Site-specific labelling of native peptides and proteins: chemical and enzymatic strategies

• Antonio Angelastro,
• Jonathan Bargh,
• Subhajit Guria,
• Victor Laserna and
• Louis Luk

Beilstein J. Org. Chem. 2026, 22, 857–881, doi:10.3762/bjoc.22.67

• dielectrophilic glyoxal-based amidine formation (27, Scheme 10a) [97][98][99][100], aldehyde generation via phenanthrenequinones 28 (Scheme 10b) [101], diketopinic acid (DKPA, 29) conjugation (Scheme 10c) [102], and methylglyoxal (30)-mediated arginine–lysine cyclisation (Scheme 10d) [103]. A different approach

The trans-influence in gold chemistry from a catalytic perspective

• Manfred Bochmann

Beilstein J. Org. Chem. 2026, 22, 838–856, doi:10.3762/bjoc.22.66

• a range of phosphine ligands in the LAuCl/AgSbF6 system (Scheme 1). Cyclisation of the diene–allene substrate 1 gives a mixture of 2, 2' and 3. Depending on the phosphine or phosphite employed, the product ratio 3/2 + 2' was found to vary from 9.5:1 to 1:9. These ratios result in a series of ΔΔG

Synthesis and uranyl(VI) extraction performance of a calix[4]pyrrole–tetrahydroxamic acid receptor

• Sara Karnib,
• Rana Baydoun,
• Wissam Zaidan,
• Nancy AlHaddad,
• Omar El Samad,
• Bilal Nsouli,
• Francine Cazier-Dennin and
• Pierre-Edouard Danjou

Beilstein J. Org. Chem. 2026, 22, 486–494, doi:10.3762/bjoc.22.36

• was first examined, since the ester functions as the synthetic precursor to the corresponding acid used in the preparation of PCP derivatives from PCP [21]. For that, PCP was initially synthesized through the molecular cyclisation of pyrrole with 4-hydroxyacetophenone in methanol in the presence of

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

• . This review aims to explore intriguing examples of such ring-size alterations in all aspects of terpenoid synthesis. The current state-of-the-art regarding proposed biosynthetic pathways for terpenoids with unusual carbon skeleta, occurring either during initial cyclisation or subsequent oxidative

• skeleton terpenoids can be broken down to several stages. First, a linear polyolefin precursor containing multiples of C5 will be assembled. In general, the cyclisation precursor will interact with a cyclase or synthase enzyme and form a “primary reactive species” which undergoes programmed termination to

• deliver the most common terpene frameworks. In some instances, these enzymes can also effect ring contraction or expansion directly during the initial cyclisation mechanism [19][20][21][22][23][24][25][26][27]. In many cases though, the ring-altering reaction instead takes place at a later stage of

Preparation of spirocyclic oxindoles by cyclisation of an oxime to a nitrone and dipolar cycloaddition

• Beth L. Ritchie,
• Alexandra Longcake and
• Iain Coldham

Beilstein J. Org. Chem. 2025, 21, 1890–1896, doi:10.3762/bjoc.21.146

• an important class of compounds with significant biological activities. Spirocyclic derivatives are present in a variety of natural products. We describe here the formation of spirooxindoles using an intermolecular nitrone cycloaddition reaction. The nitrone dipole was prepared in situ by cyclisation

• condensation, cyclisation, and cycloaddition as an efficient strategy for the rapid formation of complex spirocyclic products that could have value for the formation of novel, bioactive oxindoles. Keywords: cascade; cycloaddition; oxindole; spirocycle; stereochemistry; Introduction The Alstonia alkaloids are

• aldehyde 4 could now be tested in the cascade chemistry. This entails the addition of hydroxylamine to form the oxime, followed by cyclisation (with displacement of the tosylate) to give the nitrone for the desired dipolar cycloaddition reaction. Related chemistry (without the oxindole) with a halide

Photoswitches beyond azobenzene: a beginner’s guide

• Michela Marcon,
• Christoph Haag and
• Burkhard König

Beilstein J. Org. Chem. 2025, 21, 1808–1853, doi:10.3762/bjoc.21.143

• the hydrazone, affording the T-type azo-switch 122-E− (Scheme 38). Diarylethenes Diarylethenes are a fascinating class of robust photoswitches. Their main features include thermal stability, a high quantum yield of cyclisation, excellent resistance to fatigue, and a picosecond-range response to

• parallel excited state because of the larger dipole moment [121]. To enrich the population of the antiparallel (and thus increasing the quantum yield of cyclisation), bulky substituents on the 2,2’ positions of benzothiophene were added [122], also the antiparallel conformation could be fixed by

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

• of the resulting alkene [36]. In addition, this intramolecular etherification corresponds to the 4-exo-tet cyclisation which, in terms of kinetics, is the least favoured n-exo-tet cyclisation mode where n ≤ 7 [37]. Nevertheless, the Williamson etherification still remains one of the most common

• and the resulting 1,3-acetyloxyiodides 37 were treated in situ with methanolic t-BuOK to promote the cyclisation. Best results for the oxetane formation were obtained for dimethylphenylvinylsilane, while alkenes without the silyl group afforded mainly homoallylic alcohols 39, presumably through an

• epoxides and heteroatom-stabilised carbocations generated from alkenes (Scheme 10). In 1992, Arjona and co-workers examined the synthesis of 4,7-dioxatricyclo[3.2.1.03,6]octane derivatives 45 via intramolecular cyclisation of the corresponding hydroxyalkene precursors 44 using arylsulphenyl and arylselenyl

Visible-light-promoted radical cyclisation of unactivated alkenes in benzimidazoles: synthesis of difluoromethyl- and aryldifluoromethyl-substituted polycyclic imidazoles

• Yujun Pang,
• Jinglan Yan,
• Nawaf Al-Maharik,
• Qian Zhang,
• Zeguo Fang and
• Dong Li

Beilstein J. Org. Chem. 2025, 21, 234–241, doi:10.3762/bjoc.21.15

Hypervalent iodine-mediated intramolecular alkene halocyclisation

• Charu Bansal,
• Oliver Ruggles,
• Albert C. Rowett and
• Alastair J. J. Lennox

Beilstein J. Org. Chem. 2024, 20, 3113–3133, doi:10.3762/bjoc.20.258

• , the intramolecular HVI-mediated halocyclisation of alkenes using carbon, oxygen, nitrogen or sulfur nucleophiles. Herein, we describe the examples reported in the literature, which are organised by the different halogens involved and the internal nucleophiles. Keywords: cyclisation; halogenation

• , we have collected and described the HVI-mediated halocyclisation reactions reported in the literature for each halide. We have organised the examples firstly by the halide nucleophile, and then secondly by the internal heteroatom nucleophile involved in the cyclisation. The selectivity (chemo-, regio

• proposed by the authors (Scheme 3). Activation of the HVI reagent by H-bonding leads to ligand exchange to give an aminofluoro iodonium intermediate A. Cyclisation occurs via nitrogen attack on the alkene to then give aziridinium intermediate B. Subsequent nucleophilic attack by fluoride on the more

Synthesis of benzo[f]quinazoline-1,3(2H,4H)-diones

• Ruben Manuel Figueira de Abreu,
• Peter Ehlers and
• Peter Langer

Beilstein J. Org. Chem. 2024, 20, 2708–2719, doi:10.3762/bjoc.20.228

• of palladium-catalysed Sonogashira–Hagihara and Suzuki–Miyaura cross-coupling reactions (Scheme 1). The final cyclisation step is accomplished by an acid-mediated cycloisomerisation. The synthesis of starting materials 4 was carried out by our previously reported protocol [65]. While compounds 4a–f

• are known compounds, the synthesis of derivatives 4g–i has not been previously reported (Scheme 2). Yields were generally lower in case of the presence of electron-withdrawing substituents. Subsequently, the cyclisation of 4a–i to 5a–i by acid-mediated cycloisomerisation was studied. In our first

• when located at position 5 of the uracil ring, which might be due to the protonation of the amine under the employed reaction conditions, making the aryl ring less feasible for the SEAr reaction (Scheme 4b). The same effects may explain the cyclisation of 4f to 5f, while 4c is not converted to the

Harnessing unprotected deactivated amines and arylglyoxals in the Ugi reaction for the synthesis of fused complex nitrogen heterocycles

• Javier Gómez-Ayuso,
• Pablo Pertejo,
• Tomás Hermosilla,
• Israel Carreira-Barral,
• Roberto Quesada and
• María García-Valverde

Beilstein J. Org. Chem. 2024, 20, 1758–1766, doi:10.3762/bjoc.20.154

• benzodiazepine with different bases (caesium carbonate, sodium hydroxide). However, the complete cyclisation to the pyrrolobenzodiazepinone was not achieved, so the described three-step strategy starting from 2-nitrobenzoic acid (Ugi reaction/cyclization to pyrrolidine/reduction sequence) remains as a better

Synthesis of polycyclic aromatic quinones by continuous flow electrochemical oxidation: anodic methoxylation of polycyclic aromatic phenols (PAPs)

• Hiwot M. Tiruye,
• Solon Economopoulos and
• Kåre B. Jørgensen

Beilstein J. Org. Chem. 2024, 20, 1746–1757, doi:10.3762/bjoc.20.153

• our previous work [18]. Substrates 3a–c and 6c were synthesised by photochemical cyclisation of stilbenes [58], while 3d was prepared according to literature [59]. The substrates were oxidized under galvanic (constant current) conditions in a Syrris Asia flow system with a 225 μL electrochemical flow

Synthesis and optical properties of bis- and tris-alkynyl-2-trifluoromethylquinolines

• Stefan Jopp,
• Franziska Spruner von Mertz,
• Peter Ehlers,
• Alexander Villinger and
• Peter Langer

Beilstein J. Org. Chem. 2024, 20, 1246–1255, doi:10.3762/bjoc.20.107

• methyl groups and are known to protect from metabolic oxidation [16]. Fluorine-containing arenes are metabolically more stable as compared to non-fluorinated arenes and they show a higher lipophilicity. Known synthetic approaches towards 2-trifluoromethylquinolines include the cyclisation of anilines

• with trifluoroacetoacetate [17], the synthesis from enaminoketones [18] and the gold-catalysed cyclisation of propargylanilines [19]. Aside from its pharmaceutical significance quinoline compounds have been reported as strongly fluorescent with applications as electroluminescence materials [20][21][22

Stability trends in carbocation intermediates stemming from germacrene A and hedycaryol

• Naziha Tarannam,
• Prashant Kumar Gupta,
• Shani Zev and
• Dan Thomas Major

Beilstein J. Org. Chem. 2024, 20, 1189–1197, doi:10.3762/bjoc.20.101

• (GPP, FPP, GGPP) undergo highly complex cyclisation cascades forming terpenes and terpenoids that often have great structural complexity. For class I terpene synthases this multistep process is initiated by a heterolytic C–O bond cleavage, separating the diphosphate and isoprenoid ion pairs [3][4]. The

Synthesis of 1,4-azaphosphinine nucleosides and evaluation as inhibitors of human cytidine deaminase and APOBEC3A

• Maksim V. Kvach,
• Stefan Harjes,
• Harikrishnan M. Kurup,
• Geoffrey B. Jameson,
• Elena Harjes and
• Vyacheslav V. Filichev

Beilstein J. Org. Chem. 2024, 20, 1088–1098, doi:10.3762/bjoc.20.96

• obtained in 84% yield after silica gel purification. Removal of the Boc protecting group from 6 in the presence of trifluoroacetic acid in DCM at room temperature overnight, followed by cyclisation in boiling pyridine/triethylamine, led to 4-hydroxy-1,4-azaphosphinan-2,4-dione (7) in 84% yield. The free

• than 90% purity, as determined by 31P NMR. Compound 10 was used in the next step without further purification. A linear amide 11 was obtained in 47% yield by reacting phosphinate 10 with chloroacetamide in the presence of a large excess of HMDS in acetonitrile at 70 °C for two days. A cyclisation of

• , providing a linear nucleoside 17 as a mixture of two anomers, which were successfully separated on a silica gel column. Finally, cyclisation of a linear nucleoside was accomplished in the presence of a catalytic amount of the Lewis acid TMSOTf in 64% yield. Unfortunately, cyclisation was accompanied by

(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

• addition and cyclisation. Styrenyl (to 79a,c–e), acrylonitrile (to 79b) as well as acrylate-based alkenes could be inserted under the reaction conditions. The authors were then able to apply their method to the synthesis of isosteres of both boscalid (isostere = 80) and norharmane (isostere = 81) (Scheme

• ]propellane (129). Gassman reported the initial synthesis of [3.1.1]propellane (129) in 1980 [61], and this was recently optimised by Uchiyama (Scheme 13A) [47]. Cyclisation to the bridged structure 126 was achieved by enolate formation and intramolecular nucleophilic substitution of iodide diester 125. A

• substrate. Employing Lewis acid catalysis Deng and co-workers reported an alternative pathway to indole-derived BCHs. Polysubstituted BCHs were accessed by nucleophilic addition of the indole to the activated bicyclobutane followed by a Mannich cyclisation [81]. The synthesis of wide variety of tri- and

A laterally-fused N-heterocyclic carbene framework from polysubstituted aminoimidazo[5,1-b]oxazol-6-ium salts

• Andrew D. Gillie,
• Matthew G. Wakeling,
• Bethan L. Greene,
• Louise Male and
• Paul W. Davies

Beilstein J. Org. Chem. 2024, 20, 621–627, doi:10.3762/bjoc.20.54

• -step ynamide annulation and imidazolium ring-formation sequence. Metalation with Au(I), Cu(I) and Ir(I) at the C2 position provides an L-shaped NHC ligand scaffold that has been validated in gold-catalysed alkyne hydration and arylative cyclisation reactions. Keywords: annulation; carbene; gold

• [20][21] that should facilitate access to different groups at the oxazole C-2 position allowing a range of imidazolium-forming cyclisation strategies to be explored. Glorius and co-workers reported the formation of symmetrical NHCs by imidazolium ring formation from bisoxazoline motifs [22] but

• introduce a formamide motif in place of the amine or imine to allow the use of more forcing cyclisation conditions (Scheme 1a, path c). Oxazole 8a was obtained in good yield from 1a using only a slight excess of nitrenoid 7 and 2 mol % catalyst loading. Heating 8a in the presence of POCl3 afforded the 3

Tying a knot between crown ethers and porphyrins

• Maksym Matviyishyn and
• Bartosz Szyszko

Beilstein J. Org. Chem. 2023, 19, 1630–1650, doi:10.3762/bjoc.19.120

• synthesising these systems, including cyclisation reactions, self-assembly, and their remarkable reactivity. The potential applications of porphyrin-crown ether hybrids are also highlighted. Moreover, the discussion identifies the challenges associated with synthesising and characterising hybrids, outlining

Functions of enzyme domains in 2-methylisoborneol biosynthesis and enzymatic synthesis of non-natural analogs

• Binbin Gu,
• Lin-Fu Liang and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2023, 19, 1452–1459, doi:10.3762/bjoc.19.104

• of the 2-methylisoborneol synthase was investigated through enzyme incubations with several substrate analogs, giving access to two C12 monoterpenoids. Implications on the stereochemical course of the terpene cyclisation by 2-methylisoborneol synthase are discussed. Keywords: biosynthesis; enzymes

• model was proposed that proceeds through the S-adenosylmethionine (SAM) dependent methylation of geranyl diphosphate (GPP) to 2-methyl-GPP (2-Me-GPP), followed by a terpene cyclisation to 1 (Scheme 1A) [10]. The cyclisation cascade requires isomerisation to (R)-2-methyllinalyl diphosphate [22], followed

• by two sequential cyclisation reactions to A and B, and terminal quenching with water. This hypothesis was confirmed by the discovery of the biosynthetic genes coding for a GPP methyltransferase (GPPMT) and a type I terpene synthase termed 2-methylisoborneol synthase (2MIBS) [23][24]. Interestingly

Functional characterisation of twelve terpene synthases from actinobacteria

• Anuj K. Chhalodia,
• Houchao Xu,
• Georges B. Tabekoueng,
• Binbin Gu,
• Kizerbo A. Taizoumbe,
• Lukas Lauterbach and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2023, 19, 1386–1398, doi:10.3762/bjoc.19.100

• 26 can be well understood from the cyclisation mechanism towards 23 (Scheme 1B). After substrate ionisation to A a 1,10-cyclisation leads to the (E,E)-germacradienyl cation (B) that can either be deprotonated to 24 or captured with water to yield 26. Both compounds are important neutral intermediates

• in sesquiterpene biosynthesis that can be reactivated by reprotonation for a second cyclisation to eudesmane (6,6-bicyclic) or guaiane (7,5-bicyclic) sesquiterpene hydrocarbons or alcohols, respectively [46][47]. Starting from 26, such a protonation induced cyclisation can lead to C that is the

• as plasticisers. A) Cope rearrangement of 24 and 26. B) Cyclisation mechanism from FPP to 23, identifying compound 26 as a biosynthetic intermediate and 24 as a side product. Terpene synthase homologs characterised in this study. Supporting Information Supporting Information File 162: Additional

Synthesis of ether lipids: natural compounds and analogues

• Marco Antônio G. B. Gomes,
• Alicia Bauduin,
• Chloé Le Roux,
• Romain Fouinneteau,
• Wilfried Berthe,
• Mathieu Berchel,
• Hélène Couthon and
• Paul-Alain Jaffrès

Beilstein J. Org. Chem. 2023, 19, 1299–1369, doi:10.3762/bjoc.19.96

• was converted to the epoxide 7.5 by cyclisation in the presence of potassium carbonate in methanol, thus producing the interesting building block 7.5. A second option, optimized to avoid the formation of epoxide, used a hindered base and the reactive benzyltriflate as electrophile to achieve under

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

• [36] via the polyphosphoric acid (PPA) catalysed cyclisation of 2,2'-diaminobibenzyl (20) at elevated temperatures (Scheme 3) [37][38]. 1.3 Catalytic dehydrogenation An early synthesis of 5H-dibenzo[b,f]azepine (1a) involved the gas phase dehydrogenation of 10,11-dihydro-5H-dibenzo[b,f]azepine (2a) to

• , several methods of carboxamidation were tested, thus allowing the authors to synthesize carbamazepine (CBZ) derivatives of 43. 3 Metal-catalysed cyclisation Diverse metal-catalysed coupling methods exist for the preparation of the dibenzo[b,f]heteropine ring system. The following approaches are broadly

• -pyridobenzazepines 60b via cyclisation of 2,2'-dihalostilbene analogue 58 through a Pd-catalysed double Buchwald–Hartwig amination. The stilbene analogues 58 were prepared by a Wittig reaction with reported yields of the desired Z-isomer around 55%. The amination step was performed on a series of primary alkylamines

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

• initial formation from farnesyl diphosphate, these neutral intermediates can become reprotonated for a second cyclisation to reach the bicyclic eudesmane and guaiane skeletons. This review summarises the accumulated knowledge on eudesmane and guaiane sesquiterpene hydrocarbons and alcohols that

• is terminated by deprotonation to yield a terpene hydrocarbon or by nucleophilic attack of water to generate a terpene alcohol. For the precursor of sesquiterpenes FPP six initial cyclisation modes are possible (Scheme 1). After ionisation to A either a 1,10-cyclisation to the (E,E)-germacradienyl

• cation (B) or a 1,11-cyclisation to the (E,E)-humulyl cation (C) is possible. Reattack of diphosphate at C-3 results in nerolidyl diphosphate (NPP) that can undergo a conformational change by rotation around the C-2/C-3 single bond, which allows reionisation to D. This intermediate can react in a 1,10

Cytochrome P450 monooxygenase-mediated tailoring of triterpenoids and steroids in plants

• Karan Malhotra and
• Jakob Franke

Beilstein J. Org. Chem. 2022, 18, 1289–1310, doi:10.3762/bjoc.18.135

• polycyclic skeletons of triterpenoids and steroids are created by oxidosqualene cyclases (OSCs) from the universal substrate 2,3-oxidosqualene [5]. As different folding modes (chair–boat–chair vs chair–chair–chair) and different ring sizes can occur during this cyclisation cascade, resulting triterpene and

• catalyse the initial C22,16 dihydroxylation of cholesterol (3) [35]; in contrast, the related CYP DzCYP90B71 was found to catalyse only the first hydroxylation at C22 [66]. This step is followed by a rate-limiting cyclisation step through unstable furostanol intermediate 14 that involves CYP-catalysed

Synthesis of tryptophan-dehydrobutyrine diketopiperazine and biological activity of hangtaimycin and its co-metabolites

• Houchao Xu,
• Anne Wochele,
• Minghe Luo,
• Gregor Schnakenburg,
• Yuhui Sun,
• Heike Brötz-Oesterhelt and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2022, 18, 1159–1165, doi:10.3762/bjoc.18.120

• -butyloxycarbonyl (Boc)-protected threonine using bis(2-oxo-3-oxazolidinyl)phosphinic chloride (BOP-Cl) [14][15] and Hünig’s base to give 9. Cleavage of the Boc group with 5% TFA followed by basic treatment resulted in the cyclisation to the dioxopiperazine 10. Acetylation and subsequent treatment with LiClO4 and

• 13. Removal of the Cbz group by catalytic hydrogenation proceeded with spontaneous cyclisation to 14. With this material, the elimination of the MOM group smoothly proceeded by treatment with KH and 18-crown-6 in THF at 25 °C to 15, that upon removal of the Boc group with TFA and 1,3-dimethoxybenzene