Ring contraction and ring expansion reactions in terpenoid biosynthesis and their application to total synthesis

• Nicolas Kratena,
• Nicolas Heinzig and
• Peter Gärtner

Beilstein J. Org. Chem. 2026, 22, 289–343, doi:10.3762/bjoc.22.21

• provide the essential carbocation stabilisation (likely via cation–π interactions) and thus facilitate alkyl migration. The resulting 5,7-bicyclic carbocation 14b is further elaborated and decorated by selective oxidation reactions to build up the family of pseudolaric acids. For example, after a second

On the aromaticity and photophysics of 1-arylbenzo[a]imidazo[5,1,2-cd]indolizines as bicolor fluorescent molecules for barium tagging in the study of double-beta decay of ¹³⁶Xe

• Eric Iván Velazco-Cabral,
• Fernando Auria-Luna,
• Juan Molina-Canteras,
• Miguel A. Vázquez,
• Iván Rivilla and
• Fernando P. Cossío

Beilstein J. Org. Chem. 2025, 21, 1627–1638, doi:10.3762/bjoc.21.126

• fluorescent moieties constitute promising candidates to detect Ba2+ cations [8][9] (Figure 2). Another essential component is an aza-crown ether of appropriate dimensions to capture the barium cation. In addition, one para-disubstituted phenyl (or aryl) group is installed to generate selective cation–π

• interactions. Finally, a spacer (denoted as X and Y in Figure 2) and a linker (denoted as Z) to anchor the sensor to a suitable surface via a covalent interaction are required. Ideally, different configurations and conformations of the fluorophore in the free and chelated states would result in a bicolor

Binding of tryptophan and tryptophan-containing peptides in water by a glucose naphtho crown ether

• Gianpaolo Gallo and
• Bartosz Lewandowski

Beilstein J. Org. Chem. 2025, 21, 541–546, doi:10.3762/bjoc.21.42

• moiety in the amino acid. This hypothesis is also supported by the significant quenching of the fluorescence of the naphthalene unit in 1 upon addition of increasing amounts of H-Trp-OH to the solution. Additionally, cation–π interactions between the ammonium cation of the guest and the naphthalene of

Synthesis and characterization of water-soluble C₆₀–peptide conjugates

• Yue Ma,
• Lorenzo Persi and
• Yoko Yamakoshi

Beilstein J. Org. Chem. 2024, 20, 777–786, doi:10.3762/bjoc.20.71

• ) showed much smaller aggregation (dotted green line, ≈12 nm), providing a transparent solution, while C60–oligo-Arg (5c) remained insoluble over the tested pH value range (4.0–9.2). This was presumably due to the strong cation–π interactions between the cationic Arg moieties and the aromatic C60, which is

Anion–π catalysis on carbon allotropes

• M. Ángeles Gutiérrez López,
• Mei-Ling Tan,
• Giacomo Renno,
• Augustina Jozeliūnaitė,
• J. Jonathan Nué-Martinez,
• Javier Lopez-Andarias,
• Naomi Sakai and
• Stefan Matile

Beilstein J. Org. Chem. 2023, 19, 1881–1894, doi:10.3762/bjoc.19.140

• available so far on anion–π catalysis on carbon allotropes. Review Anion–π catalysis on fullerenes The use of fullerenes in catalysis is surprisingly underdeveloped [45][46][47][48][49][50][51]. Anion–π and cation–π interactions on fullerenes attract similarly little attention until today [52][53][54][55

• intermediate XIV [24][25]. Thus, this reaction can be considered as the anion–π catalysis counterpart of the steroid cyclizations catalyzed in nature with the charge inverted, conventional cation–π interactions [15]. Anion–π catalysis on fullerene dimers The strength of anion–π interactions increases with face

• strong anion–π and cation–π interactions [112][113][114][115] and accelerate and direct the electron displacement during the reaction. Possible limitations at high concentrations in suspension from catalyst depolarization by multiple binding (Figure 1) naturally do not apply to permanent polarization by

Unraveling the role of prenyl side-chain interactions in stabilizing the secondary carbocation in the biosynthesis of variexenol B

• Moe Nakano,
• Rintaro Gemma and
• Hajime Sato

Beilstein J. Org. Chem. 2023, 19, 1503–1510, doi:10.3762/bjoc.19.107

• of the neighboring prenyl side chain interacts and promptly induce C–C bond formation. There have been no reports published where, as in our case, the cation is stabilized without bond formation. We have also considered other transannular cation–π interactions in this system. In this case, the

• gets close, it would easily form a C–C bond. Therefore, we believe that no other transannular cation–π interactions need to be considered in this system. In systems without cation–π interactions, such as in the biosynthesis of variediene [39] and spiroviolene [40], bonds around the secondary

• carbocation are strongly influenced by hyperconjugation. In particular, C–C bonds containing a secondary cation are shortened to about 1.45 Å, showing a slight double bond character. On the other hand, in intermediates such as IM2a and IM2b, which have cation–π interactions, the surrounding bonds are hardly

Phenanthridine–pyrene conjugates as fluorescent probes for DNA/RNA and an inactive mutant of dipeptidyl peptidase enzyme

• Josipa Matić,
• Tana Tandarić,
• Marijana Radić Stojković,
• Filip Šupljika,
• Zrinka Karačić,
• Ana Tomašić Paić,
• Lucija Horvat,
• Robert Vianello and
• Lidija-Marija Tumir

Beilstein J. Org. Chem. 2023, 19, 550–565, doi:10.3762/bjoc.19.40

• further promoted by favorable cation–π interactions. In other words, in Phen-Py-1+ and Phen-Py-2+, stacked structures account for around 96% and 98% of population, respectively, while in unionized Phen-Py-1 and Phen-Py-2 these cluster around 84% in both cases (Figure 2). This is further supported by

• that a higher degree of aromatic surfaces overlapping and cation–π interactions also yield excimer fluorescence quenching [30][32][33]. This conclusion is strongly in line with experimental insight reported here and helps in explaining the observed excimer fluorescence quenching with a decrease in the

• + and Phen-Py-2+, where π–π stacking contacts are further promoted by the favorable cation–π interactions. Phenanthridine–pyrene conjugate Phen-Py-1 showed excimer fluorescence that was red shifted compared to the emission of a single phenanthridine or pyrene chromophore. This excimer fluorescence was

A resorcin[4]arene hexameric capsule as a supramolecular catalyst in elimination and isomerization reactions

• Tommaso Lorenzetto,
• Fabrizio Fabris and
• Alessandro Scarso

Beilstein J. Org. Chem. 2022, 18, 337–349, doi:10.3762/bjoc.18.38

• 1375 Å3 that is accessible [23] for cationic guests [24] thanks to extended cation–π interactions [25] as well as other electron poor molecules [26][27]. After our seminal work on the use of 16 as a nanoreactor to bind an Au(I) catalyst and to impart unique product [28] as well as substrate [29

• all these reactions is the combination of its weak Brønsted acidity [42] that, by protonation of the substrate, leads to the formation of cationic species [43] and the stabilization of the latter through cation–π interactions [44] within the electron-rich aromatic cavity of the capsule, thus providing

Site-selective reactions mediated by molecular containers

• Rui Wang and
• Yang Yu

Beilstein J. Org. Chem. 2022, 18, 309–324, doi:10.3762/bjoc.18.35

• demonstrated a relatively high affinity towards cationic guests through cation–π interactions, which was crucial for the catalysis of many of the organic reactions. And similarly, the ionic form of the host made it water-soluble and reactions could be conducted in water. In this particular example, the

Halides as versatile anions in asymmetric anion-binding organocatalysis

• Lukas Schifferer,
• Martin Stinglhamer,
• Kirandeep Kaur and
• Olga García Macheño

Beilstein J. Org. Chem. 2021, 17, 2270–2286, doi:10.3762/bjoc.17.145

• the seleniranium ring. Through favorable cation–π interactions with the catalyst, the (S,S)-intermediate reacts faster than its opposing enantiomer, allowing for excellent yields up to 95% and high enantioselectivities up to 91% ee. In contrast to the previous example, in which the chloride anion was

• with aromatic residues of the enzymes. In fact, these additional stabilizing effects can be exploited in the design of more effective noncovalent catalytic structures for anion-binding catalysis. In this regard, cation–π interactions have been used to develop several types of anion binding-catalyzed

• varying aromatic residues were synthesized to elucidate if interactions with the anionic and cationic species could simultaneously be achieved. Hence, in 2010, they successfully showed that such rather small catalysts can mimic nature’s principle of cation–π interactions, allowing for a highly

Insight into functionalized-macrocycles-guided supramolecular photocatalysis

• Minzan Zuo,
• Krishnasamy Velmurugan,
• Kaiya Wang,
• Xueqi Tian and
• Xiao-Yu Hu

Beilstein J. Org. Chem. 2021, 17, 139–155, doi:10.3762/bjoc.17.15

• supramolecular hosts to recognize guests through hydrophobicity, π–π stacking, cation–π interactions, ion–dipole interactions, etc. [31][32]. Due to the cavity-shape limitation of calixarenes, most calixarene-based photocatalytic systems are mainly based on the fabrication of hybrid materials for energy transfer

Halide metathesis in overdrive: mechanochemical synthesis of a heterometallic group 1 allyl complex

• Ross F. Koby,
• Nicholas R. Rightmire,
• Nathan D. Schley,
• Timothy P. Hanusa and
• William W. Brennessel

Beilstein J. Org. Chem. 2019, 15, 1856–1863, doi:10.3762/bjoc.15.181

• the only recoverable material; extracted with toluene, it crystallized from solution as the solvate. A single crystal X-ray study analysis reveals bent polymeric chains of alternating K+ cations and [A']− anions. Each potassium is capped with a toluene molecule, bonded through cation–π interactions

• centroid distance is 2.99 Å, which is typical for K+…(arene) cation–π interactions [22][23]. The enthalpy of binding (∆H°) of an isolated K…(benzene or toluene) unit is almost 80 kJ mol−1 (see calculated value in Table 1, entry 5); the energy is reduced by about 40% when the ring is bound to the neutral

Host–guest interactions between p-sulfonatocalix[4]arene and p-sulfonatothiacalix[4]arene and group IA, IIA and f-block metal cations: a DFT/SMD study

• Valya K. Nikolova,
• Cristina V. Kirkova,
• Silvia E. Angelova and
• Todor M. Dudev

Beilstein J. Org. Chem. 2019, 15, 1321–1330, doi:10.3762/bjoc.15.131

• anchoring points for the positively charged guests. Cation–π interactions between the monoatomic cations and p-sulfonatocalix[4]arene in water are supposed (but not proven) to take part in the inclusion complex formation [31]. Mendes et al. have carried out molecular dynamics (MD) simulations of association

Recent advances in hypervalent iodine(III)-catalyzed functionalization of alkenes

• Xiang Li,
• Pinhong Chen and
• Guosheng Liu

Beilstein J. Org. Chem. 2018, 14, 1813–1825, doi:10.3762/bjoc.14.154

• enantioselectivity (52 vs 53). Moreover, the more electron-deficient 3,4,5-trifluorophenyl analog 54 was found to be less enantioselective. The authors proposed that the benzylic groups can stabilize the cationic intermediates and/or transition states through cation–π interactions, which play an important role in

A conformationally adaptive macrocycle: conformational complexity and host–guest chemistry of zorb[4]arene

• Liu-Pan Yang,
• Song-Bo Lu,
• Arto Valkonen,
• Fangfang Pan,
• Kari Rissanen and
• Wei Jiang

Beilstein J. Org. Chem. 2018, 14, 1570–1577, doi:10.3762/bjoc.14.134

• large amplitude conformational responses to the electronic substituents on the guests. Instead of a linear free-energy relationship, ZB4 follows a parabolic free-energy relationship. This is explained by invoking the influence of secondary C–H···O hydrogen bonds on the primary cation···π interactions

• structures may provide an explanation for their surprising binding behaviors. Multiple non-covalent interactions, including C–H···O hydrogen bonds, cation···π, C–H···π and π···π interactions, are involved in all the cases. Undoubtedly, cation···π interactions between the core quaternary ammonium ions of the

• the substituents to leverage the primary cation···π interactions and thus the final binding affinities. This may explain the parabolic distribution of binding affinities over the Hammett parameters (σp) of the substituents as shown in Figure 2. The conformational adaptivity or flexibility allows ZB4

A detailed view on 1,8-cineol biosynthesis by Streptomyces clavuligerus

• Jan Rinkel,
• Patrick Rabe,
• Laura zur Horst and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2016, 12, 2317–2324, doi:10.3762/bjoc.12.225

• involved in the stabilisation of cationic intermediates, e.g., by cation–π interactions [8][9][10]. The overall process usually generates an enantiomerically pure (poly)cyclic terpene with several stereogenic centres. A large variety of carbon skeletons is accessible, e g., more than 120 skeletons each

From supramolecular chemistry to the nucleosome: studies in biomolecular recognition

• Marcey L. Waters

Beilstein J. Org. Chem. 2016, 12, 1863–1869, doi:10.3762/bjoc.12.175

• -helices; aromatic interactions; β-hairpin peptides; cation–π interactions; dynamic combinatorial chemistry; histone; molecular recognition in water; nucleosome; π–π-stacking; post-translational modification; supramolecular chemistry; Review Childhood influences When thinking about how to start writing

• Dennis Dougherty’s work on cation–π interactions (Figure 3a) [2]. Thus, while at the time I was intent on studying organometallic chemistry, my interest in supramolecular chemistry increased the more I read through graduate school, and particularly molecular recognition in aqueous solution, which I

• postdoc. Seminal work probing the electrostatic component of π–π stacking and edge-face aromatic interactions as well as cation–π interactions was being published at the time, as well as tantalizing suggestions about their relevance in biological structure and function. In particular, I was inspired by

My maize and blue brick road to physical organic chemistry in materials

• Anne J. McNeil

Beilstein J. Org. Chem. 2016, 12, 229–238, doi:10.3762/bjoc.12.24

• , might be gelators. We searched the CSD for molecules containing a mercury atom (Hg2+) that was involved in an intermolecular cation–π interaction [28]. We identified molecule 2a, which exhibited 1D π–cation–π interactions in the solid state (Figure 3). Graduate student Kelsey (King) Carter synthesized

An overview of the synthetic routes to the best selling drugs containing 6-membered heterocycles

• Marcus Baumann and
• Ian R. Baxendale

Beilstein J. Org. Chem. 2013, 9, 2265–2319, doi:10.3762/bjoc.9.265

• aromatic amino acid residues, Tyr652 and Phe656, that are located in an exposed area within the S6 domain pointing towards the possibility of cation π-interactions explaining the affinity of drugs containing tertiary amines towards hERG channels. Consequently, careful and early-stage examination of hERG

Anion–π interactions influence pK_a values

• Christopher J. Cadman and
• Anna K. Croft

Beilstein J. Org. Chem. 2011, 7, 320–328, doi:10.3762/bjoc.7.42

• suggest that these interactions can be further enhanced through co-operative interactions [19][20]. In addition to guiding binding interactions, aromatic groups are also able to direct reaction outcomes. This has been well established in the field of cation–π interactions, where early experiments by Cram

Molecular recognition of organic ammonium ions in solution using synthetic receptors

• Andreas Späth and
• Burkhard König

Beilstein J. Org. Chem. 2010, 6, No. 32, doi:10.3762/bjoc.6.32

• develop synthetic receptors for their selective molecular recognition. The type of host compounds for organic ammonium ion binding span a wide range from crown ethers to calixarenes to metal complexes. Typical intermolecular interactions are hydrogen bonds, electrostatic and cation–π interactions

• are used to enhance further the binding and selectivity with a binding mechanism that can be understood on the combined efforts of several non-covalent interactions such as hydrogen bonding, electrostatic interactions, hydrophobic interactions [20][21][22], cation–π interactions, π–π staking