaction occurs from the opposite face, the other regioisomer is exclusively formed by severe electrostatic repulsion in a perepoxide intermediate. We explain this unusual behavior by the fixed geometry of spiro compounds and call it a "spiro effect" in singlet oxygen ene reactions.}, language = {en} } @article{HaubitzFudickarLinkeretal.2020, author = {Haubitz, Toni and Fudickar, Werner and Linker, Torsten and Kumke, Michael Uwe}, title = {pH-sensitive fluorescence switching of pyridylanthracenes}, series = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, volume = {124}, journal = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, number = {52}, publisher = {American Chemical Society}, address = {Washington}, issn = {1089-5639}, doi = {10.1021/acs.jpca.0c09911}, pages = {11017 -- 11024}, year = {2020}, abstract = {9,10-substituted anthracenes are known for their useful optical properties like fluorescence, which makes them frequently used probes in sensing applications. In this article, we investigate the fundamental photophysical properties of three pyridyl-substituted variants. The nitrogen atoms in the pyridinium six-membered rings are located in the ortho-, meta-, and para-positions in relation to the anthracene core. Absorption, fluorescence, and transient absorption measurements were carried out and were complemented by theoretical calculations. We monitored the photophysics of the anthracene derivatives in chloroform and water investigating the protonated as well as their nonprotonated forms. We found that the optical properties of the nonprotonated forms are strongly determined by the anthracene chromophore, with only small differences to other 9,10-substituted anthracenes, for example diphenyl anthracene. In contrast, protonation leads to a strong decrease in fluorescence intensity and lifetime. Transient absorption measurements and theoretical calculations revealed the formation of a charge-transfer state in the protonated chromophores, where electron density is shifted from the anthracene moiety toward the protonated pyridyl substituents. While the para- and ortho-derivatives' charge transfer is still moderately fluorescent, the meta-derivative is affected much stronger and shows nearly no fluorescence. This nitrogen-atom-position-dependent sensitivity to hydronium activity makes a combination of these fluorophores very attractive for pH-sensing applications covering a broadened pH range.}, language = {en} } @article{MaiLindeLinker2020, author = {Mai-Linde, Yasemin and Linker, Torsten}, title = {Radical clock probes to determine carbohydrate radical stabilities}, series = {Organic letters}, volume = {22}, journal = {Organic letters}, number = {4}, publisher = {American Chemical Society}, address = {Washington}, issn = {1523-7060}, doi = {10.1021/acs.orglett.0c00111}, pages = {1525 -- 1529}, year = {2020}, abstract = {Carbohydrate radical stabilities in the 1- and 2-position have been determined by a radical clock approach, starting from cyclopropanated sugars with xanthates as precursors. Various hexoses and pentoses afforded 1-deoxy sugars as main products, indicating that anomeric radicals are more stable than radicals in the 2-position. An additional influence of the configurations on radical stabilities has been observed. Our results should be interesting for the understanding of 1,2-radical rearrangements in carbohydrate chemistry and offer an easy access to deoxy-vinyl sugars.}, language = {en} } @article{FudickarRoderListeketal.2021, author = {Fudickar, Werner and Roder, Phillip and Listek, Martin and Hanack, Katja and Linker, Torsten}, title = {Pyridinium alkynylanthracenes as sensitizers for photodynamic therapy}, series = {Photochemistry and photobiology}, volume = {98}, journal = {Photochemistry and photobiology}, number = {1}, publisher = {Wiley}, address = {Hoboken}, issn = {0031-8655}, doi = {10.1111/php.13554}, pages = {193 -- 201}, year = {2021}, abstract = {Photodynamic therapy (PDT) is a mild but effective method to treat certain types of cancer upon irradiation with visible light. Here, three isomeric methylpyridinium alkynylanthracenes 1op were evaluated as sensitizers for PDT. Upon irradiation with blue or green light, all three compounds show the ability to initiate strand breaks of plasmid DNA. The mayor species responsible for cleavage is singlet oxygen (O-1(2)) as confirmed by scavenging reagents. Only isomers 1m and 1p can be incorporated into HeLa cells, whereas isomer 1o cannot permeate through the membrane. While isomer 1m targets the cell nucleus, isomer 1p assembles in the cellular cytoplasm and impacts the cellular integrity. This is in accordance with a moderate toxicity of 1p in the dark, whereas 1m exhibits no dark toxicity. Both isomers are suitable as PDT reagents, with a CC50 of 3 mu m and 75 nm, for 1p and 1m, respectively. Thus, derivative 1m, which can be easily synthesized, becomes an interesting candidate for cancer therapy.}, language = {en} } @article{FudickarLinker2021, author = {Fudickar, Werner and Linker, Torsten}, title = {Photooxygenation of naphthalene}, series = {ChemPhotoChem}, volume = {5}, journal = {ChemPhotoChem}, number = {11}, publisher = {Wiley-VCH}, address = {Weinheim}, doi = {10.1002/cptc.202100097}, pages = {1004 -- 1008}, year = {2021}, abstract = {The photooxygenation of naphthalene to the corresponding endoperoxide (EPO) under various conditions is described. Substantial conversion is only observed at -10 degrees C and after more than two days, indicating that the [4+2] cycloaddition of singlet oxygen to this acene proceeds much more slowly than corresponding reactions of substituted naphthalenes, a rate constant of k = 5.4 +/- 0.3 M(-1)s(-1) was determined by competition kinetics. Another problem is the thermal lability and photochemical cleavage of the naphthalene EPO. We investigated the mechanism of this radical process depending on the light source and sensitizer in comparison to known cyclohexadiene EPO. Thus, bisepoxides and keto epoxides are formed after homolysis of the O-O bond by irradiation with sodium lamps or blue LEDs and subsequent cyclization. This process is accelerated by the sensitizers methylene blue and 9,10-dicyanoanthracene, indicating an electron transfer mechanism. Finally, the cleavage of the peroxidic bond is inhibited with red LEDs, and photooxygenation under such conditions affords 20 \% EPO. Thus, we could demonstrate that contrary to literature statements singlet oxygen does indeed react with naphthalene.}, language = {en} } @article{KruegerBramborgKellingetal.2021, author = {Kr{\"u}ger, Tobias and Bramborg, Andrea and Kelling, Alexandra and Sperlich, Eric and Linker, Torsten}, title = {Birch Reduction of Arenes as an Easy Entry to γ-Spirolactones}, series = {European journal of organic chemistry}, volume = {2021}, journal = {European journal of organic chemistry}, number = {46}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1434-193X}, doi = {10.1002/ejoc.202101018}, pages = {6383 -- 6391}, year = {2021}, abstract = {A convenient method for the synthesis of γ-spirolactones in only 2-3 steps is described. Birch reduction of inexpensive and commercially available aromatic carboxylic acids in the presence of ethylene oxide affords hydroxy acids, which undergo direct lactonization during work-up. Suitable precursors are methyl-substituted benzoic acids, naphthoic, and dicarboxylic acids. Subsequent hydrogenation proceeds smoothly with Pd/C as catalyst and saturated γ-spirolactones are isolated in excellent yields and stereoselectivities. Thus, up to 3 new stereogenic centers can be constructed as sole diastereomers from achiral benzoic acids. Furthermore, it is possible to control the degree of saturation with Raney nickel or Wilkinson's catalyst to obtain products with 1 double bond. Overall, more than 30 new γ-spirolactones have been synthesized in analytically pure form.}, language = {en} } @article{KruegerKellingSchildeetal.2016, author = {Kr{\"u}ger, Tobias and Kelling, Alexandra and Schilde, Uwe and Linker, Torsten}, title = {Simple Synthesis of gamma-Spirolactams by Birch Reduction of Benzoic Acids}, series = {European journal of organic chemistry}, journal = {European journal of organic chemistry}, number = {6}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1434-193X}, doi = {10.1002/ejoc.201601650}, pages = {1074 -- 1077}, year = {2016}, abstract = {A convenient synthesis of gamma-spirolactams in only two steps was developed. Birch reduction of benzoic acids and immediate alkylation with chloroacetonitrile afforded cyclohexadienes in high yields. The products could be isolated by crystallization on a large scale in analytically pure form. Subsequent hydrogenation with platinum(IV) oxide as the catalyst reduced the nitrile functionality and the double bonds in the same step with excellent stereoselectivity. The relative configurations were determined unequivocally by X-ray analyses. Direct cyclization of the intermediary formed amino acids afforded the desired gamma-spirolactams in excellent overall yields. The procedure is characterized by few steps, cheap reagents, and can be performed on a large scale, interesting for industrial processes.}, language = {en} } @article{TianHuZhangetal.2018, author = {Tian, Guang-Zong and Hu, Jing and Zhang, Heng-Xi and Rademacher, Christoph and Zou, Xiao-Peng and Zheng, Hong-Ning and Xu, Fei and Wang, Xiao-Li and Linker, Torsten and Yin, Jian}, title = {Synthesis and conformational analysis of linear homo- and heterooligomers from novel 2-C-branched sugar amino acids (SAAs)}, series = {Scientific reports}, volume = {8}, journal = {Scientific reports}, publisher = {Nature Publ. Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/s41598-018-24927-6}, pages = {8}, year = {2018}, abstract = {Sugar amino acids (SAAs), as biologically interesting structures bearing both amino and carboxylic acid functional groups represent an important class of multifunctional building blocks. In this study, we develop an easy access to novel SAAs in only three steps starting from nitro compounds in high yields in analytically pure form, easily available by ceric (IV) mediated radical additions. Such novel SAAs have been applied in the assembly of total nine carbopeptoids with the form of linear homo-and heterooligomers for the structural investigations employing circular dichroism (CD) spectroscopy, which suggest that the carbopeptoids emerge a well-extended, left (or right)-handed conformation similar to polyproline II (PPII) helices. NMR studies also clearly demonstrated the presence of ordered secondary structural elements. 2D-ROESY spectra were acquired to identify i+1NH <-> (C1H)-C-i, (C2H)-C-i correlations which support the conformational analysis of tetramers by CD spectroscopy. These findings provide interesting information of SAAs and their oligomers as potential scaffolds for discovering new drugs and materials.}, language = {en} } @article{GeroldingerTonnerFudickaretal.2018, author = {Geroldinger, Gerald and Tonner, Matthias and Fudickar, Werner and De Sarkar, Sritama and Dighal, Aishwarya and Monzote, Lianet and Staniek, Katrin and Linker, Torsten and Chatterjee, Mitali and Gille, Lars}, title = {Activation of anthracene endoperoxides in leishmania and impairment of mitochondrial functions}, series = {Molecules}, volume = {23}, journal = {Molecules}, number = {7}, publisher = {MDPI}, address = {Basel}, issn = {1420-3049}, doi = {10.3390/molecules23071680}, pages = {22}, year = {2018}, abstract = {Leishmaniasis is a vector-borne disease caused by protozoal Leishmania. Because of resistance development against current drugs, new antileishmanial compounds are urgently needed. Endoperoxides (EPs) are successfully used in malaria therapy, and experimental evidence of their potential against leishmaniasis exists. Anthracene endoperoxides (AcEPs) have so far been only technically used and not explored for their leishmanicidal potential. This study verified the in vitro efficiency and mechanism of AcEPs against both Leishmania promastigotes and axenic amastigotes (L. tarentolae and L. donovani) as well as their toxicity in J774 macrophages. Additionally, the kinetics and radical products of AcEPs' reaction with iron, the formation of radicals by AcEPs in Leishmania, as well as the resulting impairment of parasite mitochondrial functions were studied. Using electron paramagnetic resonance combined with spin trapping, photometry, and fluorescence-based oximetry, AcEPs were demonstrated to (i) show antileishmanial activity in vitro at IC50 values in a low micromolar range, (ii) exhibit host cell toxicity in J774 macrophages, (iii) react rapidly with iron (II) resulting in the formation of oxygen- and carbon-centered radicals, (iv) produce carbon-centered radicals which could secondarily trigger superoxide radical formation in Leishmania, and (v) impair mitochondrial functions in Leishmania during parasite killing. Overall, the data of different AcEPs demonstrate that their structures besides the peroxo bridge strongly influence their activity and mechanism of their antileishmanial action.}, language = {en} } @article{FudickarLinker2018, author = {Fudickar, Werner and Linker, Torsten}, title = {Release of Singlet Oxygen from Aromatic Endoperoxides by Chemical Triggers}, series = {Angewandte Chemie : a journal of the Gesellschaft Deutscher Chemiker ; International edition}, volume = {57}, journal = {Angewandte Chemie : a journal of the Gesellschaft Deutscher Chemiker ; International edition}, number = {39}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1433-7851}, doi = {10.1002/anie.201806881}, pages = {12971 -- 12975}, year = {2018}, abstract = {The generation of reactive singlet oxygen under mild conditions is of current interest in chemistry, biology, and medicine. We were able to release oxygen from dipyridylanthracene endoperoxides (EPOs) by using a simple chemical trigger at low temperature. Protonation and methylation of such EPOs strongly accelerated these reactions. Furthermore, the methyl pyridinium derivatives are water soluble and therefore serve as oxygen carriers in aqueous media. Methylation of the EPO of the ortho isomer affords the parent form directly without increasing the temperature under very mild conditions. This exceptional behavior is ascribed to the close contact between the nitrogen atom and the peroxo group. Singlet oxygen is released upon this reaction, and can be used to oxygenate an acceptor such as tetramethylethylene in the dark with no heating. Thus, a new chemical source of singlet oxygen has been found, which is triggered by a simple stimulus.}, language = {en} } @article{BauchFudickarLinker2021, author = {Bauch, Marcel and Fudickar, Werner and Linker, Torsten}, title = {Stereoselective [4+2] Cycloaddition of Singlet Oxygen to Naphthalenes Controlled by Carbohydrates}, series = {Molecules : a journal of synthetic chemistry and natural product chemistry}, volume = {16}, journal = {Molecules : a journal of synthetic chemistry and natural product chemistry}, number = {4}, publisher = {MDPI}, address = {Basel}, issn = {1420-3049}, doi = {10.3390/molecules26040804}, pages = {17}, year = {2021}, abstract = {Stereoselective reactions of singlet oxygen are of current interest. Since enantioselective photooxygenations have not been realized efficiently, auxiliary control is an attractive alternative. However, the obtained peroxides are often too labile for isolation or further transformations into enantiomerically pure products. Herein, we describe the oxidation of naphthalenes by singlet oxygen, where the face selectivity is controlled by carbohydrates for the first time. The synthesis of the precursors is easily achieved starting from naphthoquinone and a protected glucose derivative in only two steps. Photooxygenations proceed smoothly at low temperature, and we detected the corresponding endoperoxides as sole products by NMR. They are labile and can thermally react back to the parent naphthalenes and singlet oxygen. However, we could isolate and characterize two enantiomerically pure peroxides, which are sufficiently stable at room temperature. An interesting influence of substituents on the stereoselectivities of the photooxygenations has been found, ranging from 51:49 to up to 91:9 dr (diastereomeric ratio). We explain this by a hindered rotation of the carbohydrate substituents, substantiated by a combination of NOESY measurements and theoretical calculations. Finally, we could transfer the chiral information from a pure endoperoxide to an epoxide, which was isolated after cleavage of the sugar chiral auxiliary in enantiomerically pure form.}, language = {en} } @article{FudickarLinker2019, author = {Fudickar, Werner and Linker, Torsten}, title = {Theoretical insights into the effect of solvents on the [4+2] cycloaddition of singlet oxygen to substituted anthracenes}, series = {Journal of physical organic chemistry}, volume = {32}, journal = {Journal of physical organic chemistry}, number = {7}, publisher = {Wiley}, address = {Hoboken}, issn = {0894-3230}, doi = {10.1002/poc.3951}, pages = {9}, year = {2019}, abstract = {The [4 + 2] cycloadditions of singlet oxygen to 9,10-diphenylanthracene (1) and the meta and para isomers of 9,10-dipyridylanthracene (2m/p) and 9,10-methoxyphenylanthracene (3m/p) have been studied by density functional calculations in the gas phase at the UB3LYP/6-31G* level and for the first time in solvents at the conductor-like polarizable continuum model (CPCM) UM062X/6-31G* level. The differences in calculated transition state (TS) energies derived from this method are in line with experimentally observed reactivity orders in solution. For the gas-phase reaction, the first TS of the stepwise pathway (TS1) has biradical character, and its energy lies below the energy of the TS of the concerted path (TSconc). In contrast, in the solvent acetonitrile, TS1 resembles a zwitterion and lies significantly higher than the TSconc. Thus, a concerted mechanism applies in solvents, and the energy gap between the TS of the two processes decreases with decreasing polarity. A change from a pyridyl against a methoxyphenyl substituent in the para position causes a maximal reduction of the activation barrier by approximately 1.7 kcal/mol, resulting in a fivefold increased reactivity.}, language = {en} } @article{FudickarVomdranLinker2006, author = {Fudickar, Werner and Vomdran, Katja and Linker, Torsten}, title = {Auxiliary controlled singlet-oxygen ene reactions of cyclohexenes}, series = {Tetrahedron}, volume = {62}, journal = {Tetrahedron}, number = {46}, publisher = {Elsevier}, address = {Oxford}, issn = {0040-4020}, doi = {10.1016/j.tet.2006.07.104}, pages = {10639 -- 10646}, year = {2006}, abstract = {The photooxygenation of homochiral cyclohexene ketals, which are easily available from 2-cyclohexenone and L-tartrates, affords hydroperoxides and after reduction the corresponding allylic alcohols in good yields and high regioselectivities. This can be rationalized by electronic repulsions in a perepoxide intermediate and provides evidence for unfavorable 1,3 diaxial interactions with a dioxolane oxygen atom. Only low stereoselectivities were observed, due to the flexibility of the cyclohexene ring. However, the diastereomers could be separated and after cleavage of the auxiliary, 4-hydroxy-2-cyclohexen-1-one was isolated in enantiomerically pure form, which can serve as a building block for natural product synthesis.}, language = {en} } @article{BauchKlaperLinker2017, author = {Bauch, Marcel and Klaper, Matthias and Linker, Torsten}, title = {Intermediates in the cleavage of endoperoxides}, series = {Journal of physical organic chemistry}, volume = {30}, journal = {Journal of physical organic chemistry}, publisher = {Wiley}, address = {Hoboken}, issn = {0894-3230}, doi = {10.1002/poc.3607}, pages = {6}, year = {2017}, abstract = {The decomposition of anthracene endoperoxides has been investigated under various conditions. Thermolyses proceed via radical intermediates and afford anthracenes and rearrangement products, depending on the substitution pattern. Interestingly, not only the O-O but also the C-O bond can be cleaved homolytically. Under basic conditions fragmentations take place, affording anthraquinone, and reactive oxygen species. This mechanism explains the often observed decomposition of endoperoxides during work-up. Finally, an acid-catalyzed cleavage has been observed under release of hydrogen peroxide. The results should be interesting for the mechanistic understanding of peroxide decomposition and the endoperoxides might serve as mild sources of reactive oxygen species for future applications. Copyright (C) 2016 John Wiley \& Sons, Ltd.}, language = {en} } @article{FudickarPavasheLinker2017, author = {Fudickar, Werner and Pavashe, Prashant and Linker, Torsten}, title = {Thiocarbohydrates on Gold Nanoparticles: Strong Influence of Stereocenters on Binding Affinity and Interparticle Forces}, series = {Chemistry - a European journal}, volume = {23}, journal = {Chemistry - a European journal}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0947-6539}, doi = {10.1002/chem.201700846}, pages = {8685 -- 8693}, year = {2017}, abstract = {Carbohydrates carrying thiol groups at the C-2 position have been attached to gold nanoparticles (AuNPs) with stereocenters in close proximity to the surface for the first time. Their configurations can be clearly distinguished by the tendency of particle aggregation. AuNP surface plasmon resonance (SPR), X-ray photoelectron spectroscopy (XPS), and IR spectroscopy indicate that the thiocarbohydrates replace citrate molecules at different rates, causing aggregation and eventually precipitation. A quantitative formulation of this aggregation process shows that reactivities can vary by several magnitudes. Adsorption isotherms and kinetics also demonstrate that the number of thiocarbohydrates varies by a factor of two. Molecular mechanics force field (MMFF) calculations reveal their relative orientations. Based on these models, the different binding behavior can be ascribed to attractive van der Waals forces and hydrogen bonds. Such interactions occur either between the carbohydrate and AuNPs, by lateral intermolecular forces at the surface, or by interparticle attraction, in analogy to cell-surface carbohydrates of biological recognition systems. Aggregation of NPs therefore act as an indicator to differentiate between various carbohydrates with defined configurations.}, language = {en} } @article{KleinpeterWernerLinker2017, author = {Kleinpeter, Erich and Werner, Peter and Linker, Torsten}, title = {Synthesis and NMR spectroscopic conformational analysis of benzoic acid esters of mono- and 1,4-dihydroxycyclohexane, 4-hydroxycyclohexanone and the -ene analogue - The more polar the molecule the more stable the axial conformer}, series = {Tetrahedron}, volume = {73}, journal = {Tetrahedron}, publisher = {Elsevier}, address = {Oxford}, issn = {0040-4020}, doi = {10.1016/j.tet.2017.04.029}, pages = {3801 -- 3809}, year = {2017}, abstract = {para-Substituted benzoic acid esters of cyclohexanol, 1,4-dihydroxycyclohexane, 4-hydroxy-cyclohexanone and of the corresponding exo-methylene derivative were synthesized and the conformational equilibria of the cyclohexane skeleton studied by low temperature H-1 and C-13 NMR spectroscopy. The geometry optimized structures of the axial/equatorial chair conformers were computed at the DFT level of theory. Only one preferred conformation of the ester group was obtained for both the axial and the equatorial conformer, respectively. The content of the axial conformer increases with growing polarity of the 6-membered ring moiety; hereby, in addition, the effect of sp(2) hybridization/polarity of C(4)= O/C(4)= CH2 on the present conformational equilibria is critically evaluated. Another dynamic process could be studied, for the first time in this kind of compounds. (C) 2017 Elsevier Ltd. All rights reserved.}, language = {en} } @article{BauchKrtitschkaLinker2017, author = {Bauch, Marcel and Krtitschka, Angela and Linker, Torsten}, title = {Photooxygenation of oxygen-substituted naphthalenes}, series = {Journal of physical organic chemistry}, volume = {30}, journal = {Journal of physical organic chemistry}, publisher = {Wiley}, address = {Hoboken}, issn = {0894-3230}, doi = {10.1002/poc.3734}, pages = {6803 -- 6813}, year = {2017}, abstract = {The reaction of oxygen-substituted naphthalenes with singlet oxygen (O-1(2)) has been investigated, and labile endoperoxides have been isolated and characterized at -78 degrees C for the first time. Low-temperature kinetics by UV spectroscopy revealed that alkoxy and silyloxy substituents remarkably increase the rate of photooxygenations compared to 1,4-dimethylnaphthalene, whereas acyloxy-substituted acenes are inert towards O-1(2). The reactivities nicely correlate with HOMO energies and free activation energies, which we determined by density functional theory calculations. The lability of the isolated endoperoxides is due to their very fast back reaction to the corresponding naphthalenes even at -20 degrees C under release of O-1(2), making them to superior sources of this reactive species under very mild conditions. Finally, a carbohydrate-substituted naphthalene has been synthesized, which reacts reversibly with O-1(2) and might be applied for enantioselective oxidations in future work.}, language = {en} }