TY - JOUR A1 - Fudickar, Werner A1 - Linker, Torsten T1 - Synthesis of Pyridylanthracenes and Their Reversible Reaction with Singlet Oxygen to Endoperoxides JF - The journal of organic chemistry N2 - The ortho, meta, and para isomers of 9,10-dipyridylanthracene 1 have been synthesized and converted into their endoperoxides 1-O-2 upon oxidation with singlet oxygen. The kinetics of this reaction can be controlled by the substitution pattern and the solvent: in highly polar solvents, the meta isomer is the most reactive, whereas the ortho isomer is oxidized fastest in nonpolar solvents. Heating of the endoperoxides affords the parent anthracenes by release of singlet oxygen. Y1 - 2017 U6 - https://doi.org/10.1021/acs.joc.7b01765 SN - 0022-3263 VL - 82 SP - 9258 EP - 9262 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Heydenreich, Matthias A1 - Koch, Andreas A1 - Krtitschka, Angela A1 - Krüger, Tobias A1 - Linker, Torsten T1 - NMR spectroscopic conformational analysis of 4-methylene-cyclohexyl pivalateThe effect of sp(2) hybridization JF - Magnetic resonance in chemistry N2 - The conformational equilibrium of the axial/equatorial conformers of 4-methylene-cyclohexyl pivalate is studied by dynamic NMR spectroscopy in a methylene chloride/freon mixture. At 153K, the ring interconversion gets slow on the nuclear magnetic resonance timescale, the conformational equilibrium (-G degrees) can be examined, and the barrier to ring interconversion (G(#)) can be determined. The structural influence of sp(2) hybridization on both G degrees and G(#) of the cyclohexyl moiety can be quantified. KW - 4-methylene-cyclohexyl pivalate KW - conformational analysis KW - dynamic NMR spectroscopy KW - exo-methylene conformational effect at cyclohexane KW - quantum chemical calculations Y1 - 2017 U6 - https://doi.org/10.1002/mrc.4630 SN - 0749-1581 SN - 1097-458X VL - 55 SP - 1073 EP - 1078 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Fudickar, Werner A1 - Linker, Torsten T1 - Imaging by sensitized oxygenations of photochromic anthracene films BT - Examination of effects that improve performance and reversibility JF - Chemistry - a European journal N2 - The aliphatic anthracene compound 1 and the oligomeric anthracene 2 were synthesized. Thin films of 1 and 2 mixed with the sensitizers tetraphenylporphyrin (TPP) and methylene blue (MB) were irradiated with visible light in air. Upon formation of singlet oxygen, the anthracene units were converted quantitatively to the corresponding endoperoxides. Heating of the irradiated samples afforded the parent anthracenes with high yields. Here, we demonstrate that the kinetics and reversibility of this reaction strongly depend on the microenvironment of the anthracene groups in the two compounds. The photooxidation of thin films of I is accompanied by interesting changes in the morphology of the film and allows the first application of 1 as a nondestructive negative-tone photo-resist for lithography and as an oxidizing ink. The morphology of 2 remained unchanged after photooxidation as a result of the stabilizing oligomer backbone. This stabilizing effect significantly improves the photochromic performance of 2. The reversibility of the photooxidation is very high (> 90%) for oligomeric films of 2 after several cycles of irradiation and beating. Decomposition of the anthracene and a loss of the activity of the sensitizer diminish slightly the performance of the monomeric species. KW - anthracenes KW - lithography KW - photochromism KW - singlet oxygen KW - thin films Y1 - 2006 U6 - https://doi.org/10.1002/chem.200600387 SN - 0947-6539 VL - 12 SP - 9276 EP - 9283 PB - WILEY‐VCH CY - Weinheim ER - TY - JOUR A1 - Linker, Torsten T1 - Addition of Heteroatom Radicals to endo-Glycals JF - Chemistry N2 - Radical reactions have found many applications in carbohydrate chemistry, especially in the construction of carbon–carbon bonds. The formation of carbon–heteroatom bonds has been less intensively studied. This mini-review will summarize the efforts to add heteroatom radicals to unsaturated carbohydrates like endo-glycals. Starting from early examples, developed more than 50 years ago, the importance of such reactions for carbohydrate chemistry and recent applications will be discussed. After a short introduction, the mini-review is divided in sub-chapters according to the heteroatoms halogen, nitrogen, phosphorus, and sulfur. The mechanisms of radical generation by chemical or photochemical processes and the subsequent reactions of the radicals at the 1-position will be discussed. This mini-review cannot cover all aspects of heteroatom-centered radicals in carbohydrate chemistry, but should provide an overview of the various strategies and future perspectives KW - radicals KW - carbohydrates KW - heteroatoms KW - synthesis Y1 - 2020 U6 - https://doi.org/10.3390/chemistry2010008 SN - 2624-8549 VL - 2 IS - 1 SP - 80 EP - 92 PB - MDPI CY - Basel ER - TY - JOUR A1 - Klaper, Matthias A1 - Wessig, Pablo A1 - Linker, Torsten T1 - Base catalysed decomposition of anthracene endoperoxide JF - Chemical communications N2 - Catalytic amounts of a weak base are sufficient to induce the decomposition of anthracene endoperoxides to anthraquinone. The mechanism has been elucidated by isolation of intermediates in combination with DFT calculations. The whole process is suitable for the convenient generation of hydrogen peroxide under very mild conditions. Y1 - 2016 U6 - https://doi.org/10.1039/c5cc08606j SN - 1359-7345 SN - 1364-548X VL - 52 SP - 1210 EP - 1213 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Bauch, Marcel A1 - Böttcher, Dominique A1 - Bornscheuer, Uwe T. A1 - Linker, Torsten T1 - Enzymatic Cleavage of Aryl Acetates JF - ChemCatChem : heterogeneous & homogeneous & bio- & nano-catalysis ; a journal of ChemPubSoc Europe N2 - Seven enzymes have been screened for the cleavage of aryl acetates. Phenyl and naphthyl acetates react with lipases and esterases, whereas the sterically demanding anthracene acetate gave a conversion only with porcine liver esterase and esterase 2 from Bacillus subtilis (BS2). These two enzymes have been employed on a preparative (0.5 mmol) scale and afforded cleavage products in 91 and 94% yields, even for anthracene acetate. Thus, this method is superior to chemical cleavage with catalytic amounts of sodium methoxide (Zemplen conditions), which gave only low conversions. Finally, regioselectivity has been achieved with an anthracene bisacetate, in which an ethyl group controls the cleavage of the first acetate. This indicates that steric interactions play a crucial role in the enzymatic cleavage of aryl acetates, which might be interesting for future applications or the development of enzyme inhibitors. KW - arenes KW - enzyme catalysis KW - regioselectivity KW - steric hindrance KW - substituent effects Y1 - 2016 U6 - https://doi.org/10.1002/cctc.201600678 SN - 1867-3880 SN - 1867-3899 VL - 8 SP - 2853 EP - 2857 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Pavashe, Prashant A1 - Elamparuthi, Elangovan A1 - Hettrich, Cornelia A1 - Moeller, Heiko M. A1 - Linker, Torsten T1 - Synthesis of 2-Thiocarbohydrates and Their Binding to Concanavalin A JF - The journal of organic chemistry N2 - A convenient and general synthesis of 2-thiocarbohydrates via cerium ammonium nitrate oxidation of the thiocyanate ion is described. Radical addition to glycals proceeds with excellent regio- and good stereoselectivities in only one step, deprotection affords water-soluble 2-thio saccharides. Binding studies to Con A have been performed by isothermal titration calorimetry (ITC) and saturation transfer difference (STD) NMR spectroscopy. The 2-thiomannose derivative binds even stronger to Con A than the natural substrate, offering opportunities for new lectin or enzyme inhibitors. Y1 - 2016 U6 - https://doi.org/10.1021/acs.joc.6b00987 SN - 0022-3263 VL - 81 SP - 8595 EP - 8603 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Schilde, Uwe A1 - Kelling, Alexandra A1 - Umbreen, Sumaira A1 - Linker, Torsten T1 - Crystal structures of three bicyclic carbohydrate derivatives JF - Acta crystallographica, Section E, Crystallographic communications N2 - The title compounds, [(1R,3R,4R,5R,6S)-4,5-bis(acetyloxy)-7-oxo-2-oxabicyclo-[4.2.0]octan-3-yl]methyl acetate, C14H18O8, (I), [(1S,4R,5S,6R)-5-acetyloxy-7-hydroxyimino-2-oxobicyclo[4.2.0] octan-4-yl acetate, C11H15NO6, (II), and [(3aR, 5R, 6R, 7R, 7aS)-6,7-bis(acetyloxy)-2-oxooctahydropyrano[3,2-b]pyrrol-5-yl] methyl acetate, C14H19NO8, (III), are stable bicyclic carbohydrate derivatives. They can easily be synthesized in a few steps from commercially available glycals. As a result of the ring strain from the four-membered rings in (I) and (II), the conformations of the carbohydrates deviate strongly from the ideal chair form. Compound (II) occurs in the boat form. In the five-membered lactam (III), on the other hand, the carbohydrate adopts an almost ideal chair conformation. As a result of the distortion of the sugar rings, the configurations of the three bicyclic carbohydrate derivatives could not be determined from their NMR coupling constants. From our three crystal structure determinations, we were able to establish for the first time the absolute configurations of all new stereocenters of the carbohydrate rings. KW - crystal structure KW - carbohydrate derivatives KW - conformation KW - configuration Y1 - 2016 U6 - https://doi.org/10.1107/S2056989016018727 SN - 2056-9890 VL - 72 SP - 1839 EP - + PB - International Union of Crystallography CY - Chester ER - TY - JOUR A1 - Krueger, Tobias A1 - Kelling, Alexandra A1 - Linker, Torsten A1 - Schilde, Uwe T1 - Crystal structures of three cyclohexane‑based γ‑spirolactams BT - determination of configurations and conformations JF - BMC Chemistry N2 - The title compounds, 2-azaspiro[4.5]deca-1-one, C₉H₁₅NO, (1a), cis-8-methyl-2-azaspiro[4.5]deca-1-one, C₁₀H₁₇NO, (1b), and trans-8-methyl-2-azaspiro[4.5]deca-1-one, C₁₀H₁₇NO, (1c), were synthesized from benzoic acids 2 in only 3 steps in high yields. Crystallization from n-hexane afforded single crystals, suitable for X-ray diffraction. Thus, the configurations, conformations, and interesting crystal packing effects have been determined unequivocally. The bicyclic skeleton consists of a lactam ring, attached by a spiro junction to a cyclohexane ring. The lactam ring adopts an envelope conformation and the cyclohexane ring has a chair conformation. The main difference between compound 1b and compound 1c is the position of the carbonyl group on the 2-pyrrolidine ring with respect to the methyl group on the 8-position of the cyclohexane ring, which is cis (1b) or trans (1c). A remarkable feature of all three compounds is the existence of a mirror plane within the molecule. Given that all compounds crystallize in centrosymmetric space groups, the packing always contains interesting enantiomer-like pairs. Finally, the structures are stabilized by intermolecular N–H···O hydrogen bonds. KW - 2-Azaspiro[4.5]deca-1-ones KW - Cis- and trans-form KW - Configuration KW - Conformation KW - Lactams Y1 - 2019 U6 - https://doi.org/10.1186/s13065-019-0586-7 SN - 2661-801X VL - 13 IS - 69 PB - Springer International Publishing CY - Basel ER - TY - JOUR A1 - Umbreen, Sumaira A1 - Linker, Torsten T1 - Simple Synthesis of Conformationally Fixed Glycosamine Analogues by Beckmann Rearrangement at the Carbohydrate Ring JF - Chemistry - a European journal N2 - Conformationally fixed carbohydrate analogues are promising small-molecule inhibitors for hydrolases like O-GlcNAcase (OGA); however, their synthesis usually requires many steps. Herein we describe cycloadditions of dichloroketene to various glycals and subsequent Beckmann rearrangements, which offer an easy and stereoselective entry to glycosamine derivatives in good yields. The reactions are applicable for hexoses, pentoses, and disaccharides, and transformations to the corresponding imidates proceed smoothly. First biological tests reveal that such imidates indeed inhibit human OGA. KW - carbohydrates KW - cycloaddition KW - enzyme inhibitors KW - rearrangement KW - selective syntheses Y1 - 2015 U6 - https://doi.org/10.1002/chem.201406546 SN - 0947-6539 SN - 1521-3765 VL - 21 IS - 20 SP - 7340 EP - 7344 PB - Wiley-VCH CY - Weinheim ER -