Effect of ring size on photoisomerization properties of stiff stilbene macrocycles

• Sandra Olsson,
• Óscar Benito Pérez,
• Magnus Blom and
• Adolf Gogoll

Beilstein J. Org. Chem. 2019, 15, 2408–2418, doi:10.3762/bjoc.15.233

• a six carbon chain) showed some degree of isomerization when irradiated. DFT calculations of the energy differences between the E- and Z-isomers show the same trend as the experimental results. Interestingly the DFT study highlights that the energy difference between the E- and Z-isomers of even the

• the resulting macrocycle. Keywords: DFT; molecular mechanics; photostability; photo-switch; ring-strain; stiff stilbene; Introduction The stiff stilbene (SS) molecule has drawn a lot of interest due to its photodynamic properties [1]. Stiff stilbenes typically undergo light triggered isomerization

• ). Photoisomerization Photoisomerizing the (Z)-1a–d to the (E)-1a–d isomers requires to stretch the linker. The isomerization was achieved by irradiation of a degassed solution of (Z)-1a–d in chloroform or deuterated chloroform using either a 280 or 300 nm filter (Scheme 3). The conversion was followed by UV–vis or 1H

Reversible switching of arylazopyrazole within a metal–organic cage

• Anton I. Hanopolskyi,
• Soumen De,
• Michał J. Białek,
• Yael Diskin-Posner,
• Liat Avram,
• Moran Feller and
• Rafal Klajn

Beilstein J. Org. Chem. 2019, 15, 2398–2407, doi:10.3762/bjoc.15.232

• switching to the Z isomer was accompanied by the release of one of the two guests from the cage and the formation of a 1:1 cage/Z-arylazopyrazole inclusion complex. DFT calculations suggest that this process involves a dramatic change in the conformation of the cage. Back-isomerization was induced with

• green light and resulted in the initial 1:2 cage/E-arylazopyrazole complex. This back-isomerization reaction also proceeded in the dark, with a rate significantly higher than in the absence of the cage. Keywords: arylazopyrazoles; coordination cages; inclusion complexes; molecular switches

• affected by their confinement to the surfaces of inorganic nanoparticles. Among other examples, the isomerization kinetics of azobenzene could be tuned by a factor of >5000 by the molecules with which it was co-adsorbed on gold nanoparticles [23], and donor–acceptor Stenhouse adduct (DASA) switches were

Synthesis of acremines A, B and F and studies on the bisacremines

• Nils Winter and
• Dirk Trauner

Beilstein J. Org. Chem. 2019, 15, 2271–2276, doi:10.3762/bjoc.15.219

• should then close the tetrahydrofuran ring of the natural product. Upon irradiation of 5 using a Hg lamp, however, the only productive pathway which could be observed was isomerization of the disubstituted double bond (Table 1, entries 17 and 18). Again, we attempted to promote the reaction by tethering

Recent advances in transition-metal-catalyzed incorporation of fluorine-containing groups

• Xiaowei Li,
• Xiaolin Shi,
• Xiangqian Li and
• Dayong Shi

Beilstein J. Org. Chem. 2019, 15, 2213–2270, doi:10.3762/bjoc.15.218

• formation of fluorinated products with an overall retention of the stereochemical configuration suggests a mechanism wherein a palladium-π-allyl intermediate undergoes a rapid π-σ-π isomerization. In 2013, the first example of an allylic C–H fluorination reaction of simple alkenes with Et3N·3HF as a

Harnessing enzyme plasticity for the synthesis of oxygenated sesquiterpenoids

• Melodi Demiray,
• David J. Miller and
• Rudolf K. Allemann

Beilstein J. Org. Chem. 2019, 15, 2184–2190, doi:10.3762/bjoc.15.215

• group could react at C10 to induce a fast 1,11-cyclisation, forming cation 25, which effectively competes with the isomerization of 11 to 8-methoxy-NDP. A subsequent [1,3]-hydride shift leads to 24 (Scheme 3A). Direct deprotonation of 22 at C15 forms the minor reaction product (E)-8-methoxy-β-farnesene

Azologization and repurposing of a hetero-stilbene-based kinase inhibitor: towards the design of photoswitchable sirtuin inhibitors

• Christoph W. Grathwol,
• Nathalie Wössner,
• Sören Swyter,
• Adam C. Smith,
• Enrico Tapavicza,
• Robert K. Hofstetter,
• Anja Bodtke,
• Manfred Jung and
• Andreas Link

Beilstein J. Org. Chem. 2019, 15, 2170–2183, doi:10.3762/bjoc.15.214

• physicochemical properties upon irradiation with light, represent one major approach to this. One of the most common light-driven transformations exploited in molecular photoswitches is the E–Z isomerization of double bonds [18]. In this context, the photochemistry of stilbenes and the closely related azobenzenes

• provided deeper insights and clarified the differential behaviour observed in the UV–vis spectra of 2b and 2f after UV irradiation. As anticipated, UV irradiation lead to E→Z isomerization of the C=C double bond in both compounds. The (Z)-isomers were found to be slightly more polar than the respective (E

• reversibly between two states comprising high amounts of (E)-11 and (Z)-11, respectively. The other part of the hypothesis was, that by this photoinduced isomerization a considerable drop of activity would occur due to the conformational change and the resulting changed geometry and polarity. However, this

Characterization of two new degradation products of atorvastatin calcium formed upon treatment with strong acids

• Jürgen Krauß,
• Monika Klimt,
• Markus Luber,
• Peter Mayer and
• Franz Bracher

Beilstein J. Org. Chem. 2019, 15, 2085–2091, doi:10.3762/bjoc.15.206

• dehydration of the δ-hydroxy group and some epimers resulting from acid-catalyzed isomerization reactions. In contrast, Vukkum et al. [13] describe, besides lactones 2 and 3, an α,β-unsaturated carboxylic acid 4. Treatment under more drastic conditions (6 M HCl, reflux, 3 h) was reported to result mainly in

A review of the total syntheses of triptolide

• Xiang Zhang,
• Zaozao Xiao and
• Hongtao Xu

Beilstein J. Org. Chem. 2019, 15, 1984–1995, doi:10.3762/bjoc.15.194

• isomer 43. Treatment of 43 with methoxide ions in methanol at room temperature for 15 min gave the desired C-5 trans-butenolide 8 (40%) along with its C-5 cis-epimer 44 (60%), which is the sole product from the base-catalyzed isomerization of 43. Epoxidation of 43 gave a C-4,5-epoxide intermediate, which

• isomerization of olefin 43, the benzylic oxidation of 8, the use of m-CPBA to introduce the C-9,11 epoxide and the non-stereoselective reduction of the C-14 carbonyl group using sodium borohydride, caused an unacceptable overall yield (1.6%). This pioneering work undoubtedly established the basis for the future

• under the optimal photoredox conditions to provide tetracyclic intermediate 19i (dr = 1:1, Scheme 8, route L), with a cis A-/B-ring connection rather than the desired trans connection. Treatment of 19i with H2SO4, followed by RuCl2(PPh3)3-catalyzed double bond isomerization gave the known intermediate

Norbornadiene-functionalized triazatriangulenium and trioxatriangulenium platforms

• Roland Löw,
• Talina Rusch,
• Tobias Moje,
• Fynn Röhricht,
• Olaf M. Magnussen and
• Rainer Herges

Beilstein J. Org. Chem. 2019, 15, 1815–1821, doi:10.3762/bjoc.15.175

• , while the functional groups are protruding upright and freestanding from the central carbon atoms. Azobenzene derivatized TATA’s are known to exhibit extremely fast cis→trans isomerization on metal surfaces, via a peculiar non-adiabatic singlet→triplet→singlet mechanism. We now prepared norbornadienes

• (NBD) and quadricyclanes (QC) attached to TATA and TOTA platforms which can be used to check if these accelerated rates and the spin change mechanism also apply to [2 + 2] cycloreversions (QC→NBD). Keywords: [2 + 2] cycloaddition; [2 + 2] cycloreversion; norbornadiene; photochemical isomerization

• ; quadricyclane; self-assembled monolayers; TATA platform; thermal isomerization; TOTA platform; Introduction Recently, we discovered that the thermochemically forbidden cis–trans isomerization of azobenzenes can be efficiently catalysed by a very peculiar mechanism on bulk gold [1]. In heterogeneous catalysis

Recent advances on the transition-metal-catalyzed synthesis of imidazopyridines: an updated coverage

• Gagandeep Kour Reen,
• Ashok Kumar and
• Pratibha Sharma

Beilstein J. Org. Chem. 2019, 15, 1612–1704, doi:10.3762/bjoc.15.165

• abstraction from the sp3 carbon atom leading to the formation of six-membered Cu(III) species 42. Furthermore, consecutive isomerization/oxidation/reductive elimination leads to the generation of final compound 37 with regeneration of the Cu(I) catalyst (Scheme 14). The presence of EDGs as compared to EWGs on

• activation of the C–H bond in the alkyne by copper on ZnAl2O4 initiated an intramolecular nucleophilic attack of pyridine nitrogen to the triple bond (135). This was followed by aromatic isomerization to form imidazo[1,2-a]pyridines (Scheme 46). The importance of Cu for this reaction is mentioned in Table 4

Synthesis of ([1,2,4]triazolo[4,3-a]pyridin-3-ylmethyl)phosphonates and their benzo derivatives via 5-exo-dig cyclization

• Aleksandr S. Krylov,
• Artem A. Petrosian,
• Julia L. Piterskaya,
• Nataly I. Svintsitskaya and
• Albina V. Dogadina

Beilstein J. Org. Chem. 2019, 15, 1563–1568, doi:10.3762/bjoc.15.159

• nucleophilic substitution of chlorine in the chloroethynylphosphonate to form ynamine intermediate A, isomerization of which provides ketenimine B. Further formation of the imine tautomer C enables an intramolecular 5-exo-dig cyclization to furnish the title [1,2,4]triazolo[4,3-a]pyridines (Scheme 6

An azobenzene container showing a definite folding – synthesis and structural investigation

• Abdulselam Adam,
• Saber Mehrparvar and
• Gebhard Haberhauer

Beilstein J. Org. Chem. 2019, 15, 1534–1544, doi:10.3762/bjoc.15.156

• , whereas by the use of visible light the stretched trans,trans-isomer is formed. By means of quantum chemical calculations and CD spectroscopy we could show that the trans→cis isomerization is spatially directed; that means that one of the two different macrocycles performs a definite clockwise rotation to

• the other, caused by irradiation with UV light. For the cis→trans isomerization counterclockwise rotations are found. Furthermore, quantum chemical calculations reveal that the energy of the cis,cis-isomer is only slightly higher than the energy of the cis,trans-isomer. This effect can be explained by

• , the trans→cis isomerization is triggered by UV light whereas the cis→trans back relaxation takes place by visible light or heat [30][42]. Due to the high reversibility, the simple synthesis and the high photostability azobenzene derivatives are the most commonly used switching units. A further

Diazocine-functionalized TATA platforms

• Roland Löw,
• Talina Rusch,
• Fynn Röhricht,
• Olaf Magnussen and
• Rainer Herges

Beilstein J. Org. Chem. 2019, 15, 1485–1490, doi:10.3762/bjoc.15.150

• Abstract Recently, it has been shown that the thermochemical cis→trans isomerization of azobenzenes is accelerated by a factor of more than 1000 by electronic coupling to a gold surface via a conjugated system with 11 bonds and a distance of 14 Å. The corresponding molecular architecture consists of a

• to the trans-form, which slowly returns back to the stable cis-isomer. To investigate the thermal trans→cis isomerization as a function of the conjugation to the metal surface, we connected the acetylene spacer in meta (weak conjugation) and in para (strong conjugation) position. Both isomers form

• ordered monolayers on Au(111) surfaces. Keywords: cis–trans isomerization; diazocine; molecular switch; photochrome; self-assembled monolayers; TATA platform; Introduction Catalysts increase chemical reaction rates by lowering the activation energies and thus create more favorable reaction pathways [1

Reversible end-to-end assembly of selectively functionalized gold nanorods by light-responsive arylazopyrazole–cyclodextrin interaction

• Maximilian Niehues,
• Patricia Tegeder and
• Bart Jan Ravoo

Beilstein J. Org. Chem. 2019, 15, 1407–1415, doi:10.3762/bjoc.15.140

• -functionalized with the CD, no assembly could be observed after addition of the divalent azobenzene linker. Moreover, the assemblies could only once be disassembled by the combination of UV irradiation and physical forces by sonication. The light-induced back-isomerization of azobenzenes did not form similar end

• photophysical properties like nearly quantitative isomerization, straight forward synthesis in excellent yields and very long Z-isomer half-life times up to 1000 days due to less steric repulsion [37]. In previous reports we could show that the AAP guest inclusion properties to β-CD are comparable to

Precious metal-free molecular machines for solar thermal energy storage

• Meglena I. Kandinska,
• Snejana M. Kitova,
• Vladimira S. Videva,
• Stanimir S. Stoyanov,
• Stanislava B. Yordanova,
• Stanislav B. Baluschev,
• Silvia E. Angelova and
• Aleksey A. Vasilev

Beilstein J. Org. Chem. 2019, 15, 1096–1106, doi:10.3762/bjoc.15.106

• constants of metal complexes were determined and are in good agreement with the literature data for reference dyes. The temporal evolution of trans-to-cis isomerization was observed in a real-time regime. The dyes demonstrated a low intrinsic fluorescence of their Ba2+ complexes and high yield of E/Z

• depends strongly on the Ba2+ concentration. The cis isomers formed upon irradiation are thermally unstable and revert to the trans isomers in the dark [23]. The rates of trans-to-cis isomerization were determined for several Ba2+ concentrations in the range of 2 × 10−3 M up to 1 M at a fixed dye

• ), as for the free dye 4b, increased with the irradiation time, suggesting that isomerization from trans to a cis form of the free dye or its Ba2+ complex proceeded until a photostationary state was reached. As can be seen from Figure 4 the trans-to-cis isomerization takes place to a higher extent in

Multicomponent reactions (MCRs): a useful access to the synthesis of benzo-fused γ-lactams

• Edorta Martínez de Marigorta,
• Jesús M. de Los Santos,
• Ana M. Ochoa de Retana,
• Javier Vicario and
• Francisco Palacios

Beilstein J. Org. Chem. 2019, 15, 1065–1085, doi:10.3762/bjoc.15.104

• Heck cyclization reaction between the substituted alkyne and aryl bromide in 133 takes place to form a cyclic palladium intermediate 134 with E-configuration, resulting from a syn-addition mechanism of this step. The addition of a silver salt reduces the probability of isomerization of the double bond

Easy, efficient and versatile one-pot synthesis of Janus-type-substituted fullerenols

• Marius Kunkel and
• Sebastian Polarz

Beilstein J. Org. Chem. 2019, 15, 901–905, doi:10.3762/bjoc.15.87

• from the isomerization and fragmentation of the oxygen species in addition to the fragmentation of the substituents. The signals can be assigned with a general formula [M − xH2O − yH − zO − v(HNR)]a−/+. The molecular ion peak of the janus-fullerenol with 1 as substituent can be identified at m/z 1798.6

Coordination chemistry and photoswitching of dinuclear macrocyclic cadmium-, nickel-, and zinc complexes containing azobenzene carboxylato co-ligands

• Jennifer Klose,
• Tobias Severin,
• Peter Hahn,
• Alexander Jeremies,
• Jens Bergmann,
• Daniel Fuhrmann,
• Jan Griebel,
• Bernd Abel and
• Berthold Kersting

Beilstein J. Org. Chem. 2019, 15, 840–851, doi:10.3762/bjoc.15.81

• the meta-stable cis-form relaxes back thermally to the more stable trans from (Figures S50 and S51 in Supporting Information File 1). Thus, the cis–trans isomerization appears to be not constrained by the [Cd2L]2+ fragment, as revealed by the similar half-lives (τ1/2 cis–trans free deprotonated

• + entity, and trans-configured azo groups. NMR and UV–vis studies clearly show that the complexes retain their mixed-ligand nature in the solution state. Irradiation of the Cd complex 1 with a LED UV lamp leads to a trans-to-cis isomerization which appears to proceed within the coordination sphere of the

• irradiated with a UV LED lamp (365 nm) for 1 to 30 s after irradiation. Concentration of solutions: 10−5 M. Synthesis of complexes 1–9. Cis/trans isomerization process of the bound azo-carboxylato co-ligand in [Cd2L(μ-azo-H)]ClO4 (1) in acetonitrile mediated by irradiation with 365 nm UV light. The amino

A diastereoselective approach to axially chiral biaryls via electrochemically enabled cyclization cascade

• Hong Yan,
• Zhong-Yi Mao,
• Zhong-Wei Hou,
• Jinshuai Song and
• Hai-Chao Xu

Beilstein J. Org. Chem. 2019, 15, 795–800, doi:10.3762/bjoc.15.76

• groups including aryltrimethylsilane (3p), silyl ether (3q–s), ester (3t) and cylic ketal (3u). Note that all the diastereomers were separable by flash column chromatography. Heating a solution of the major isomer of 3n in MeCN at 80 °C for 4 h did not lead to isomerization, suggesting that the

• in MeCN at 80 °C for 4 h resulted in a mixture of diastereomers in a ratio of 3:1. These results suggest that the sterically demanding substituents at R1 and R2 positions (cf. Table 1) are critical to ensure good stereoselectivity during the product formation and to prevent isomerization after the

Azologization of serotonin 5-HT₃ receptor antagonists

• Karin Rustler,
• Galyna Maleeva,
• Piotr Bregestovski and
• Burkhard König

Beilstein J. Org. Chem. 2019, 15, 780–788, doi:10.3762/bjoc.15.74

• photo-induced trans–cis isomerization representing the time of birth of the azobenzene photoswitch. Benefiting of their accessible synthesis, large change in polarity and geometry upon switching, excellent photochromic properties and tuneability, azobenzenes are amongst the most widely used photochromic

• such an extent, that no new maximum representing the cis-isomer was observed and thus cis–trans isomerization only occurs thermally and is not triggerable by irradiation with visible light. Back-isomerization was triggered by irradiation with visible light (5a, 5b, 16a, 16b, 23, and 28) of the

• indicated wavelength or by thermal relaxation (5a, 5b, 12a, 16a–d, 23, and 28). The irradiation times were determined by following the UV–vis spectrum upon isomerization until no more changes in absorption were observed and the photostationary state (PSS) was reached. The points of intersection in the

Synthesis of polydicyclopentadiene using the Cp₂TiCl₂/Et₂AlCl catalytic system and thin-layer oxidation of the polymer in air

• Zhargolma B. Bazarova,
• Ludmila S. Soroka,
• Alex A. Lyapkov,
• Мekhman S. Yusubov and
• Francis Verpoort

Beilstein J. Org. Chem. 2019, 15, 733–745, doi:10.3762/bjoc.15.69

• group vibration bands in the infrared spectra. At the same time, structuring and isomerization occur in layers generating changes in their physical properties, in particular, the decrease of layer permeability for atmospheric air. In its turn, this leads to the transition of the oxidation from a kinetic

Synthesis of functionalized diazocines for application as building blocks in photo- and mechanoresponsive materials

• Widukind Moormann,
• Daniel Langbehn and
• Rainer Herges

Beilstein J. Org. Chem. 2019, 15, 727–732, doi:10.3762/bjoc.15.68

• were investigated by NMR and UV–vis spectroscopy and the results are listed in Table 1. Photostationary states (PSS) as well as half-lives (t1/2) were determined in acetonitrile at 300 K and 298.15 K, respectively. The (cis → trans) and (trans → cis) isomerization were achieved by irradiation into the

• appropriate n–π* bands at 385 and 530 nm. As a result of electronic decoupling the absorption bands are well separated and the photostationary states of diazocines 4–7 are considerably improved compared to 3,3’- (2) and 4,4’-diaminodiazocine 3 (Figure 3). The (cis → trans) isomerization of diazocines 4–7 was

• achieved after 2 min of irradiation at 385 nm in yields of 74–85%. All trans-diazocines were converted quantitatively to the cis-configuration either by thermal relaxation or by irradiation at 530 nm. In general, the (trans → cis) isomerization can be accomplished with wavelengths between 520 and 620 nm

The LANCA three-component reaction to highly substituted β-ketoenamides – versatile intermediates for the synthesis of functionalized pyridine, pyrimidine, oxazole and quinoxaline derivatives

• Tilman Lechel,
• Roopender Kumar,
• Mrinal K. Bera,
• Reinhold Zimmer and
• Hans-Ulrich Reissig

Beilstein J. Org. Chem. 2019, 15, 655–678, doi:10.3762/bjoc.15.61

• D thus accomplishing the final connectivity of the three components. Enol/carbonyl tautomerization gives the isolated β-ketoenamide KE with E-configuration being the result of the intramolecular acyl transfer. Even after storage of β-ketoenamides there is no evidence that an isomerization to the

Synthesis and selected transformations of 2-unsubstituted 1-(adamantyloxy)imidazole 3-oxides: straightforward access to non-symmetric 1,3-dialkoxyimidazolium salts

• Grzegorz Mlostoń,
• Małgorzata Celeda,
• Katarzyna Urbaniak,
• Marcin Jasiński,
• Vladyslav Bakhonsky,
• Peter R. Schreiner and
• Heinz Heimgartner

Beilstein J. Org. Chem. 2019, 15, 497–505, doi:10.3762/bjoc.15.43

• -aromatization of the imidazole ring. The same mechanism governs sulfur-transfer reactions with cycloaliphatic thioketones yielding the corresponding imidazole-2-thiones [13]. The isomerization of N-oxides 1 into the corresponding imidazol-2-ones can be easily performed by treatment with acetic anhydride at room

• 8b–d in 66%, 62%, and 39% yield, respectively. As pointed out in the introduction, one of the most typical reactions of 2-unsubstituted imidazole N-oxides is their isomerization to imidazol-2-ones. In many reported cases, this transformation can be performed by treatment with acetic anhydride

• ppm belongs to HN(3). The 13C NMR spectrum confirmed the structure of 9 by the signal of the C(2)=O group found at 153.0 ppm. The corresponding band in the IR spectrum appeared at 1702 cm−1as the strongest absorption. Thermal isomerization of 7b was also tested. However, boiling in bromobenzene for 15

Aqueous olefin metathesis: recent developments and applications

• Valerio Sabatino and
• Thomas R. Ward

Beilstein J. Org. Chem. 2019, 15, 445–468, doi:10.3762/bjoc.15.39

• multiple tedious steps, primarily because metal complexes’ impurities in the final product may cause isomerization or decomposition of the products and may be toxic. The latter is a very critical issue for the pharmaceutical industry, as the amount of ruthenium in APIs (active pharmaceutical ingredients

• -metathesis of allyl alcohol (59) and the cis–trans isomerization of cis-butenedienol (Z-58). Metathesis catalysts bearing quaternary ammonium groups provide an attractive alternative to classical ruthenium catalysts. Although they do not represent a great improvement in terms of catalytic activity, they

• diallylmalonate 31 in DCM (Scheme 11). Upon reaction completion, the catalyst is extracted from the organic reaction mixture with D2O and (re)-used for the isomerization of cis-butenediol Z-58 in water. Recently the removal of a water-soluble catalyst from reaction mixtures was also achieved with catalyst 12