A dynamic combinatorial library for biomimetic recognition of dipeptides in water

• Florian Klepel and
• Bart Jan Ravoo

Beilstein J. Org. Chem. 2020, 16, 1588–1595, doi:10.3762/bjoc.16.131

• amount of different receptor candidates to 20. The exchange was subsequently quenched by protonation of the thiolates and the samples were analyzed by HPLC–MS . Pre-separation was performed on a ZIC-HILIC column, which is known for good peptide selectivity [22]. However, complete separation of all

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

Recent synthesis of thietanes

• Jiaxi Xu

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

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

• class of substrates that usually undergo a protonation to form C(sp3) radicals. Alkenyl or aryl radical ions are generally accessed through SET. The presence of electron-donating groups facilitates the oxidation of the precursor to the radical cations, while electron-poor alkynes and arenes can undergo

• intermediate of important reactions, such as the pinacol [101] or the McMurry coupling [102]. Recently, photocatalysis has been used to access ketyl radicals through the reduction of ketones with a suitable transition metal-based photocatalyst [103] or organic dye [104]. The protonation of this type of

• radicals occurs on the heteroatom and leads to versatile neutral C(sp3) radicals. Such processes can also occur via concerted proton-coupled electron transfer mechanisms [105]. Similarly, the photocatalyzed reduction of imines followed by protonation, as well as the reduction of iminium compounds, gives

Synthesis and properties of quinazoline-based versatile exciplex-forming compounds

• Rasa Keruckiene,
• Simona Vekteryte,
• Ervinas Urbonas,
• Matas Guzauskas,
• Eigirdas Skuodis,
• Dmytro Volyniuk and
• Juozas V. Grazulevicius

Beilstein J. Org. Chem. 2020, 16, 1142–1153, doi:10.3762/bjoc.16.101

• photoluminescence and electroluminescence based on blue emissive quinazoline derivatives obtained through controlled acid protonation were employed in a single-layered white OLED with EQEs of 1.4% and 3% [9]. These reports proved that by using an asymmetric quinazoline acceptor, highly efficient TADF materials for

Recent applications of porphyrins as photocatalysts in organic synthesis: batch and continuous flow approaches

• Rodrigo Costa e Silva,
• Luely Oliveira da Silva,
• Aloisio de Andrade Bartolomeu,
• Timothy John Brocksom and
• Kleber Thiago de Oliveira

Beilstein J. Org. Chem. 2020, 16, 917–955, doi:10.3762/bjoc.16.83

• porphyrins as photocatalysts under sunlight irradiation furnished the corresponding sulfoxides with high chemoselectivity (up to 100%), scalability (up to 2.6 mmol) and high yields (up to 100%) [94]. According to the authors, the protonation of the porphyrins causes a red-shift of the photosensitizer with an

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

• lead to the formation of the desired product and the regeneration of benzaldehyde (8). In most cases, p-toluenesulfonic acid (TsOH) was also used in order to improve the yield through the protonation and activation of benzothiazole. The proposed reaction mechanism, as explained above, is depicted in

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

• isomerization in the presence of methanesulfonic acid. The thermodynamic data and the olefinicity values (OL%) calculated for the corresponding protonated derivatives 1+H+ and 4+H+ showed that indeed this protonation perturbs the original equilibrium significantly. The protonated 2-phospholene oxide 4+H+ became

• -catalyzed reactions. Scheme 3 shows the computed reaction mechanism of the acidic transformation, which starts from the complex formed from MeSO3H and the 3-phospholene oxide 16A, involving in a protonation equilibrium with 16B. In the next step, the acid migrates to position C(4), and the olefinic carbon

Recent advances in Cu-catalyzed C(sp³)–Si and C(sp³)–B bond formation

• Balaram S. Takale,
• Ruchita R. Thakore,
• Elham Etemadi-Davan and
• Bruce H. Lipshutz

Beilstein J. Org. Chem. 2020, 16, 691–737, doi:10.3762/bjoc.16.67

• . Systematic kinetic studies using NMR experiments suggested that protonation of the intermediate 252 occurs from the sterically favored side, leading to the kinetically stable cis product (Scheme 42). Nonetheless, some epimerization under the reaction conditions took place leading to the thermodynamically

• through subsequent C–C bond formation (Scheme 72) [135]. The proposed mechanism involves the formation of L–Cu–O–t-Bu 458 which reacts with B2pin2 to generate L-Cu-Bpin 459. The subsequent coordination to the aldehyde results in the intermediate 461. Upon protonation of 461 the desired enantiomerically

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

• diethoxymethylsilane (4 equiv) as a reductant and t-AmOH (1 equiv) as a protonation reagent in DMA as the solvent at 25 °C for 15 h, the product 2a was produced in 98% yield (1H NMR analysis) with a promising enantioselectivity of 69% ee (Table 1, entry 1). When the phenolic hydroxy group of L1 was changed to a

• silane PMHS gave only trace amounts of the product. The nature of the alcoholic protonation reagent also had a strong impact. The presence of a tertiary alcohol, t-AmOH or t-BuOH, was essential for the reaction to occur with a reasonable yield, while iPrOH and MeOH markedly suppressed the reaction (Table

• 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

Photophysics and photochemistry of NIR absorbers derived from cyanines: key to new technologies based on chemistry 4.0

• Bernd Strehmel,
• Christian Schmitz,
• Ceren Kütahya,
• Yulian Pang,
• Anke Drewitz and
• Heinz Mustroph

Beilstein J. Org. Chem. 2020, 16, 415–444, doi:10.3762/bjoc.16.40

• . The nitrogens in the cyanine chain still exhibit low nucleophilicity since electron density distributes over the entire methine chain. Thus, protonation of the nitrogen in the cyanine moiety occurs with less efficiency compared to amines [117]. This strategy helped to enable cationic

Recent developments in photoredox-catalyzed remote ortho and para C–H bond functionalizations

• Rafia Siddiqui and
• Rashid Ali

Beilstein J. Org. Chem. 2020, 16, 248–280, doi:10.3762/bjoc.16.26

• 59 are obtained by the reduction of the Co(II) catalyst 53 by radical 58. In order to form the desired cyclized product, rearomatization of 59 takes place, and protonation of 55 assists the regeneration of 56, with parallel hydrogen release, as shown in Figure 10. C–H cyclization Synthesis of

• efficient brominations, they employed sodium bromide in the presence of oxygen. The activation of the photocatalyst through protonation was shown by cyclic voltammetry, and the other interactions were revealed by emission quenching experiments and UV–vis spectroscopy. As can be seen in Scheme 21, the group

[1,3]/[1,4]-Sulfur atom migration in β-hydroxyalkylphosphine sulfides

• Katarzyna Włodarczyk,
• Piotr Borowski and
• Marek Stankevič

Beilstein J. Org. Chem. 2020, 16, 88–105, doi:10.3762/bjoc.16.11

• was investigated by molecular modeling (Scheme 9). The first step of the rearrangement involved the protonation of 20, which afforded intermediate I, where the proton inserted between the sulfur and the oxygen atom. This intermediate appeared to be stable and failed to undergo further transformation

• , according to calculations. The reason of this behavior was most probably the cyclic structure of the intermediate. However, if the intermediate I can undergo a second protonation process, intermediate III forms through transition state II. Water elimination from III proceeded with slight increase in energy

• the γ-mercaptoalkylphosphine sulfides from β-hydroxyalkylphosphine sulfides 16 and 21 and, to some extent, 8 and 11. In these cases, DFT calculations allowed to propose a slightly different mechanism (see Supporting Information File 1). In this case, a water molecule, formed after protonation of the

A review of asymmetric synthetic organic electrochemistry and electrocatalysis: concepts, applications, recent developments and future directions

• Munmun Ghosh,
• Valmik S. Shinde and
• Magnus Rueping

Beilstein J. Org. Chem. 2019, 15, 2710–2746, doi:10.3762/bjoc.15.264

• forms chiral ion pair B. The intermediate then undergoes enantioselective protonation by Yoh-H+ to generate Yoh-bound 9' (complex C). Subsequent release of the reduced product 9' and reprotonation of yohimbine complete the cycle (Scheme 9) [32][33]. In a continuation of their previous work [34], Wang

• ) abstracts a proton from the cocatalyst (n-butanol), which results in the protonation of the quiniclidine N-atom to give CN–H. At the same time, compound 27 undergoes a one-electron reduction at the cathode to afford radical anion 30, which is protonated by CN–H and generates 31. This results in the

Emission solvatochromic, solid-state and aggregation-induced emissive α-pyrones and emission-tuneable 1H-pyridines by Michael addition–cyclocondensation sequences

• Natascha Breuer,
• Irina Gruber,
• Christoph Janiak and
• Thomas J. J. Müller

Beilstein J. Org. Chem. 2019, 15, 2684–2703, doi:10.3762/bjoc.15.262

• furnishing directly 1H-pyridine 5a after protonation. For examining the influence of the electronic nature of the alkynone 3 on the product formation, a range of differently substituted alkynones 3 (for experimental details on their preparation, see chapters 2.1 and 2.2 in Supporting Information File 1

Acid-catalyzed rearrangements in arenes: interconversions in the quaterphenyl series

• Sarah L. Skraba-Joiner,
• Carter J. Holt and
• Richard P. Johnson

Beilstein J. Org. Chem. 2019, 15, 2655–2663, doi:10.3762/bjoc.15.258

• . DFT computations with inclusion of implicit solvation support a complex network of phenyl and biphenyl shifts, with barriers to rearrangement in the range of 10–21 kcal/mol. Consistent with experiments, the lowest energy arenium ion located on this surface is due to protonation of m,p'-quaterphenyl

• believed to occur through the intermediacy of ipso arenium ions 4–6 which connect through 1,2-phenyl shifts. The term "ipso" was first proposed by Perrin and Skinner to explain unusual results in electrophilic substitution reactions; this refers to protonation at the site of a substituent [24]. Ipso

• protonation is the essential step in arenium ion rearrangements. Our interest in this field arose from an accidental rediscovery of rearrangements in the terphenyl series (1–3; Scheme 1), by heating 1 with AlCl3 – a reaction independently discovered twice before [22][23]! We confirmed earlier observations of

Nanangenines: drimane sesquiterpenoids as the dominant metabolite cohort of a novel Australian fungus, Aspergillus nanangensis

• Heather J. Lacey,
• Cameron L. M. Gilchrist,
• Andrew Crombie,
• John A. Kalaitzis,
• Daniel Vuong,
• Peter J. Rutledge,
• Peter Turner,
• John I. Pitt,
• Ernest Lacey,
• Yit-Heng Chooi and
• Andrew M. Piggott

Beilstein J. Org. Chem. 2019, 15, 2631–2643, doi:10.3762/bjoc.15.256

• -type sesquiterpenoids from farnesyl diphosphate is proposed to proceed via the protonation-initiated mechanism (class II terpene synthases) [24], which is distinct from the ionisation-initiated mechanism (class I) terpene synthases, where a carbocation is generated by the release of a diphosphate group

• [33][34][35]. Specifically, AstC contains a DxDTT motif [21], which is a variant of the DxDD motif known to be involved in class II-type protonation-initiated terpene cyclisation [36]. Importantly, the DDxxD motif in AstC contains a substitution of the second Asp for Asn, leading to a loss of

Current understanding and biotechnological application of the bacterial diterpene synthase CotB2

• Ronja Driller,
• Daniel Garbe,
• Norbert Mehlmer,
• Monika Fuchs,
• Keren Raz,
• Dan Thomas Major,
• Thomas Brück and
• Bernhard Loll

Beilstein J. Org. Chem. 2019, 15, 2355–2368, doi:10.3762/bjoc.15.228

• result of heterolytic cleavage of the pyrophosphate–hydrocarbon bond or protonation of a double bond, (2) propagation of the carbocation along the forming terpene skeleton as a result of ring formations, hydride and/or methyl shifts, de- and reprotonation of intermediates, the creation of a terminal

• metal cluster in class I terpenoid cyclases, class II reactions proceed via a protonation of the terminal carbon–carbon double bond of an isoprenoid substrate [3]. In addition to the differences in the activation mechanism, the two classes of TPSs have an unrelated overall fold. Class I TPSs establish

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

• ][22]. For benzanilides Tu [21] proposed a mechanism involving protonation of the amide oxygen, followed by 1,3 proton shift to the ring carbon next to the amide carbonyl group, followed by elimination of protonated phenyl isocyanate under re-aromatization. For the pyrrole derived substrate

• investigated here, even direct protonation at C-3 of the pyrrole by strong acid is most likely, due to the significant basicity of pyrroles. Delocalization of the positive charge (A ↔ A’) as shown in Scheme 2 will support the initial ring protonation step. The X-ray analysis of compound 7 revealed that the

1,2,3,4-Tetrahydro-1,4,5,8-tetraazaanthracene revisited: properties and structural evidence of aromaticity loss

• Arnault Heynderickx,
• Sébastien Nénon,
• Olivier Siri,
• Vladimir Lokshin and
• Vladimir Khodorkovsky

Beilstein J. Org. Chem. 2019, 15, 2059–2068, doi:10.3762/bjoc.15.203

• yellowish color of 5 stems in fact from the admixture of 3. We first envisaged the protonation of 3 in order to isolate the corresponding diiminium salts which should have a stronger electron-withdrawing character (compared to 3). Acidification of 3 in ethyl acetate by an excess of HCl or HBF4 yielded red

• chemistry also reproduce the distortion, albeit to a smaller extent: 1.07 and 0.2 degrees. The frequency calculation reproduces the experimental IR spectrum reasonably well (Supporting Information File 1, Figure S1). The X-ray structure determination of 6a confirms that the protonation occurs at 5,8

• rigid and the molecules can become more or less planar to adopt the constrains of the molecular packing. Thus, the structures of salts 6a and 7a resemble more those of cyanine dyes and the structure of 3 those of merocyanines. The protonation positions of 3 are correctly predicted by the calculations

Genome mining in Trichoderma viride J1-030: discovery and identification of novel sesquiterpene synthase and its products

• Xiang Sun,
• You-Sheng Cai,
• Yujie Yuan,
• Guangkai Bian,
• Ziling Ye,
• Zixin Deng and
• Tiangang Liu

Beilstein J. Org. Chem. 2019, 15, 2052–2058, doi:10.3762/bjoc.15.202

• three types based on their amino acid sequence. Type I TPSs are metal-dependent enzymes that initiate cyclisation by the elimination of diphosphate groups from precursors and carbocation formation, and type II TPSs initiate the catalytic process by the protonation of an olefinic double bond [7]. The

Reactions of 2-carbonyl- and 2-hydroxy(or methoxy)alkyl-substituted benzimidazoles with arenes in the superacid CF₃SO₃H. NMR and DFT studies of dicationic electrophilic species

• Dmitry S. Ryabukhin,
• Alexey N. Turdakov,
• Natalia S. Soldatova,
• Mikhail O. Kompanets,
• Alexander Yu. Ivanov,
• Irina A. Boyarskaya and
• Aleksander V. Vasilyev

Beilstein J. Org. Chem. 2019, 15, 1962–1973, doi:10.3762/bjoc.15.191

• [2]. Thus, the development of further syntheses of benzimidazole derivatives and the study of their properties are important goals for chemistry, medicine and materials science. Since George Olah proposed the concept of generating superelectrophilic intermediates through the protonation by Brønsted

• ][20][21][22][23]. These carbonyl-substituted heteroarenes possess basic sites (nitrogen or oxygen atoms of the heterocyclic system), which are fully protonated in acid, so that upon subsequent protonation of the carbonyl oxygen, more reactive dicationic electrophiles can be generated. Previously

• and Discussion The protonation of formyl and acetylbenzimadazoles 1 and 2 gave N,O-diprotonated species I and II, respectively (see Table 1). Protonation of the hydroxy group of benzimidazoles 3–8 in strong acids gave dicationic species III, V, VII, and VIII, the dehydration of the latter resulted in

Morphology-tunable and pH-responsive supramolecular self-assemblies based on AB₂-type host–guest-conjugated amphiphilic molecules for controlled drug delivery

• Yang Bai,
• Cai-ping Liu,
• Di Chen,
• Long-hai Zhuo,
• Huai-tian Bu and
• Wei Tian

Beilstein J. Org. Chem. 2019, 15, 1925–1932, doi:10.3762/bjoc.15.188

• β-CD at pH 5.0 (Figure 3b). Second, the UV–vis results (Figure 3c) showed that the absorption band at λ = 248 nm decreased and the absorption band at λ = 281, 273 nm shifted to 275, 268 nm when the solution pH was changed from 7.5 to 5.0, owing to the protonation of BM inclusion complexes. In

• addition, the fluorescence spectra indicated that the maximum emission wavelength shifted from 297 nm at pH 7.4 to 370 nm at pH 5.0 (Figure 3d), indicating as well the protonation of BM inclusion complexes. These results further proved that the self-assembly morphology transitions of supramolecular self

Inherent atomic mobility changes in carbocation intermediates during the sesterterpene cyclization cascade

• Hajime Sato,
• Takaaki Mitsuhashi,
• Mami Yamazaki,
• Ikuro Abe and
• Masanobu Uchiyama

Beilstein J. Org. Chem. 2019, 15, 1890–1897, doi:10.3762/bjoc.15.184

• ) triggering the biosynthetic cyclization cascade by the elimination of pyrophosphate or by protonation; (ii) preorganization of the substrate to generate the reactive conformation; (iii) protection of reactive intermediates from water; and (iv) termination of the reaction by deprotonation or hydration. We

The cyclopropylcarbinyl route to γ-silyl carbocations

• Xavier Creary

Beilstein J. Org. Chem. 2019, 15, 1769–1780, doi:10.3762/bjoc.15.170

• cation (a cation containing hypercoordinated carbon) that could be derived from protonation of bicyclobutane [20]. Another potential mode of stabilization is by an interaction of the cationic center with the adjacent strained cyclobutyl bonds as in 7b. A second class of carbocations that this article