3-Acetoxy-fatty acid isoprenyl esters from androconia of the ithomiine butterfly Ithomia salapia

• Florian Mann,
• Daiane Szczerbowski,
• Lisa de Silva,
• Melanie McClure,
• Marianne Elias and
• Stefan Schulz

Beilstein J. Org. Chem. 2020, 16, 2776–2787, doi:10.3762/bjoc.16.228

• explained by the different stabilization of the respective ions (Figure 3). The abundance of m/z 68 is higher in isoprenyl esters due to the more stable allyl radical cation (Figure 3A). In contrast, prenyl ester fragmentation produces a stabilized allyl cation m/z 69 (Figure 3B), while isoprenyl esters

Synthesis and characterization of S,N-heterotetracenes

• Astrid Vogt,
• Florian Henne,
• Christoph Wetzel,
• Elena Mena-Osteritz and
• Peter Bäuerle

Beilstein J. Org. Chem. 2020, 16, 2636–2644, doi:10.3762/bjoc.16.214

• [22][27], organic field-effect transistors [28], or sensors [23]. The interesting structure–property relationships of the S,N-heteroacene series inspired as well theoretical work. In this respect, De Simone et al. computed electronic spectra of the neutral, charged radical cationic, and dicationic

• TTA, SN4 9, and SN4'' 33 irreversible oxidation waves were obtained due to follow-up reactions of the formed radical cations and peak potentials gradually decrease from 0.81 V for TTA to 0.07 V for SN4'' 33 (Figure 2, right). As expected, the oxidation potential is substantially influenced by the

Anion exchange resins in phosphate form as versatile carriers for the reactions catalyzed by nucleoside phosphorylases

• Julia N. Artsemyeva,
• Ekaterina A. Remeeva,
• Tatiana N. Buravskaya,
• Irina D. Konstantinova,
• Roman S. Esipov,
• Anatoly I. Miroshnikov,
• Natalia M. Litvinko and
• Igor A. Mikhailopulo

Beilstein J. Org. Chem. 2020, 16, 2607–2622, doi:10.3762/bjoc.16.212

• based on the kinetin amount) KR as yellowish powder of 98.0% purity (HPLC). UV (MeOH) λmax, nm (ε): 266 (18,675), 211 (23,900); λmin, nm (ε): 233 nm (3,340); 1H NMR (500 MHz, DMSO-d6) δ (purine numbering) purine: 8.41 (s, 1H, H-8), 8.37 (s, 1H, H-2), 8.27 (br.s, 1H, C6-NH); furfuryl radical: 7.56 (d, J

Synthesis of 4-substituted azopyridine-functionalized Ni(II)-porphyrins as molecular spin switches

• Jannis Ludwig,
• Tobias Moje,
• Fynn Röhricht and
• Rainer Herges

Beilstein J. Org. Chem. 2020, 16, 2589–2597, doi:10.3762/bjoc.16.210

• procedure of Nishimura et al. using mercury acetate in trifluoroacetic acid and anisole as radical scavenger (Scheme 5) [34]. Cleavage of the tert-butyl protection group yielded the disulfide 1g, which was reduced with catecholborane in tetrahydrofuran to give the free thiol 1h, which readily reoxidizes

Thermodynamic and electrochemical study of tailor-made crown ethers for redox-switchable (pseudo)rotaxanes

• Henrik Hupatz,
• Marius Gaedke,
• Hendrik V. Schröder,
• Julia Beerhues,
• Arto Valkonen,
• Fabian Klautzsch,
• Sebastian Müller,
• Felix Witte,
• Kari Rissanen,
• Biprajit Sarkar and
• Christoph A. Schalley

Beilstein J. Org. Chem. 2020, 16, 2576–2588, doi:10.3762/bjoc.16.209

• spectra. These bands are indicative of the radical anion NDI•− (Figure 4a–c, red curves) [59]. The radical character of NDIC8 was additionally confirmed by spectroelectrochemical EPR measurements, which showed an isotropic signal with a g-value of 2.004 (Figure S17 in Supporting Information File 1). Upon

Synthesis of novel fluorinated building blocks via halofluorination and related reactions

• Attila Márió Remete,
• Tamás T. Novák,
• Melinda Nonn,
• Matti Haukka,
• Ferenc Fülöp and
• Loránd Kiss

Beilstein J. Org. Chem. 2020, 16, 2562–2575, doi:10.3762/bjoc.16.208

• . Since the treatment of the reaction mixture at reflux conditions significantly increased the yield, the formation of 22 may follow a radical pathway. Using a modified literature protocol [35], the tricyclic imide 24 was also prepared and investigated. In halofluorination reactions, the behavior of 24

Water-soluble host–guest complexes between fullerenes and a sugar-functionalized tribenzotriquinacene assembling to microspheres

• Si-Yuan Liu,
• Xin-Rui Wang,
• Man-Ping Li,
• Wen-Rong Xu and
• Dietmar Kuck

Beilstein J. Org. Chem. 2020, 16, 2551–2561, doi:10.3762/bjoc.16.207

• loss of the entire tentacle as an C9H14N3O5· radical (244 u) from the [M + Na]+ ion followed by H atom transfer. Again, accurate mass measurements were not obtained. As another surprise, the high-resolution ESI-(−) mass spectrum of TBTQ-(OG)6 (see Figure S15 and Table S4, Supporting Information File 1

Recent developments in enantioselective photocatalysis

• Callum Prentice,
• James Morrisson,
• Andrew D. Smith and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2020, 16, 2363–2441, doi:10.3762/bjoc.16.197

• quantifier of the efficiency of a photochemical process. The quantum yield (Φ) of a reaction is often used as a mechanistic tool to probe whether a chain reaction is active (Φ > 1) or not (Φ < 1), although it should be noted that a reaction with Φ < 1 could also still include a radical chain process with an

• use of light would result in a greener reaction. where (a) is the definition of quantum yield and (b) is the quantum yield of a radical chain reaction. Review Enantioselective photocatalysis Amine catalysis Much of the history of amine catalysis used in photochemical reactions can be found in a review

• 1 with amine catalyst 3 to give enamine intermediate 4. The initiation step is proposed to be a reductive quench of the photocatalyst using 4 as a sacrificial reductant to give [Ru]•−, which can then reduce 2 to give electrophilic radical 2•. Addition of 2• to another molecule of 4 generates α-amino

Synthetic approaches to bowl-shaped π-conjugated sumanene and its congeners

• Shakeel Alvi and
• Rashid Ali

Beilstein J. Org. Chem. 2020, 16, 2212–2259, doi:10.3762/bjoc.16.186

• -methoxybenzyl chloride (PMB-Cl) in the presence of 30% NaOH(aq) and tetrabutylammonium bromide (TBAB) in a minimum amount of THF (Scheme 5) [33]. On the other occasion, tribromosumanene 36 has also been synthesized via a radical mechanism from pristine sumanene (2) in the presence of NBS and CCl4 (Scheme 5) [14

• were treated with tert-butyl hydroperoxide (TBHP), two flanking benzene rings were found to be cleaved to afford 174 and 175 in impressive yields (Scheme 45) [82][83][84]. Moreover, they observed radical cations 172 and 173 formation when the same compounds were reacted with Br2 or 2,3,5,6-tetrafluoro

Photosensitized direct C–H fluorination and trifluoromethylation in organic synthesis

• Shahboz Yakubov and
• Joshua P. Barham

Beilstein J. Org. Chem. 2020, 16, 2151–2192, doi:10.3762/bjoc.16.183

• catalysis and photochemistry. Where transition metal catalysis provides one strategy for C–H bond activation, transition-metal-free photochemical C–H fluorination can provide a complementary selectivity via a radical mechanism that proceeds under milder conditions than thermal radical activation methods

• chemoselectivity. Overall, chemo- and regioselective C(sp3)–H fluorinations continue to challenge chemists. Most direct C(sp3)–H fluorinations are reported to proceed under radical pathways involving hydrogen atom transfer (HAT), although proton-coupled electron transfer (PCET) has also been reported [44][49][50

• dictated by the thermodynamic driving force of the hydrogen atom transfer step (which depends on the stability of the resulting radical) or by the relative BDE of C–H bonds. Thus, it is possible to homolytically cleave stronger C–H bonds in the presence of weaker C–H bonds if the polarity of the stronger C

Access to highly substituted oxazoles by the reaction of α-azidochalcone with potassium thiocyanate

• Mysore Bhyrappa Harisha,
• Pandi Dhanalakshmi,
• Rajendran Suresh,
• Raju Ranjith Kumar and
• Shanmugam Muthusubramanian

Beilstein J. Org. Chem. 2020, 16, 2108–2118, doi:10.3762/bjoc.16.178

• unambiguously indicate that the reaction proceeds through a radical pathway. Potassium persulfate helps to generate a thiocyanate radical and in the absence of potassium persulfate the reaction did not proceed. This experiment supports the role of potassium persulfate as an oxidant. Based on these experiments

• , the following plausible mechanism for the formation of 2,4,5-trisubstituted oxazoles can be proposed (Scheme 7). It is known that the thiocyanate radical is generated from potassium thiocyanate by the reaction with potassium persulfate [72]. The N-end of thiocyanate radical reacts with the C=N bond to

A complementary approach to conjugated N-acyliminium formation through photoredox-catalyzed intermolecular radical addition to allenamides and allencarbamates

• Olusesan K. Koleoso,
• Matthew Turner,
• Felix Plasser and
• Marc C. Kimber

Beilstein J. Org. Chem. 2020, 16, 1983–1990, doi:10.3762/bjoc.16.165

• radical addition, using photoredox catalysis, to allenamides and allencarbamates is reported. This transformation synthesizes N-acyl-N’-aryl-N,N’-allylaminals, and proceeds by a conjugated N-acyliminium intermediate that previously has principally been generated by electrophilic activation methods. The

• radical adds to the central carbon of the allene giving a conjugated N-acyliminium that undergoes nucleophilic addition by arylamines and alcohols. Keywords: allenamide; allene; intermolecular; N-acyliminium; photoredox; Introduction Allenamides (Scheme 1, 1) and their congeners have attracted

• potentially unlocking new chemical reactivity yet unseen in the context of electrophilic activation. We envisaged that a route to the conjugated N-acyliminium intermediate could be achieved via an intermolecular addition of an electrophilic radical to an allenamide (1) followed by an oxidation process on the

Metal-free synthesis of phosphinoylchroman-4-ones via a radical phosphinoylation–cyclization cascade mediated by K₂S₂O₈

• Qiang Liu,
• Weibang Lu,
• Guanqun Xie and
• Xiaoxia Wang

Beilstein J. Org. Chem. 2020, 16, 1974–1982, doi:10.3762/bjoc.16.164

• afforded the title compounds in moderate yields. Keywords: chroman-4-ones; diphenylphosphine oxide; metal-free; potassium persulfate; radical cyclization; Introduction The chroman-4-one framework is a privileged structural motif found in numerous natural products and pharmaceuticals with extraordinary

• oxa-Michael additions of 2’-hydroxychalcones [9][10], or through condensation cyclization reactions of o-hydroxyacetophenones with ketones/aldehydes [11][12], in addition to other alternative transformations [13][14]. Moreover, radical cascade cyclizations of o-allyloxybenzaldehydes by employing

• appropriate radical precursors through visible-light promoted systems [15][16], transition-metal-catalyzed systems [17][18], or transition-metal-free systems [19][20], have emerged as a powerful strategy for the synthesis of diversely functionalized chroman-4-one derivatives. Organophosphorus compounds are

Regiodivergent synthesis of functionalized pyrimidines and imidazoles through phenacyl azides in deep eutectic solvents

• Paola Vitale,
• Luciana Cicco,
• Ilaria Cellamare,
• Filippo M. Perna,
• Antonio Salomone and
• Vito Capriati

Beilstein J. Org. Chem. 2020, 16, 1915–1923, doi:10.3762/bjoc.16.158

• -azido ketones in iPrOH in the presence of potassium ethylxanthate as a catalyst [28], (b) by exploiting the reaction of arylglyoxals with an excess amount of ammonium acetate in water [29], (c) by the cathodic reduction of 2-azido-1-phenylethanone in a DMF/LiClO4 medium [30], (d) by radical chain

When metal-catalyzed C–H functionalization meets visible-light photocatalysis

• Lucas Guillemard and
• Joanna Wencel-Delord

Beilstein J. Org. Chem. 2020, 16, 1754–1804, doi:10.3762/bjoc.16.147

• previous functionalization, thus considerably reduce waste generation and a number of synthetic steps. In parallel, transformations involving photoredox catalysis promote radical reactions in the absence of radical initiators. They are conducted under particularly mild conditions while using the visible

• catalytic system is extremely appealing. In that perspective, the scope of this review aims to present innovative reactions combining C–H activation and visible-light induced photocatalysis. Keywords: C–H activation; C–H functionalization; dual catalysis; photoredox catalysis; radical chemistry

• -transfer-type reactions under mild conditions and in the absence of toxic radical precursors [44][45][46]. Considering, on one hand, the fundamentally appealing properties of both, metal-catalyzed C–H functionalization reactions and visible-light-induced photocatalysis, and, on the other hand, their

Et₃N/DMSO-supported one-pot synthesis of highly fluorescent β-carboline-linked benzothiophenones via sulfur insertion and estimation of the photophysical properties

• Dharmender Singh,
• Vipin Kumar and
• Virender Singh

Beilstein J. Org. Chem. 2020, 16, 1740–1753, doi:10.3762/bjoc.16.146

• 2-nitrobenzaldehyde (C) could not be achieved with KOH in MeOH or Cs2CO3 in DMSO. To probe the reaction mechanism, a control experiment was conducted with model substrate 1bA in the presence of a radical scavenger (TEMPO) to check the possibility of a radical mechanism vs an electrophilic addition

• of sulfur [69] (Scheme 7). It was observed that the reaction could not be completed even after 24 h in the presence of TEMPO whereas only 20 min were required for completion under standard conditions. Thus, it is assumed that the reaction proceeds through a radical pathway. Based on our observations

• during the present study and previous reports [69], a plausible mechanism for the formation of the benzothiophenone ring is depicted in Figure 2. It is anticipated that an initial formation of trisulfur radical anion (S3·−) occurs via the reaction of elemental sulfur with triethylamine in DMSO. The

Pauson–Khand reaction of fluorinated compounds

• Jorge Escorihuela,
• Daniel M. Sedgwick,
• Alberto Llobat,
• Mercedes Medio-Simón,
• Pablo Barrio and
• Santos Fustero

Beilstein J. Org. Chem. 2020, 16, 1662–1682, doi:10.3762/bjoc.16.138

• (Scheme 23) [59]. The authors rationalized the formation of 45 by considering that the inherent weakening of the C—X bond going down the halogen series may favor the generation of radical A with chloride and bromide derivatives, especially given the tertiary position of the halide and the stoichiometric

Facile synthesis of 7-alkyl-1,2,3,4-tetrahydro-1,8-naphthyridines as arginine mimetics using a Horner–Wadsworth–Emmons-based approach

• Rhys A. Lippa,
• John A. Murphy and
• Tim N. Barrett

Beilstein J. Org. Chem. 2020, 16, 1617–1626, doi:10.3762/bjoc.16.134

• occur via a radical pathway. When iodide 29 was replaced by a superior oxidant in 1,2-dibromoethane, formation of dimer 32 increased (17% isolated yield). This supports the previously reported proposals that dimerisation occurs via single-electron oxidation of the 2-picolyl anion by the halide

• /pseudohalide oxidant followed by recombination of the resulting picolyl radical [28][29][30]. Whilst trace oxygen may be involved, as indicated by the presence of alcohol 31, no dimerisation or alcohol formation was seen in the absence of a halide/pseudohalide-based oxidant, during C-phosphorylation or

Photoredox-catalyzed silyldifluoromethylation of silyl enol ethers

• Vyacheslav I. Supranovich,
• Vitalij V. Levin and
• Alexander D. Dilman

Beilstein J. Org. Chem. 2020, 16, 1550–1553, doi:10.3762/bjoc.16.126

• )trimethylsilane followed by a reduction of the primary products with sodium borohydride is described. An 18 W, 375 nm LED was used as the light source. The reaction is performed in the presence of a gold photocatalyst, which effects the generation of a (trimethylsilyl)difluoromethyl radical via cleavage of the

• carbon–bromine bond. Keywords: difluoroalkylation; organofluorine compounds; photocatalysis; radical addition; silicon reagents; Findings Fluorinated silicon reagents have found widespread use for the introduction of fluorinated fragments [1][2][3][4][5]. Typically, these reagents work under strongly

• basic conditions required to activate inert C–Si bonds with the generation of carbanionic species. On the other hand, radical reactions offer different synthetic opportunities originating from the neutral character of the intermediates [6][7] and, accordingly, radical fluoroalkylation processes have

Heterogeneous photocatalysis in flow chemical reactors

• Christopher G. Thomson,
• Ai-Lan Lee and
• Filipe Vilela

Beilstein J. Org. Chem. 2020, 16, 1495–1549, doi:10.3762/bjoc.16.125

• photocatalyst for the reduction of aryl halides [193]. The material could undergo consecutive photoinduced electron transfers (ConPET) in which the material enters an excited state and is reduced by a sacrificial electron donor (NEt3). The resulting Zn-PDI radical anion then undergoes a second photon absorption

• photons [50]. They reported a photocatalytic atom transfer radical polymerisation for the synthesis of cross-linked PMMA using the upconversion photocatalyst system and either a blue laser or a NIR (730 nm) laser. The blue laser would excite the photocatalyst component directly, whilst the NIR laser

4-Hydroxy-3-methyl-2(1H)-quinolone, originally discovered from a Brassicaceae plant, produced by a soil bacterium of the genus Burkholderia sp.: determination of a preferred tautomer and antioxidant activity

• Dandan Li,
• Naoya Oku,
• Yukiko Shinozaki,
• Yoichi Kurokawa and
• Yasuhiro Igarashi

Beilstein J. Org. Chem. 2020, 16, 1489–1494, doi:10.3762/bjoc.16.124

• in liquid production cultures, quenched hydroxy radical-induced chemiluminescence emitted by luminol by 86%. Because some Burkholderia species are pathogenic to plants and animals, the above result suggests that 1 is a potential antioxidant to counteract reactive oxygen species-based immune response

• extinction assay [41][42]. This assay quantifies the presence of the most detrimental ROS, hydroxy radical [43][44], as intensity of luminescence emitted by oxidation of luminol. Compound 1 at 10 μM, which is lower than a concentration in production liquid cultures (15 μM), decreased luminescence to 14% of

Photocatalyzed syntheses of phenanthrenes and their aza-analogues. A review

• Alessandra Del Tito,
• Havall Othman Abdulla,
• Davide Ravelli,
• Stefano Protti and
• Maurizio Fagnoni

Beilstein J. Org. Chem. 2020, 16, 1476–1488, doi:10.3762/bjoc.16.123

• Phenanthrenes and their aza-analogues have important applications in materials science and in medicine. Aim of this review is to collect recent reports describing their synthesis, which make use of radical cyclizations promoted by a visible light-triggered photocatalytic process. Keywords: phenanthrenes

• inexpensive visible light (or sunlight, when possible) irradiation [29]. In general terms, photocatalysis smoothly gives access to reactive radical intermediates [30], mainly carbon-centered [31][32][33], or nitrogen-centered radicals [34][35]. In turn, these species have been extensively employed in radical

• heteroarene analogues via the intermediacy of a radical. However, some interesting approaches carried out under photomediated or photocatalyst-free conditions have been likewise included for the sake of completeness. Review 1 Synthesis of phenanthrenes The photocatalyzed synthesis of the phenanthrene skeleton

An overview on disulfide-catalyzed and -cocatalyzed photoreactions

• Yeersen Patehebieke

Beilstein J. Org. Chem. 2020, 16, 1418–1435, doi:10.3762/bjoc.16.118

• excellent radical properties also made these thiyl radicals powerful HAT catalysts. They have increasingly been proven useful in various types of organic photoreactions, such as cyclizations, anti-Markovnikov additions, aromatic olefin carbonylations, isomerizations, etc. They are a class of green, economic

• , mild, and chemoselective radical catalysts that deserve more attention. The present review highlights the recent progress in the field of disulfide-catalyzed and -cocatalyzed photocatalytic reactions for different reaction types. Keywords: cycloaddition; disulfide catalyst; isomerization; oxidation

• ; photocatalysis; thiyl radical; Introduction Organic disulfides are often used as the skeleton for drugs, pesticides, rubber auxiliaries, polymers, and electronic materials [1]. Over the past decade, organic disulfide-involving photoreactions have attracted increasing attention. Disulfides have versatile

Distinctive reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines under photoredox conditions

• Shrikant D. Tambe,
• Kwan Hong Min,
• Naeem Iqbal and
• Eun Jin Cho

Beilstein J. Org. Chem. 2020, 16, 1335–1342, doi:10.3762/bjoc.16.114

• unprecedented reductive coupling of N-benzylidene-[1,1'-biphenyl]-2-amines with an aliphatic amine. The presence of a phenyl substituent in the aniline moiety of the substrate was critical for the reactivity. The reaction proceeded via radical–radical cross-coupling of α-amino radicals generated by proton

• extensively explored under visible-light photocatalysis [10][11][12][13][14][15][16]. N-Benzylidenes can undergo facile single-electron reduction to generate α-amino radical intermediates, which can participate in diverse processes, depending upon the nature of the substrates and the reaction conditions

• (Scheme 1a). Various amine systems are generated from such intermediates via a wide range of processes, including hydrogen atom abstraction [17][18], the addition to unsaturated compounds [19][20][21], radical–radical coupling [22][23][24][25][26][27][28][29][30], and cyclization reactions [31][32][33

Highly selective Diels–Alder and Heck arylation reactions in a divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles from 2-formylpyrrole

• Carlos H. Escalante,
• Eder I. Martínez-Mora,
• Carlos Espinoza-Hicks,
• Alejandro A. Camacho-Dávila,
• Fernando R. Ramos-Morales,
• Francisco Delgado and
• Joaquín Tamariz

Beilstein J. Org. Chem. 2020, 16, 1320–1334, doi:10.3762/bjoc.16.113

• electronic density provided by the nitrogen lone-pair, which plausibly stabilizes the cationic or radical species formed by the oxidant reagent during the aromatization process [46], as efficiently occurred with derivatives 9. Finally, in order to obtain the aromatic indole-based pentacycles 12, the