Perspectives on push–pull chromophores derived from click-type [2 + 2] cycloaddition–retroelectrocyclization reactions of electron-rich alkynes and electron-deficient alkenes

• Michio Yamada

Beilstein J. Org. Chem. 2024, 20, 125–154, doi:10.3762/bjoc.20.13

• -tetracyano-p-quinodimethane (TCNQ), results in the formation of a zwitterionic intermediate, wherein the negative charge evolves into a stabilized carbanion (dicyanomethide anion). Subsequently, the ring closure of the zwitterionic intermediate generates the corresponding cyclobutene intermediate. Finally

• also performed theoretical calculations, unveiling that the ring-opening step presented a higher energy barrier compared with the ring-formation step, particularly when the DCV compounds lacked aryl substituents. Subsequently, they successfully isolated the corresponding cyclobutene intermediate 13

• charge-transfer complex AB. Subsequently, this complex forms a zwitterion intermediate (C1), followed by the formation of a cyclobutene intermediate (C2), ultimately forming the product P. However, the researchers strongly advocated for an alternative pathway, which entails the formation of a complex

Biphenylene-containing polycyclic conjugated compounds

• Cagatay Dengiz

Beilstein J. Org. Chem. 2023, 19, 1895–1911, doi:10.3762/bjoc.19.141

• –Alder reactions (Scheme 5). Furthermore, the researchers conducted a comprehensive investigation into the optical and electrochemical characteristics of these compounds. The key component employed in the production process, known as 3,4-bis(methylene)cyclobutene, was generated through the application of

• flash vacuum pyrolysis to 1,5-hexadiyne. When 3,4-bis(methylene)cyclobutene undergoes a reaction with dienes like 1,3-diphenylisobenzofuran and 1,3-diphenylisonaphthofuran, it selectively produces compounds 22a and 22b. In the subsequent step, the exocyclic methylidene groups react with bisarynes, which

Radical chemistry in polymer science: an overview and recent advances

• Zixiao Wang,
• Feichen Cui,
• Yang Sui and
• Jiajun Yan

Beilstein J. Org. Chem. 2023, 19, 1580–1603, doi:10.3762/bjoc.19.116

• monomer to generate a cyclobutene radical cation intermediate. The thermodynamically instable intermediate subsequently forms the propagating radical cation species via a ring-opening process. The reduced PC•− terminates the catalytic loop by reducing the propagating species to provide a polymer chain

Enolates ambushed – asymmetric tandem conjugate addition and subsequent enolate trapping with conventional and less traditional electrophiles

• Péter Kisszékelyi and
• Radovan Šebesta

Beilstein J. Org. Chem. 2023, 19, 593–634, doi:10.3762/bjoc.19.44

• stereoselective conjugate borylation of cyclobutene 1-carboxyester 175 (Scheme 45A) [87]. As a result, the cis-β-boronyl cyclobutylcarboxyester 176 was prepared on a gram scale with 80% yield and an excellent 99% ee (dr >20:1). Subsequent transformation to the corresponding trifluoroborate salt 177 resulted in a

Combining the best of both worlds: radical-based divergent total synthesis

• Kyriaki Gennaiou,
• Antonios Kelesidis,
• Maria Kourgiantaki and
• Alexandros L. Zografos

Beilstein J. Org. Chem. 2023, 19, 1–26, doi:10.3762/bjoc.19.1

• quenchers (e.g., PhSSPh) to the reaction mixture. When monosubstitution of the aryl group is present, the formed radical cation, the product of the photooxidation of the cinnamyl ether, readily cyclizes to cyclobutene radical cation 217. The latter cleaves the benzylic C–C bond to produce the 1,4-radical

• cation cis-218. On the other hand, when polysubstitution with methoxy groups is present, the cation in 216 is delocalized, inhibiting the production of cyclobutene 217. Thus, radical cyclization according to the Beckwith–Houk model [108][109] via transition states TSI and TSII would take place, leading

Menadione: a platform and a target to valuable compounds synthesis

• Acácio S. de Souza,
• Ruan Carlos B. Ribeiro,
• Dora C. S. Costa,
• Fernanda P. Pauli,
• David R. Pinho,
• Matheus G. de Moraes,
• Fernando de C. da Silva,
• Luana da S. M. Forezi and
• Vitor F. Ferreira

Beilstein J. Org. Chem. 2022, 18, 381–419, doi:10.3762/bjoc.18.43

• -LED light [111]. Under these conditions, menadione (10) and terminal alkynes 66 underwent a [2 + 2] cycloaddition reaction generating compounds containing cyclobutene rings (67a–c), that are important precursors in natural products syntheses. It is important to note that the choice of the blue-LED

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

• cyclobutene and several cationic intermediates and mainly yield 2-(1-phenylvinyl)indenes. In a similar reaction cascade, a fulvene derivative was obtained with 1,4-diphenylbutadiene as the substrate. Keywords: alkynes; aromatic substitution; cyclization; cycloaddition; iminium salts; Introduction In recent

• unpurified 1,4-cyclohexadien-1-iminium salt 4-Ch and 1,4-cyclohexadien-1-yl ketone 5-Ch indicated the presence of a minor byproduct. In the case of 5-Ch, obtained as an oil, the two components could not be separated by column chromatography; however, the 1H NMR spectrum suggested the cyclobutene structure 5

• presumably renders an ionic [2 + 2] cycloaddition pathway competitive with the Diels–Alder reaction. The few reported examples of cyclobutene formation from alkynes and unactivated 1,3-dienes include the sensitized photocycloaddition of phenylacetylene and DMBD [30] and the gold(I)-catalyzed reaction of

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

• chiral cyclobutene 180, providing highly functionalized cyclobutanes 182 and an access to the enantioenriched cyanosulfones 183, resulting from cyclobutane ring opening, or the new tetrasubstituted cyclobutanes 184 (Scheme 42). The cyclobutenes 180 were subjected to a photocatalyzed radical addition, and

• radical reenters the catalytic cycle. Various alkyl substituents on the cyclobutene ring, such as substituents bearing a chlorine atom, or silyl ethers were well-tolerated. However, bulky groups (e.g., TBDMS) led to lower yields. Methyl, ethyl, or aryl sulfones were all compatible with this methodology

The cyclopropylcarbinyl route to γ-silyl carbocations

• Xavier Creary

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

• (Scheme 8), along with a small amount of cyclobutene (35). The rate of 31 (Table 1) is not substantially enhanced relative to the unsubstituted cyclopropylcarbinyl mesylate (33). The small rate enhancement factor of 3.56 is consistent with a small inductive stabilization of the initially formed cationic

• bond c to the cationic center gives rearranged cation 44, the source of the rearranged acetate 39. Ring expansion via migration of bond b in 43 gives the β-trimethylsilyl-stabilized cyclobutyl cation 45, and subsequent desilylation provides cyclobutene (35). Alternatively, cyclobutyl to homoallylic

Oxidative radical ring-opening/cyclization of cyclopropane derivatives

• Yu Liu,
• Qiao-Lin Wang,
• Zan Chen,
• Cong-Shan Zhou,
• Bi-Quan Xiong,
• Pan-Liang Zhang,
• Chang-An Yang and
• Quan Zhou

Beilstein J. Org. Chem. 2019, 15, 256–278, doi:10.3762/bjoc.15.23

• diphenyl diselenides 12 for the synthesis of 2-phenylseleny-3,3-diarylcyclobutenes 13 under visible light irradiation (Scheme 5) [51]. The desired products 13 contained a cyclobutene group and a selenium atom, which makes the products possess unique biological and pharmaceutical activities. The mechanism

Selective ring-opening metathesis polymerization (ROMP) of cyclobutenes. Unsymmetrical ladderphane containing polycyclobutene and polynorbornene strands

• Yuan-Zhen Ke,
• Shou-Ling Huang,
• Guoqiao Lai and
• Tien-Yau Luh

Beilstein J. Org. Chem. 2019, 15, 44–51, doi:10.3762/bjoc.15.4

• Technology of Ministry of Education, Hangzhou Normal University, Hangzhou, Zhejiang 311121, China 10.3762/bjoc.15.4 Abstract At 0 °C in THF in the presence of Grubbs first generation catalyst, cyclobutene derivatives undergo ROMP readily, whereas norbornene derivatives remain intact. When the substrate

• contains both cyclobutene and norbornene moieties, the conditions using THF as the solvent at 0 °C offer a useful protocol for the selective ROMP of cyclobutene to give norbornene-appended polycyclobutene. Unsymmetrical ladderphane having polycyclobutene and polynorbornene as two strands is obtained by

• strand is 8:1. Keywords: cyclobutene; hydrolysis; linker; metathesis; norbornene; ROMP; selectivity; unsymmetrical ladderphane; Introduction Ring-opening metathesis polymerizations (ROMP) of strained cycloalkenes offer a powerful arsenal for the synthesis of polymers having a variety of fascinating

Synthesis and supramolecular self-assembly of glutamic acid-based squaramides

• Juan V. Alegre-Requena,
• Marleen Häring,
• Isaac G. Sonsona,
• Alex Abramov,
• Eugenia Marqués-López,
• Raquel P. Herrera and
• David Díaz Díaz

Beilstein J. Org. Chem. 2018, 14, 2065–2073, doi:10.3762/bjoc.14.180

• mixture of 3,4-dimethoxy-3-cyclobutene-1,2-dione (5, 2.4 g, 16.5 mmol) in MeOH (30 mL) at room temperature. After 24 h, the solvent was removed under vacuum and the product was purified by column chromatography (SiO2, hexane/EtOAc 7:3 to hexane/EtOAc 1:1). Product 7 was obtained as a brown oil in 95

One hundred years of benzotropone chemistry

• Arif Dastan,
• Haydar Kilic and
• Nurullah Saracoglu

Beilstein J. Org. Chem. 2018, 14, 1120–1180, doi:10.3762/bjoc.14.98

Vinylphosphonium and 2-aminovinylphosphonium salts – preparation and applications in organic synthesis

• Anna Kuźnik,
• Roman Mazurkiewicz and
• Beata Fryczkowska

Beilstein J. Org. Chem. 2017, 13, 2710–2738, doi:10.3762/bjoc.13.269

• acetoacetanilide with DAAD and triphenylphosphine. The obtained ylides 87 were first converted to cyclobutene derivatives 88 via the stereoselective intramolecular Wittig reaction. The resulting strained, four-membered cyclobutene derivatives underwent electrocyclic ring opening at the boiling point of toluene to

• -(1,2-dihydroquinoline-2-ylidene)pyruvate (91), Yavari and co-workers obtained 4-(2-quinolyl)cyclobutene-1,2,3-tricarboxylic acid triesters 94 and isomeric cyclopentenone derivatives 95 with total yields of 87–94% and a ratio of 1:4 via the intramolecular Wittig reaction. The addition of

• acetylenedicarboxylate, triphenylphosphine, and ethyl 4-aryl-2,4-dioxobutanoates. Synthesis of 4-(2-quinolyl)cyclobutene-1,2,3-tricarboxylic acid triesters and isomeric cyclopentenone derivatives via resonance-stabilized ylides in the intramolecular Wittig reaction. Synthesis of 4-arylquinolines via resonance-stabilized

Synthesis of Xenia diterpenoids and related metabolites isolated from marine organisms

• Tatjana Huber,
• Lara Weisheit and
• Thomas Magauer

Beilstein J. Org. Chem. 2015, 11, 2521–2539, doi:10.3762/bjoc.11.273

• of ethyl (phenylsulfonyl)acetate, a methylsulfonyl anion equivalent, to cyclobutene ester 49 followed by a sequence consisting of saponification, regioselective decarboxylation and reesterification to afford methyl ester 50. The ester group was reduced with lithium aluminum hydride and the resulting

Recent applications of ring-rearrangement metathesis in organic synthesis

• Sambasivarao Kotha,
• Milind Meshram,
• Priti Khedkar,
• Shaibal Banerjee and
• Deepak Deodhar

Beilstein J. Org. Chem. 2015, 11, 1833–1864, doi:10.3762/bjoc.11.199

• cyclopropenylcarbinyl ether 33 with catalyst 2 (Scheme 4). Cyclobutene systems Cyclobutene is also highly strained and prone to RRM very easily. Maougal and co-workers synthesized 3,3’-bipiperidine and 3,3’-bis(1,2,3,6-tetrahydropyridine) systems through a RRM sequence [9]. In this context, they have identified

• and White [10] have reported a new and efficient method to various medium size bicyclic systems. Here, the RRM strategy has been employed with catalyst 2 starting with various cyclobutene systems containing an alkene tether (e.g., 40, 42, and 44) to generate bicyclic systems such as 41, 43, and 45

• assemble the erythrina skeleton. To this end, they have identified cyclobutene derivative 48 as a useful synthone for RRM. The cyclobutene derivative 46 has been extended via Grignard addition followed by cyclization reaction. Later, cyclobutene derivative 48 was treated with catalyst 1 in the presence of

Anionic sigmatropic-electrocyclic-Chugaev cascades: accessing 12-aryl-5-(methylthiocarbonylthio)tetracenes and a related anthra[2,3-b]thiophene

• Laurence Burroughs,
• John Ritchie,
• Mkhethwa Ngwenya,
• Dilfaraz Khan,
• William Lewis and
• Simon Woodward

Beilstein J. Org. Chem. 2015, 11, 273–279, doi:10.3762/bjoc.11.31

• transformation. All attempts to isolate 14 resulted only in the rapid formation of red oils whose mass spectra show molecular ions at (14)n (n = 1–3). Attempted direct crystallisation provided only trace amounts of cyclobutene 15 which is otherwise unstable in solution (see Supporting Information File 1). Its

Synthesis of nanodiamond derivatives carrying amino functions and quantification by a modified Kaiser test

• Gerald Jarre,
• Steffen Heyer,
• Elisabeth Memmel,
• Thomas Meinhardt and
• Anke Krueger

Beilstein J. Org. Chem. 2014, 10, 2729–2737, doi:10.3762/bjoc.10.288

• . This system has been successfully used for the functionalization of fullerenes [15]. The precursor 8 was synthesized according to Martinez et al. starting from cyclobutanone and methyl thiocyanate [16]. The cyclobutene ring opens at temperatures around 190 °C and the ortho-quinodimethane is formed in

• surface groups accessible by the reagents will be measured using this wet-chemical method. Conclusion In summary, we have established two new functionalization methods for the covalent grafting of organic moieties onto nanodiamond using pyrazine and cyclobutene precursors for the in situ formation of

One-pot three-component synthesis and photophysical characteristics of novel triene merocyanines

• Christian Muschelknautz,
• Robin Visse,
• Jan Nordmann and
• Thomas J. J. Müller

Beilstein J. Org. Chem. 2014, 10, 599–612, doi:10.3762/bjoc.10.51

• of the merocyanines 8 begins with a 1,4-dipolar cyclization of 13 furnishing the highly substituted cyclobutene intermediate 14. Finally, the conrotatory electrocyclic ring opening of the cyclobutene occurs under thermodynamic control, which is obviously only governed by steric effects as reflected

• supported by the fact that only methyl-substituted ynylideneindolone intermediates 11 enable the cyclobutene pathway, whereas all N-tosyl derivatives exclusively give the merocyanines 10 via the allenyl enol pathway. All other remote substituents on the ynylideneindolone intermediates 11 do not influence

• , on the density functional level of theory (B3LYP functional, 6-31G* basis set) [40]. For a stepwise cyclobutene formation via 1,4-dipolar cyclization the negative partial charge on the central allenyl carbon atom 6 should be relatively high. The computations of the electrostatic charges for an N

Ring opening of 2-aza-3-borabicyclo[2.2.0]hex-5-ene, the Dewar form of 1,2-dihydro-1,2-azaborine: stepwise versus concerted mechanisms

• Holger F. Bettinger and
• Otto Hauler

Beilstein J. Org. Chem. 2013, 9, 761–766, doi:10.3762/bjoc.9.86

• conrotatory electrocyclic opening of one of the cyclobutene moieties of 1, on the other hand, would result in a highly strained cis,cis,trans-cyclohexa-1,3,5-triene (trans-benzene) isomer [10][11]. Computational investigations of the isomerisation have been performed to reveal mechanistic details [12][13][14

Flow photochemistry: Old light through new windows

• Jonathan P. Knowles,
• Luke D. Elliott and
• Kevin I. Booker-Milburn

Beilstein J. Org. Chem. 2012, 8, 2025–2052, doi:10.3762/bjoc.8.229

• 85 g of isolated cyclobutene product 77. A Vycor reactor, driven by a 600 W lamp gave an 83% conversion when a 0.4 M solution was passed through at 8 mL/min. This corresponds to a productivity of 159 mmol/h, which if run over the same 24 hour period would yield 685 g of product. The higher

The chemistry of bisallenes

• Henning Hopf and
• Georgios Markopoulos

Beilstein J. Org. Chem. 2012, 8, 1936–1998, doi:10.3762/bjoc.8.225

Synthesis and in silico screening of a library of β-carboline-containing compounds

• Kay M. Brummond,
• John R. Goodell,
• Matthew G. LaPorte,
• Lirong Wang and
• Xiang-Qun Xie

Beilstein J. Org. Chem. 2012, 8, 1048–1058, doi:10.3762/bjoc.8.117

• tosylates was not successful. Next, compounds possessing the cyclobutene-fused β-carboline skeleton were assembled from the versatile allenyl intermediate 6. For this subset of compounds, acylation of the sterically hindered amine of 6{1} with the ynoic acids 8{1–8}, by using bromo-tris-pyrrolidino

• the reaction temperature from 225 °C to 160 °C afforded cyclobutene 2{1,3} in 73% yield (Table 2, entry 3). Similarly, allene-yne 9{1,5} was subjected to the lower reaction temperature (Method A) to produce cyclobutene 2{1,5} in 57% yield (Table 2, entry 5). For the final library scaffold, a small

Fine-tuning alkyne cycloadditions: Insights into photochemistry responsible for the double-strand DNA cleavage via structural perturbations in diaryl alkyne conjugates

• Wang-Yong Yang,
• Samantha A. Marrone,
• Nalisha Minors,
• Diego A. R. Zorio and
• Igor V. Alabugin

Beilstein J. Org. Chem. 2011, 7, 813–823, doi:10.3762/bjoc.7.93

• )-OH/pyridine; f. HCl(g)/MeOH. Photochemical reactions of TFP-substituted aryl alkynes with selected π-systems. In short, the reaction proceeds through the photoinduced electron transfer from thiophene to the singlet excited state of the diaryl acetylene. The initially formed cyclobutene product

Complete transfer of chirality in an intramolecular, thermal [2 + 2] cycloaddition of allene-ynes to form non-racemic spirooxindoles

• Kay M. Brummond and
• Joshua M. Osbourn

Beilstein J. Org. Chem. 2011, 7, 601–605, doi:10.3762/bjoc.7.70

• oxindoles into chiral non-racemic spirooxindoles containing an alkylidene cyclobutene moiety. The enantiomeric excesses were determined by chiral lanthanide shift NMR analysis and the transfer of chiral information from the allene to the spirooxindole was found to be greater than 95%. Keywords: alkylidene

• cyclobutene; allene; allenyloxindole; chiral lanthanide shift reagent; chiral transfer; Introduction The [2 + 2] cycloaddition reaction of allenes and alkynes provides rapid entry into synthetically challenging alkylidene cyclobutene ring systems. We, along with others, have demonstrated the intramolecular

• . Based on this analysis, the product was formed with greater than 95% enantiomeric excess (Figure 3). Our working hypothesis for the mechanism for transfer of chiral information from the allene to the spirooxindole-containing cyclobutene is that the reaction still proceeds through the thermally generated