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

Azo-dimethylaminopyridine-functionalized Ni(II)-porphyrin as a photoswitchable nucleophilic catalyst

• Jannis Ludwig,
• Julian Helberg,
• Hendrik Zipse and
• Rainer Herges

Beilstein J. Org. Chem. 2020, 16, 2119–2126, doi:10.3762/bjoc.16.179

• shielded in the cis configuration and therefore catalytically inactive. The basicity was restored by isomerization to the trans state leading to a rapid conversion to the β-nitroalcohol, the product of the Henry reaction [10][11][12]. We now present a photoswitchable catalyst whose basicity is controlled

• influence of the solvent on the switching efficiency was evaluated by UV–vis (Figure S2, Supporting Information File 1) and NMR experiments (Figure 2). It is important to note that there are two ways to define the switching efficiency of record player molecules: 1. the cis–trans isomerization of the

• coordination. On this account, isomerization and coordination were investigated separately by 1H NMR spectroscopy. The protons meta to the azo group in the azopyridine unit (H-11) are responsive to the configuration of the azo group (cis or trans) and the chemical shifts of the pyrrole protons of the porphyrin

Reactions of 3-aryl-1-(trifluoromethyl)prop-2-yn-1-iminium salts with 1,3-dienes and styrenes

• Thomas Schneider,
• Michael Keim,
• Bianca Seitz and
• Gerhard Maas

Beilstein J. Org. Chem. 2020, 16, 2064–2072, doi:10.3762/bjoc.16.173

• extent (13c) or not at all (13b) (Scheme 4). A remarkable stereochemical aspect accompanied the reaction of 1a with (E)-1-phenyl-1-propene leading to 2-((E)-1-phenylprop-1-enyl)indene 12d, where trans(Ph,Me)→cis isomerization at the olefinic bond has occurred. The E-configuration was assigned based on

Syntheses of spliceostatins and thailanstatins: a review

• William A. Donaldson

Beilstein J. Org. Chem. 2020, 16, 1991–2006, doi:10.3762/bjoc.16.166

• in a high yield and enantioselectivity (Scheme 5) [17][18]. The Rubottom oxidation [28] of 43 gave a separable mixture of the desired 44 and its C-14 epimer (≈7:1 ratio). The reductive deoxygenation of 44 proceeded via the tosylhydrazone to afford 45, which upon desilylation and alkyne isomerization

Polarity effects in 4-fluoro- and 4-(trifluoromethyl)prolines

• Vladimir Kubyshkin

Beilstein J. Org. Chem. 2020, 16, 1837–1852, doi:10.3762/bjoc.16.151

• probing of the molecular sites with relatively small dipoles of the solvent. This is why here it seems more reasonable to adhere to simple schematic descriptions such as the one shown in Figure 7. Amide-bond rotation: thermodynamics Isomerization of the amide (peptide) bond is an important issue in

• transition state would explain the energy penalty in the rotation barriers (Figure 9D). In 3,3-dimethylprolines, the isomerization into the C4-exo envelope would lead to a steric clash between one of the methyl groups and the backbone carbonyl (k decreases by a factor of 2–6) [99]; in 3,4-dehydroprolines

• , the ring is unable to adopt an envelope conformation due to an endocyclic double bond (k decreases by factor 3) [72]. Overall, the conclusions regarding the ring conformation preference in the transition state provides an entirely new insight onto the peptide-bond isomerization, and can be helpful for

One-pot synthesis of isosorbide from cellulose or lignocellulosic biomass: a challenge?

• Isaline Bonnin,
• Raphaël Mereau,
• Thierry Tassaing and
• Karine De Oliveira Vigier

Beilstein J. Org. Chem. 2020, 16, 1713–1721, doi:10.3762/bjoc.16.143

• minimized by decreasing the reaction temperature to 210 °C, improving the hydrogenation capacity by optimization of the concentration of Ru/C and decreasing the solvent volume to 40 mL. A formation of isomerization products of isohexides was observed as the isomerization of glucose is acid-catalyzed, and

• the isomerization of alditols is catalyzed by Ru/C. An isosorbide yield of 52% was obtained after 1 h at 210 °C and 50 bar of H2 from 10 wt % of cellulose and a catalyst/cellulose ratio of 1:4 in the presence of 1 g of Ru/C (5 wt % of Ru). It was also shown that a longer reaction time led to an

Synthesis of Streptococcus pneumoniae serotype 9V oligosaccharide antigens

• Sharavathi G. Parameswarappa,
• Claney L. Pereira and
• Peter H. Seeberger

Beilstein J. Org. Chem. 2020, 16, 1693–1699, doi:10.3762/bjoc.16.140

• via a two-step process of triflation and azide substitution to produce the desired trisaccharide 24 [29]. Removal of the allyl group using iridium-catalyzed isomerization and subsequent treatment with iodine in the presence of water yielded trisaccharide acceptor 25 for the late stage [2 + 3

Rearrangement of o-(pivaloylaminomethyl)benzaldehydes: an experimental and computational study

• Csilla Hargitai,
• Györgyi Koványi-Lax,
• Tamás Nagy,
• Péter Ábrányi-Balogh,
• András Dancsó,
• Gábor Tóth,
• Judit Halász,
• Angéla Pandur,
• Gyula Simig and
• Balázs Volk

Beilstein J. Org. Chem. 2020, 16, 1636–1648, doi:10.3762/bjoc.16.136

• formation of dimers 5. Rearrangement of aminoaldehydes 1 to regioisomers 2 and formation of dimer-like products 3 and 8. Proposed mechanism of the isomerization of aldehydes 1 via isoindoles 4. Proposed mechanism of the formation of dimer-like products 3a,b. Proposed mechanism for the formation of dimer

An overview on disulfide-catalyzed and -cocatalyzed photoreactions

• Yeersen Patehebieke

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

• , 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

• alkanes provided an efficient protocol for the synthesis of alkanes from carboxylic acid substrates previously inaccessible through other methods. The mechanism of this reaction is similar to the previously proposed mechanism in Scheme 19. Isomerization reactions Thiyl radical-mediated isomerizations of C

• compounds. In the synthesis of these synthons, the key transformation, the isomerization of the trans-1,2-difluorotriethylsilylethylene 57 to the cis isomer 58, is realized with phenyl disulfide under ultraviolet-light irradiation (Scheme 22) [29]. In 2009, Tsuboi and co-workers also reported a dendrimer

Recent synthesis of thietanes

• Jiaxi Xu

Beilstein J. Org. Chem. 2020, 16, 1357–1410, doi:10.3762/bjoc.16.116

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

• 5–16). None of the series of mixtures 9a–p/10a–p corresponds to the direct products of the Diels–Alder cycloadditions, since they are derivatives resulting from the double bond isomerization of adducts 17 (i.e., migration from the pyrrole-fused exo cyclic position to the endo cyclic aromatic

Oxime radicals: generation, properties and application in organic synthesis

• Igor B. Krylov,
• Stanislav A. Paveliev,
• Alexander S. Budnikov and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2020, 16, 1234–1276, doi:10.3762/bjoc.16.107

• isomers (E and Z) exist. The isomerization of oxime radicals proceeds much easier than for the corresponding oximes; the observation of individual isomers is generally possible only at low temperatures [68][69] (about 190 K). According to quantum chemical calculations, the oxime radicals have an increased

Photocatalysis with organic dyes: facile access to reactive intermediates for synthesis

• Stephanie G. E. Amos,
• Marion Garreau,
• Luca Buzzetti and
• Jerome Waser

Beilstein J. Org. Chem. 2020, 16, 1163–1187, doi:10.3762/bjoc.16.103

Fluorinated phenylalanines: synthesis and pharmaceutical applications

• Laila F. Awad and
• Mohammed Salah Ayoup

Beilstein J. Org. Chem. 2020, 16, 1022–1050, doi:10.3762/bjoc.16.91

• , m, or p-fluoro or (trifluoromethyl)phenylpyruvic acids 97a–f. The reaction was carried out in presence of 0.5 equiv of zinc(II) acetate in the presence of NaOMe. The initially formed complexes 98a–f underwent isomerization to 99a–f. Acid hydrolysis then gave the FPhe derivatives 53a,b, 53i, 81, and

• catalytic amination of (E)-cinnamic acid The enzyme phenylalanine aminomutase (PAM) from Taxus chinensis catalyzes the stereoselective isomerization of α-phenylalanine to β-phenyalanine 111a–c. Mechanistic studies showed that (E)-cinnamic acid is an intermediate in this transformation [63]. Accordingly

Cation-induced ring-opening and oxidation reaction of photoreluctant spirooxazine–quinolizinium conjugates

• Phil M. Pithan,
• Sören Steup and
• Heiko Ihmels

Beilstein J. Org. Chem. 2020, 16, 904–916, doi:10.3762/bjoc.16.82

• behavior as it can undergo isomerization and cyclization reactions. For example, the irradiation of a 5’-naphthylvinyl-substituted spirooxazine derivative led not only to a ring opening of the oxazine unit but also to an irreversible loss of the photochromic properties, resulting from a cyclization

Aldehydes as powerful initiators for photochemical transformations

• Maria A. Theodoropoulou,
• Nikolaos F. Nikitas and
• Christoforos G. Kokotos

Beilstein J. Org. Chem. 2020, 16, 833–857, doi:10.3762/bjoc.16.76

• shed more light on some mechanistic pathways and set the key foundation for the further use as photoinitiators or photocatalysts. In 1961, Hammond and co-workers studied the cis/trans isomerization of the piperylenes (1,3-pentadienes) 61 and 62 in the presence of carbonyl compounds as photosensitizers

• (Scheme 15) [50]. Among the compounds tested, 1-naphthaldehyde (63) provided a photostationary mixture rich in the trans-diene (trans/cis ratio ≈ 13). A year later, the same group further studied the cis/trans isomerization of the piperylenes 61 and 62 utilizing energy transfer from the triplet states of

• transferred to the olefins in the Paterno–Büchi reaction, they performed a photochemical reaction between benzaldehyde (8) or benzophenone and the 3-methyl-2-pentenes 69 and 70, respectively. The isomerization of the starting olefins indicated a bimolecular energy transfer mechanism (Scheme 17). Benzophenone

Preparation of 2-phospholene oxides by the isomerization of 3-phospholene oxides

• Péter Bagi,
• Réka Herbay,
• Nikolett Péczka,
• Zoltán Mucsi,
• István Timári and
• György Keglevich

Beilstein J. Org. Chem. 2020, 16, 818–832, doi:10.3762/bjoc.16.75

• -phospholene oxides in methanesulfonic acid, or via the formation of cyclic chlorophosphonium salts. Whereas mixtures of the 2- and 3-phospholene oxides formed, when the isomerization of 3-phospholene oxides was attempted under thermal conditions, or in the presence of a base. The mechanisms of the various

• double bond migration pathways were elucidated by quantum chemical calculations. Keywords: chlorophosphonium salts; isomerization; 2-phospholene oxides; 3-phospholene oxides; quantum chemistry; Introduction P-Heterocyclic derivatives are valuable targets in synthetic organophosphorus chemistry [1][2][3

• strategies for 2-phospholene oxides (Figure 1). A convenient strategy may involve the isomerization of 3-phospholene oxides to the corresponding 2-phospholene oxides. A few isomerizations resulting 2-phospholene derivatives have already been observed and published in the literature [58][59][60][61]. However

Combining enyne metathesis with long-established organic transformations: a powerful strategy for the sustainable synthesis of bioactive molecules

• Valerian Dragutan,
• Ileana Dragutan,
• Albert Demonceau and
• Lionel Delaude

Beilstein J. Org. Chem. 2020, 16, 738–755, doi:10.3762/bjoc.16.68

• , which underwent a cross-metathesis with ethyl acrylate in the presence of the Hoveyda–Grubbs second-generation-type catalyst (10 mol %) leading to the corresponding vinyl ester 8 in 63% yield, as a convenient precursor to (+)-anthramycin. The latter was obtained via an isomerization of the double bond

Synthesis of disparlure and monachalure enantiomers from 2,3-butanediacetals

• Adam Drop,
• Hubert Wojtasek and
• Bożena Frąckowiak-Wojtasek

Beilstein J. Org. Chem. 2020, 16, 616–620, doi:10.3762/bjoc.16.57

• isomerization of trans-2,3-butanediacetals to the cis-isomers has also been performed via dithiolate derivative 10 to obtain compound 13 (Scheme 2) [35]. However, attempts to isomerize dimethyl ester 9 in the same way gave a cyclic compound as a major product [36]. We have recently developed a simple and

• efficient method for the isomerization of trans-disubstituted butanediacetal derivative 11 with two different substituents (one ester group and one thioester group) to its cis derivative 14 which proceeded with 94% yield [37]. The cis-2,3-butanediacetals 12–14 have the appropriate configuration of the

• ), and (−)-monachalure (4) from diols 5–8. Isomerization of trans-2,3-butanediacetals 9–11 to cis-2,3-butanediacetals 12–14. Synthesis of diol 17 and its subsequent modifications. Synthesis of (+)-disparlure (1) and (+)-monachalure (2). Synthesis of (−)-disparlure (3) and (−)-monachalure (4). Conditions

Copper-catalyzed enantioselective conjugate reduction of α,β-unsaturated esters with chiral phenol–carbene ligands

• Shohei Mimura,
• Sho Mizushima,
• Yohei Shimizu and
• Masaya Sawamura

Beilstein J. Org. Chem. 2020, 16, 537–543, doi:10.3762/bjoc.16.50

• π-complex C undergoes 1,4-hydrocupration to afford copper enolate D. During our initial investigations, we observed configurational isomerization from 1a to (E)-1a when the reaction was conducted in the absence of alcoholic protonation reagents. This observation implied that the 1,4-hydrocupration

Architecture and synthesis of P,N-heterocyclic phosphine ligands

• Wisdom A. Munzeiwa,
• Bernard Omondi and
• Vincent O. Nyamori

Beilstein J. Org. Chem. 2020, 16, 362–383, doi:10.3762/bjoc.16.35

• -quinolyhydrazine (112) at a molar ratio of 2:1 in the presence of a base gave compound 113 in very good yield. Heating of compound 113 in toluene induced isomerization where the pendant arm is shifted to the quinoline ring via a P–C migration to obtain compound 114. The corresponding P–N and P–P rearrangements

p-Pyridinyl oxime carbamates: synthesis, DNA binding, DNA photocleaving activity and theoretical photodegradation studies

• Panagiotis S. Gritzapis,
• Panayiotis C. Varras,
• Nikolaos-Panagiotis Andreou,
• Katerina R. Katsani,
• Konstantinos Dafnopoulos,
• George Psomas,
• Zisis V. Peitsinis,
• Alexandros E. Koumbis and
• Konstantina C. Fylaktakidou

Beilstein J. Org. Chem. 2020, 16, 337–350, doi:10.3762/bjoc.16.33

• under reflux were identical, meaning that no isomerization or decomposition occurred and that the delivered product in both cases is the more thermodynamically stable. Interestingly, although carbamates are important in both medicinal and polymer chemistry, besides compound 23 [61], all the rest were

Reversible photoswitching of the DNA-binding properties of styrylquinolizinium derivatives through photochromic [2 + 2] cycloaddition and cycloreversion

• Sarah Kölsch,
• Heiko Ihmels,
• Jochen Mattay,
• Norbert Sewald and
• Brian O. Patrick

Beilstein J. Org. Chem. 2020, 16, 111–124, doi:10.3762/bjoc.16.13

• electrocyclization or E-to-Z isomerization reactions, whereas the well-established photochromic cycloaddition–cycloreversion equilibrium to establish photoswitchable DNA binders has so far been widely neglected. In fact, there is only one reported example for the use of the reversible photoinduced dimerization of

• of the initially formed Z-isomers Z-3a and Z-3b, with both processes generally being possible starting from 3b and 3c (Scheme 2). On the one hand, the photometric monitoring as well as the 1H NMR spectroscopic studies of the photoreaction of 3b indicated a preceding E-to-Z isomerization (cf

• other hand, the reaction was performed using suspensions, so that the reaction may also have taken place with undissolved solid in which the E-to-Z isomerization was most likely suppressed due to the restricted space in the confined medium. Thus, the selective formation of the dimers 4b and 4c is

Photocontrolled DNA minor groove interactions of imidazole/pyrrole polyamides

• Sabrina Müller,
• Jannik Paulus,
• Jochen Mattay,
• Heiko Ihmels,
• Veronica I. Dodero and
• Norbert Sewald

Beilstein J. Org. Chem. 2020, 16, 60–70, doi:10.3762/bjoc.16.8

• peptides or small molecules, thereby controlling geometry and functionality. E-to-Z photoisomerization usually is achieved upon irradiation at 350 nm (π–π* transition), while the Z-to-E isomerization proceeds photochemically upon irradiation at >400 nm (n–π* transition) or thermally. Photoswitchable

• substance, once located at the DNA target, with an external stimulus. Azobenzenes are photoswitchable molecules capable of generating significant structural changes in peptides or small molecules, thereby controlling their geometry and functionality upon irradiation [10][11][12][13]. While isomerization of

• replacement of the γ-aminobutyric acid linker between the two hairpin strands by a photoswitchable azobenzene motif. As the azobenzene moiety undergoes a considerable steric reorganization upon isomerization from the E- to the Z-configuration, the end-to-end distance in a 4,4'-disubstituted azobenzene changes

Starazo triple switches – synthesis of unsymmetrical 1,3,5-tris(arylazo)benzenes

• Andreas H. Heindl and
• Hermann A. Wegner

Beilstein J. Org. Chem. 2020, 16, 22–31, doi:10.3762/bjoc.16.4

• characterized in terms of their isomerization behavior using UV–vis and 1H NMR spectroscopy. The efficient synthesis of this new class of unsymmetrical tris(arylazo)benzenes opened new avenues to novel multistate switching materials. Keywords: multistate photoswitches; synthesis; tris(arylazo)benzene

• isomerization can be reversed either by irradiation with visible light (ca. 450 nm) or thermally, upon heating. The thermal back-isomerization rate can be controlled by various factors: Functional group substitutions on the phenyl rings determine the thermal half-lives of (Z)-ABs [1]. For example, o-fluoro

• their isomerization properties [3][4]. Also, the incorporation of AB units into cyclic [5] or macrocyclic structures can control the switching, depending, i.a., on symmetry and ring strain [6][7][8][9]. By combining these approaches, half-lives can be tuned from milliseconds to years. The incorporation