Oxetanes: formation, reactivity and total syntheses of natural products

• Peter Gabko,
• Martin Kalník and
• Maroš Bella

Beilstein J. Org. Chem. 2025, 21, 1324–1373, doi:10.3762/bjoc.21.101

• ] Cycloadditions Another widely used method for oxetane synthesis is the [2 + 2] cycloaddition between carbonyls and olefins (Scheme 19), and the two main variations include light-induced Paternò–Büchi reactions and Lewis acid- or base-catalysed formal [2 + 2] cycloadditions. The main advantages of these reactions

• . published a light-induced cross-selective [2 + 2] cycloaddition between 3-(arylmethylidene)oxetanes 159 and electron-deficient alkenes 160 (Scheme 40) [91]. The methodology used a commercially available iridium-based photosensitiser and blue-light irradiation at a slightly elevated temperature. The

Recent advances in synthetic approaches for bioactive cinnamic acid derivatives

• Betty A. Kustiana,
• Galuh Widiyarti and
• Teni Ernawati

Beilstein J. Org. Chem. 2025, 21, 1031–1086, doi:10.3762/bjoc.21.85

• 350–352 in good yields via benzyne intermediate 353 followed by a [2 + 2] cycloaddition with the sulfoxide affording an o-quinone intermediate 354 (Scheme 78) [132]. Furthermore, dioxolane compounds have also been subjected to synthesize cinnamic acid derivatives through CO2 release. For example, Chen

Light-enabled intramolecular [2 + 2] cycloaddition via photoactivation of simple alkenylboronic esters

• Lewis McGhie,
• Hannah M. Kortman,
• Jenna Rumpf,
• Peter H. Seeberger and
• John J. Molloy

Beilstein J. Org. Chem. 2025, 21, 854–863, doi:10.3762/bjoc.21.69

• , and control reactions support sensitization, enabling an intramolecular [2 + 2] cycloaddition to be realized accessing 3D bicyclic fragments containing a boron handle. Keywords: boron; catalysis; [2 + 2] cycloaddition; energy transfer; photochemistry; Introduction The strategic use of a photon to

• reactivity in EnT catalysis has also been intensively pursued (Figure 1B). The controlled geometric isomerization of boron-containing conjugated dienes [42], styrenes [43][44][45] and β-boryl acrylates [46][47] has been established with great effect, while elegant [2 + 2] cycloaddition strategies to readily

• . Herein, we demonstrate the sensitization of alkenylboronates to enable efficient intramolecular [2 + 2] cycloaddition using high energy photosensitizers (Figure 1C). Sensitization was quickly established and explored through the use of alkene scrambling (geometrical isomerization) reaction probes, to

Unprecedented visible light-initiated topochemical [2 + 2] cycloaddition in a functionalized bimane dye

• Metodej Dvoracek,
• Brendan Twamley,
• Mathias O. Senge and
• Mikhail A. Filatov

Beilstein J. Org. Chem. 2025, 21, 500–509, doi:10.3762/bjoc.21.37

• promising strategy for synthesizing substituted cyclobutanes without the need for ultraviolet irradiation. Keywords: bimane; [2 + 2] cycloaddition; fluorescence; topochemical polymerization; X-ray crystallography; Introduction Topochemical polymerizations refer to polymerization reactions occurring in the

• conventions. Although these dyes have been known since 1978, their photochemistry has only been briefly explored. In this work, during the synthesis of a series of these dyes (Figure 2a), we observed an unprecedented intermolecular [2 + 2] cycloaddition of syn-(ethoxycarbonyl,chloro)bimane (Cl2B), seen in

• a [2 + 2] cycloaddition reaction, with approximately 20% yield of the dimer in the crystal. This was particularly notable, given that the compound had spent minimal time in solution with limited exposure to visible light, and no exposure to ultraviolet light. To further explore this phenomenon, we

Photomechanochemistry: harnessing mechanical forces to enhance photochemical reactions

• Francesco Mele,
• Ana M. Constantin,
• Andrea Porcheddu,
• Raimondo Maggi,
• Giovanni Maestri,
• Nicola Della Ca’ and
• Luca Capaldo

Beilstein J. Org. Chem. 2025, 21, 458–472, doi:10.3762/bjoc.21.33

• alignment within the crystal drives the reaction. For example, in 2008, Vittal et al. [62] designed a mechanochemical strategy to align the C=C bonds of 4,4'-bipyridylethylene (bpe, 1.1) molecules to drive efficient [2 + 2] cycloaddition and give the corresponding cyclobutanes (1.2). This approach relied on

• are crucial for product formation. Lastly, the author noted that the inclusion of acetonitrile as a LAG agent enabled the transformation even without the presence of a photocatalyst [72][74]. In 2023, Braunschweig and co-workers reported the photomechanochemical [2 + 2] cycloaddition of acenaphthylene

• + 2] cycloaddition (Scheme 11). Regarding the experimental setup, the authors adapted a commercially available ball mill, by introducing a custom-made stainless steel safety shield to minimize unwanted light leakage. The interior surface of the shield is somewhat reflective. A hole in the milling

Emerging trends in the optimization of organic synthesis through high-throughput tools and machine learning

• Pablo Quijano Velasco,
• Kedar Hippalgaonkar and
• Balamurugan Ramalingam

Beilstein J. Org. Chem. 2025, 21, 10–38, doi:10.3762/bjoc.21.3

• . Photochemical reactions require uniform light penetration of the reaction mixture, and flow setups with uniform path lengths would be ideal for such reactions. A self-optimizing continuous-flow reactor was designed by Poscharny et al. [59] for [2 + 2]-cycloaddition reactions promoted by light. The optimization

Synthesis of acenaphthylene-fused heteroarenes and polyoxygenated benzo[j]fluoranthenes via a Pd-catalyzed Suzuki–Miyaura/C–H arylation cascade

• Merve Yence,
• Dilgam Ahmadli,
• Damla Surmeli,
• Umut Mert Karacaoğlu,
• Sujit Pal and
• Yunus Emre Türkmen

Beilstein J. Org. Chem. 2024, 20, 3290–3298, doi:10.3762/bjoc.20.273

• reaction cascade starting from diarylalkynes 8, which involves indole formation/peri-C–H annulation and N-dealkylation reactions to afford acenaphthylene-fused indole products 9 (Scheme 1b) [41]. Recently, Takeuchi and co-workers reported an effective Ir-catalyzed [2 + 2 + 2] cycloaddition between 1,8

• )-catalyzed [2 + 2 + 2] cycloaddition reactions with 1,8-dialkynylnaphthalenes to access azafluoranthenes and 2-pyridone-fused naphthalenes [27]. In 2017, we reported a Pd-catalyzed cascade reaction that involves a sequential Suzuki–Miyaura cross-coupling and a subsequent intramolecular C–H arylation between

Extension of the π-system of monoaryl-substituted norbornadienes with acetylene bridges: influence on the photochemical conversion and storage of light energy

• Robin Schulte,
• Dustin Schade,
• Thomas Paululat,
• Till J. B. Zähringer,
• Christoph Kerzig and
• Heiko Ihmels

Beilstein J. Org. Chem. 2024, 20, 3061–3068, doi:10.3762/bjoc.20.254

• . Photometric monitoring of the irradiation of 1i (A) and 1l (B) in the presence of [Ru(phen)3](PF6)2; λex = 520 nm. For clarity, the absorption of the sensitizer has been deducted. Insets: plot of absorption at long-wavelength maximum versus irradiation time t [38]. Photoinduced [2 + 2]-cycloaddition

Metal-free double azide addition to strained alkynes of an octadehydrodibenzo[12]annulene derivative with electron-withdrawing substituents

• Naoki Takeda,
• Shuichi Akasaka,
• Susumu Kawauchi and
• Tsuyoshi Michinobu

Beilstein J. Org. Chem. 2024, 20, 2234–2241, doi:10.3762/bjoc.20.191

• oxidation-controlled cyclooctyne-1,2-quinone cycloaddition (SPOCQ) was developed and employed in the same fields of chemical biology [6][7]. On the other hand, the use of the SpAAC in materials science was slow. We developed another class of metal-free click chemistry reactions, such as the [2 + 2

• ] cycloaddition–retroelectrocyclization (CA-RE) between electron-rich alkynes and electron-deficient olefins [8]. The [2 + 2] CA-RE click reactions were employed to produce a variety of functional materials, such as nonlinear optical chromophores [9][10], super acceptors [11][12], ion sensing D–A systems [13

Efficacy of radical reactions of isocyanides with heteroatom radicals in organic synthesis

• Akiya Ogawa and
• Yuki Yamamoto

Beilstein J. Org. Chem. 2024, 20, 2114–2128, doi:10.3762/bjoc.20.182

• lack of geometrical isomerization of the C–N double bond of the dithiolation products [32]. The thioselenation product 6’, an imine derivative, can be converted to a β-lactam derivative by [2 + 2] cycloaddition with ketene generated in situ. For example, thioselenation of RNC (R = (EtO)2P(O)–CH2) gave

• imine 6’ (96%), which underwent [2 + 2] cycloaddition with methoxyketene to afford β-lactam derivative 7 (79%) (Scheme 4). Selective replacement of the PhSe group of 7 with a 3-butanonyl group (34%) and the subsequent intramolecular Horner–Emmons reaction successfully led to carbacephem skeleton 8 (96

Multicomponent syntheses of pyrazoles via (3 + 2)-cyclocondensation and (3 + 2)-cycloaddition key steps

• Ignaz Betcke,
• Alissa C. Götzinger,
• Maryna M. Kornet and
• Thomas J. J. Müller

Beilstein J. Org. Chem. 2024, 20, 2024–2077, doi:10.3762/bjoc.20.178

1,2-Difluoroethylene (HFO-1132): synthesis and chemistry

• Liubov V. Sokolenko,
• Taras M. Sokolenko and
• Yurii L. Yagupolskii

Beilstein J. Org. Chem. 2024, 20, 1955–1966, doi:10.3762/bjoc.20.171

• . Photochemical [2 + 2]-cycloaddition with fluorinated aldehydes and ketones gives access to a variety of fluorinated oxygen-containing heterocycles. We hope that this article will help chemists to utilize HFO-1132 and that this olefin will find applications as a useful synthon in organic chemistry. 1,2

• higher reactivity than (Z)-1,2-difluoroethylene in this reaction. The reaction of either (E)- or (Z)-1,2-difluoroethylene with perfluoroaldehydes resulted in the formation of three isomeric oxetanes in a 1.0:1.7:1.3 ratio in a high yield of 78–94% (Scheme 22) [49]. All data reported for the [2 + 2

• ]-cycloaddition reaction of fluorinated ketones and aldehydes [49][100] were indicative of the fact that under photochemical conditions, this reaction is likely to be a stepwise process involving the formation of a biradical intermediate. Either (Z)- or (E)-1,2-difluoroethylene easily reacted with

Access to 2-oxoazetidine-3-carboxylic acid derivatives via thermal microwave-assisted Wolff rearrangement of 3-diazotetramic acids in the presence of nucleophiles

• Ivan Lyutin,
• Vasilisa Krivovicheva,
• Grigory Kantin and
• Dmitry Dar’in

Beilstein J. Org. Chem. 2024, 20, 1894–1899, doi:10.3762/bjoc.20.164

• reported in the literature are based on the construction of the β-lactam ring (Scheme 1). The main methods include the [2 + 2] cycloaddition of acyl ketenes, generated by various methods, with imines [11][12][13][14] and the Wolff rearrangement of γ-amino-α-diazo-β-ketoesters followed by intramolecular

Synthesis and characterization of 1,2,3,4-naphthalene and anthracene diimides

• Adam D. Bass,
• Daniela Castellanos,
• Xavier A. Calicdan and
• Dennis D. Cao

Beilstein J. Org. Chem. 2024, 20, 1767–1772, doi:10.3762/bjoc.20.155

• acetylenedicarboxylate (DMAD). Although this [2 + 2 + 2] cycloaddition reactivity strategy has been reported under a variety of aryne generation conditions [17][18][19], in our hands we were only able to generate practical amounts of tetraesters 3 using the method reported by Peña et al. [18]. Hydrolysis of 3 with

Novel route to enhance the thermo-optical performance of bicyclic diene photoswitches for solar thermal batteries

• Akanksha Ashok Sangolkar,
• Rama Krishna Kadiyam and
• Ravinder Pawar

Beilstein J. Org. Chem. 2024, 20, 1053–1068, doi:10.3762/bjoc.20.93

• incorporate all desired features within a single photoswitch is a focus of current research [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. The bridged bicyclic diene (BBD)-based NBD/QC couple involves a reversible [2 + 2]-cycloaddition and the chemical transformation during its

(Bio)isosteres of ortho- and meta-substituted benzenes

• H. Erik Diepers and
• Johannes C. L. Walker

Beilstein J. Org. Chem. 2024, 20, 859–890, doi:10.3762/bjoc.20.78

• ) [32]. The synthesis of the corresponding 2-aza-1,5-BCHs 13 was achieved by an intramolecular photochemical [2 + 2] cycloaddition. They were able to employ this synthetic route to synthesise a variety of 1,2-BCPs (±)-14 bearing a protected amine in the bridge position. In early 2023, MacMillan and co

• intramolecular crossed [2 + 2] cycloaddition strategy (Scheme 4C) [36]. They were able to avoid purification by column chromatography by transformation of BCH ester (±)-53 into BCH acid (±)-54, allowing isolation of pure 1,2-BCH (±)-54 by crystallisation. Through their synthesis they were able to access both

• -workers adopted a similar intramolecular [2 + 2] cycloaddition using benzophenone as a triplet sensitizer to access 1,5-BCHs (±)-59a–e as racemic mixtures of endo:exo diastereomers (Scheme 5A) [42]. Basic hydrolysis of the ester moiety followed by recrystallisation gave the diasteromerically pure acid

Enhanced reactivity of Li⁺@C₆₀ toward thermal [2 + 2] cycloaddition by encapsulated Li⁺ Lewis acid

• Hiroshi Ueno,
• Yu Yamazaki,
• Hiroshi Okada,
• Fuminori Misaizu,
• Ken Kokubo and
• Hidehiro Sakurai

Beilstein J. Org. Chem. 2024, 20, 653–660, doi:10.3762/bjoc.20.58

• derivatization of this novel material. In this study, we report the synthesis of Li+@C60 derivatives via the thermal [2 + 2] cycloaddition reaction of styrene derivatives, achieving significantly higher yields of monofunctionalized Li+@C60 compared to previously reported reactions. Furthermore, by combining

• experimental and theoretical approaches, we clarified the range of applicable substrates for the thermal [2 + 2] cycloaddition of Li+@C60, highlighting the expanded scope of this straightforward and selective functionalization method. Keywords: electron transfer; fullerene; ion-endohedral fullerene; Lewis

• acid catalyst; thermal [2 + 2] cycloaddition; Introduction Chemical functionalization of fullerenes is a fascinating and extensively studied approach, playing a pivotal role in fullerene-based materials science to introduce various characteristic functionalities [1][2][3][4][5][6][7]. Significant

Unveiling the regioselectivity of rhodium(I)-catalyzed [2 + 2 + 2] cycloaddition reactions for open-cage C₇₀ production

• Cristina Castanyer,
• Anna Pla-Quintana,
• Anna Roglans,
• Albert Artigas and
• Miquel Solà

Beilstein J. Org. Chem. 2024, 20, 272–279, doi:10.3762/bjoc.20.28

• holds significant promise for applications in the fields of medicinal chemistry, materials science, and photovoltaics. In this study, we investigate the regioselectivity of the rhodium(I)-catalyzed [2 + 2 + 2] cycloaddition reactions between diynes and C70 as a novel procedure for generating C70 bis

• to be able to produce meaningful changes in the reactivity of the cage [28][29][30][31][32]. In 2018, our group reported a catalytic process to transform C60 in bis(fulleroid) derivatives [33][34][35]. This transformation encompassed a partially intermolecular Rh-catalyzed [2 + 2 + 2] cycloaddition

• unexpectedly, the [6,6]-bond in C60 was the one involved in this [2 + 2 + 2] cycloaddition. Although there are several papers reporting the opening of a hole in C70 [39][40][41][42][43][44][45], this chemistry has been less explored than in C60. The lower D5h symmetry of C70 compared to the Ih of C60 increases

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

• Michio Yamada Department of Chemistry, Tokyo Gakugei University, Nukuikitamachi 4-1-1, Koganei, Tokyo 184-8501, Japan 10.3762/bjoc.20.13 Abstract Various push–pull chromophores can be synthesized in a single and atom-economical step through [2 + 2] cycloaddition–retroelectrocyclization (CA–RE

• of click-type [2 + 2] cycloaddition (CA)–retroelectrocyclization (RE) reactions, offering means for preparing these coveted push–pull chromophores [7]. A review conducted in 2023 comprehensively explored the optoelectronic properties of TCBDs along with their photovoltaic applications [8]. The

• reactions. In these reactions, the nucleophilic attack of the alkyne carbon of 1 occurs at the C(1) carbon of 6. When the formal [2 + 2] cycloaddition, as delineated in path A, occurs for the zwitterionic intermediate featuring the 1,1,3-tricyanoallyl anion obtained through the nucleophilic attack

Electron-beam-promoted fullerene dimerization in nanotubes: insights from DFT computations

• Laura Abella,
• Gerard Novell-Leruth,
• Josep M. Ricart,
• Josep M. Poblet and
• Antonio Rodríguez-Fortea

Beilstein J. Org. Chem. 2024, 20, 92–100, doi:10.3762/bjoc.20.10

• the reaction either via singlet excitation or via radical cation formation (Scheme 1). Estimation of the activation barrier for the [2 + 2] cycloaddition when the nanotube acts as a sensitizer is 33.5 ± 6.8 kJ mol−1. This value agrees with computational predictions for the reaction via an excited

• analyzed the interaction between C60 and the nanotube within the peapod. Next, we have found that some dimeric C60–C60 fullerene structures inside the carbon nanotube are thermodynamically favorable. Experiments indicate that, besides C60 sensitization via a singlet excited state, the [2 + 2] cycloaddition

• + 2] cycloaddition in phase 1 We analyzed in detail the energy profile for the first step in the dimerization process, that is, the reversible [2 + 2] cycloaddition to obtain dimers 1-Cs and 1-D2h. We initially considered dimerization in the gas phase to check the reliability of our methodology

Biphenylene-containing polycyclic conjugated compounds

• Cagatay Dengiz

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

• ] cycloaddition [19][20], [2 + 2 + 2] cycloaddition [21], and the Ullmann reaction [15][22] (Scheme 1). Due to the observed low yields in flash vacuum pyrolysis, the difficulty in synthesizing starting materials, such as 3, and the impractical nature of scaling up the method for large quantities, the other three

• -catalyzed formal [2 + 2] cycloaddition reactions but also the emergence of tetracene trimer 86 and tetramer 88 stemming from [2 + 2 + 2] cycloaddition reactions (Scheme 18). It is proposed that an aryne intermediate is formed after thermal activation and that the observed end products are formed from arynes

• resulting from a [2 + 2] cycloaddition and trimer 90 formed via a [2 + 2 + 2] cycloaddition pathway were observed. In contrast, when applied to Au(100), only the dimer structure 91 was generated through the [2 + 2] cycloaddition process. In a recent study in this field, Izydorczyk et al. were able to

The unique reactivity of 5,6-unsubstituted 1,4-dihydropyridine in the Huisgen 1,4-diploar cycloaddition and formal [2 + 2] cycloaddition

• Xiu-Yu Chen,
• Hui Zheng,
• Ying Han,
• Jing Sun and
• Chao-Guo Yan

Beilstein J. Org. Chem. 2023, 19, 982–990, doi:10.3762/bjoc.19.73

• -dihydropyridines in acetonitrile at room temperature afforded functionalized isoquinolino[1,2-f][1,6]naphthyridines in good yields and with high diastereoselectivity. More importantly, the formal [2 + 2] cycloaddition reaction of dialkyl acetylenedicarboxylates and 5,6-unsubstituted 1,4-dihydropyridines in

• refluxing acetonitrile gave unique 2-azabicyclo[4.2.0]octa-3,7-dienes as major products and 1,3a,4,6a-tetrahydrocyclopenta[b]pyrroles as minor products via further rearrangement. Keywords: 1,4-dihydropyridine; electron-withdrawing alkyne; formal [2 + 2] cycloaddition; Huisgen's 1,4-dipole; isoquinoline

• acetylenedicarboxylate and 5,6-unsubstituted 1,4-dihydropyridines with N–Ar groups did not give the above isoquinolino[1,2-f][1,6]naphthyridines, but the unique 2-azabicyclo[4.2.0]octa-3,7-diene-7,8-dicarboxylates were isolated in moderate yields. These products were obviously produced from the formal [2 + 2

Strategies in the synthesis of dibenzo[b,f]heteropines

• David I. H. Maier,
• Barend C. B. Bezuidenhoudt and
• Charlene Marais

Beilstein J. Org. Chem. 2023, 19, 700–718, doi:10.3762/bjoc.19.51

• ) chloride (Scheme 26). Alkyne–carbonyl metathesis is proposed to proceed via [2 + 2] cycloaddition and –reversion steps, catalysed by a Brønsted or Lewis acid, with the catalyst proposed to form a σ-complex with the carbonyl group and/or a π-complex with the alkyne [68]. 3.7 Hydroarylation The construction

Strategies to access the [5-8] bicyclic core encountered in the sesquiterpene, diterpene and sesterterpene series

• Cécile Alleman,
• Charlène Gadais,
• Laurent Legentil and
• François-Hugues Porée

Beilstein J. Org. Chem. 2023, 19, 245–281, doi:10.3762/bjoc.19.23

Germacrene B – a central intermediate in sesquiterpene biosynthesis

• Houchao Xu and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2023, 19, 186–203, doi:10.3762/bjoc.19.18

• acetone and water to a hemiacetal that can decompose to 11 (Scheme 4B) [43]. Furthermore, 1 shows an interesting photochemistry (Scheme 4C). A [2 + 2] cycloaddition of the endocyclic double bonds yields 12 whose formation is understandable from conformers 1c and 1d. The all-cis stereoisomer 14 requires a

• photochemical E/Z isomerisation to 13 prior to [2 + 2] cycloaddition. Further photochemical products from 1 include 5, 15 that may be formed through a biradical mechanism, and rearranged 16 [51]. Germacrene B (1) has planar chirality (Scheme 4D), but recovery of the starting material from an incomplete