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

• such as K+, NH4+, and Ba2+ required two crown ether cavities attached to the porphyrin to form the coordination complexes through the dimerisation of the macrocycle. The dimensions of the crown ether pocket determined the complex formation; for example, if there was a mismatch in the sizes of the

• cation and crown ether pocket, dimerisation of the crown porphyrin molecule would occur. The dimerisation led to interesting changes in the visible, NMR, ESR, and emission spectral features. Further developments by Camilleri, Gunter, Boitrel, and Osuka focused on the exploitation of meso-crowned

Group 13 exchange and transborylation in catalysis

• Dominic R. Willcox and
• Stephen P. Thomas

Beilstein J. Org. Chem. 2023, 19, 325–348, doi:10.3762/bjoc.19.28

• ‒H borylation, with an initial B‒Y/B‒H transborylation activating the precatalyst [62][63][64][65]. Zhang showed that benzoic acid decomposed HBpin to BH3 in situ to catalyse the C2‒H borylation of indoles (Scheme 4b) [66][67]. Gellrich reported the bis(pentafluorophenyl)borane-catalysed dimerisation

• of heterocycles and the proposed mechanism. b) Benzoic acid-promoted C‒H borylation of N-heterocycles and the proposed mechanism, where the active catalyst BH3 was formed in situ from HBpin decomposition. Bis(pentafluorophenyl)borane-catalysed dimerisation of allenes and the proposed mechanism

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

• hangtaimycin, starting from ʟ-tryptophan is presented. Comparison to TDD isolated from the hangtaimycin producer Streptomyces spectabilis confirmed its S configuration. The X-ray structure of the racemate shows an interesting dimerisation through hydrogen bridges. The results from bioactivity testings of

Allylic alcohols and amines by carbenoid eliminative cross-coupling using epoxides or aziridines

• Matthew J. Fleming and
• David M. Hodgson

Beilstein J. Org. Chem. 2021, 17, 2385–2389, doi:10.3762/bjoc.17.155

• alkoxide. Keywords: alkenes; aziridines; epoxides; lithiation; synthetic methods; Introduction Methods for the convergent generation of alkenes can be of significant utility in organic synthesis [1]. A relatively under-examined approach is through the interaction of two carbenoids [2]. Dimerisation of

• . Alongside the cinnamylamine 23, small amounts of the aziridine-derived carbenoid dimerisation product, 2-ene-1,4-diamine 24 [5], were observed. While the reaction profile was not altered on a solvent switch to hexane (23 (62%, E/Z = 61:39); 24 (16%)), the yield of cinnamylamine 23 was slightly improved in

• controllable by using enantiomerically pure coupling partners [2]. Such terminal epoxides and aziridines are readily available [3][5], while the corresponding α-lithio ethers can be accessed from enantioenriched α-stannyl ethers [25]. The enantiopure variants await future investigation. Dimerisation of α

Recent developments in enantioselective photocatalysis

• Callum Prentice,
• James Morrisson,
• Andrew D. Smith and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2020, 16, 2363–2441, doi:10.3762/bjoc.16.197

• forms is proposed to involve the alkoxy group rather than the N–H bond that is usually invoked. Then, upon photoexcitation, Dexter energy transfer to the maleimide is dominant to generate 3maleimide rather than energy transfer to the quinolone. While some maleimide dimerisation is observed, which

Facile synthesis of 7-alkyl-1,2,3,4-tetrahydro-1,8-naphthyridines as arginine mimetics using a Horner–Wadsworth–Emmons-based approach

• Rhys A. Lippa,
• John A. Murphy and
• Tim N. Barrett

Beilstein J. Org. Chem. 2020, 16, 1617–1626, doi:10.3762/bjoc.16.134

• achiral aldehydes (azetidine cores 25 and 27). Furthermore, key to success of the Horner–Wadsworth–Emmons olefination was premixing of the aldehyde and phosphonate 7 prior to the addition of KOt-Bu. Upon deprotonation, phosphonate 7 (in the absence of aldehyde) underwent dimerisation to olefin 28. While

• occur via a radical pathway. When iodide 29 was replaced by a superior oxidant in 1,2-dibromoethane, formation of dimer 32 increased (17% isolated yield). This supports the previously reported proposals that dimerisation occurs via single-electron oxidation of the 2-picolyl anion by the halide

• /pseudohalide oxidant followed by recombination of the resulting picolyl radical [28][29][30]. Whilst trace oxygen may be involved, as indicated by the presence of alcohol 31, no dimerisation or alcohol formation was seen in the absence of a halide/pseudohalide-based oxidant, during C-phosphorylation or

An overview of the cycloaddition chemistry of fulvenes and emerging applications

• Ellen Swan,
• Kirsten Platts and
• Anton Blencowe

Beilstein J. Org. Chem. 2019, 15, 2113–2132, doi:10.3762/bjoc.15.209

• cycloaddition proceeds through an ambimodal [6 + 4]/[4 + 6] transition state leading to both of the proposed [6 + 4] adducts, which can interconvert through a Cope rearrangement (Scheme 6) [107]. Dimerisation cycloadditions Generally, dimerization of fulvenes is an undesired process that may occur upon storage

Synthesis of 1-azaspiro[4.4]nonan-1-oxyls via intramolecular 1,3-dipolar cycloaddition

• Yulia V. Khoroshunova,
• Denis A. Morozov,
• Andrey I. Taratayko,
• Polina D. Gladkikh,
• Yuri I. Glazachev and
• Igor A. Kirilyuk

Beilstein J. Org. Chem. 2019, 15, 2036–2042, doi:10.3762/bjoc.15.200

• possible pathway of decay may include oxidation of the ene-hydroxylamine tautomeric form to vinylnitroxide; similar compounds are prone to various dimerisation reactions (Scheme 3) [11]. It is worth noting that in the mass spectrum of 7c, the [M − 1]+ ion was observed instead of the molecular ion. The easy

Design, synthesis and spectroscopic properties of crown ether-capped dibenzotetraaza[14]annulenes

• Krzysztof M. Zwoliński and
• Julita Eilmes

Beilstein J. Org. Chem. 2019, 15, 617–622, doi:10.3762/bjoc.15.57

• showed cation and solvent-induced dimerisation. More recently, Raston et al. reported the synthesis of Goedken’s macrocycle tmtaa nickel(II) complex featuring a single oligo(ethylene glycol) chain strapped across the face of the DBTAA macrocyclic core [28]. However, to the best of our knowledge, there

Synthesis and metal binding properties of N-alkylcarboxyspiropyrans

• Alexis Perry and
• Christina J. Kousseff

Beilstein J. Org. Chem. 2017, 13, 1542–1550, doi:10.3762/bjoc.13.154

• complicated by Cu2+-catalysed spiropyran dimerisation [36]. In the light of the results from Natali et al. [3] – in which evidence for involvement of a carboxylate ligand in metal binding was presented but no comparison was made against a control – it is reassuring to note that incorporation of such a ligand

Cyclisation mechanisms in the biosynthesis of ribosomally synthesised and post-translationally modified peptides

• Andrew W. Truman

Beilstein J. Org. Chem. 2016, 12, 1250–1268, doi:10.3762/bjoc.12.120

• the biosynthesis of pyrroloquinoline quinone (PQQ). RiPPs with uncharacterised mechanisms of cyclisation. Unusual heterocycles in ComX and methanobactin are indicated in red. RTD-1 is formed by the head-to-tail dimerisation of precursor peptides encoded on two separate genes. Acknowledgements The

Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions

• Rajeev S. Menon,
• Akkattu T. Biju and
• Vijay Nair

Beilstein J. Org. Chem. 2016, 12, 444–461, doi:10.3762/bjoc.12.47

• the combination of NHC 24 and a Brønsted base (4-methoxyphenolate) promoted a formal dimerisation of 2-(aroylvinyl)arylaldehydes 70 to afford benzo[a]tetrahydrofluorenones 71 [60]. This stereoselective reaction proceeds via a benzoin–Michael–Michael cascade process (Scheme 43). Further investigations

• in Cheng’s group revealed an intriguing divergent catalytic dimerisation of 2-formylcinnamates 72. Co-operative catalysis by NHC precatalyst 73 and a Lewis acid (titanium isopropoxide) afforded isochromenone derivatives 74 via a sequence of reactions initiated by a benzoin condensation. Treatment of

One-pot functionalisation of N-substituted tetrahydroisoquinolines by photooxidation and tunable organometallic trapping of iminium intermediates

• Joshua P. Barham,
• Matthew P. John and
• John A. Murphy

Beilstein J. Org. Chem. 2014, 10, 2981–2988, doi:10.3762/bjoc.10.316

• electrophilic aromatic substitution at the 2-position of the N-aryl moiety. (The isolation of enone 8a and the fact that 5a does not react with oct-1-ene under the same conditions rules out a Diels–Alder-type pathway to 8b.) Formation of side-products 7a and 7b can be rationalised by dimerisation of allylzinc

• species which, following β-hydride delivery to a second iminium (5a), generates 7a and 4a (Figure 4). Heating the allylzinc halide to promote dimerisation [42], prior to addition to 5a in THF, altered the ratio of 6aj:7a from 8:1 to 3:2 by LC–MS. In further support of this mechanism, reduction of 5a to 4a

• side-product 8a. Analogous reactivity in the formation of cyclic product 8b under enamine catalysis. LC–MS (%) yields in parenthesis. Isolated yields, %, after chromatography not in parenthesis. Mechanism for dimerisation of the allylzinc halide and β-hydride addition to 5a [36]. Oxidative C–H

A tandem Mannich addition–palladium catalyzed ring-closing route toward 4-substituted-3(2H)-furanones

• Jubi John,
• Eliza Târcoveanu,
• Peter G. Jones and
• Henning Hopf

Beilstein J. Org. Chem. 2014, 10, 1462–1470, doi:10.3762/bjoc.10.150

• and 2a as model substrates. Screening of bases revealed that Na2CO3 was more effective than either K2CO3 or KOt-Bu (Table 1, entries 1–3). When KOt-Bu was employed as base, byproduct 4 (formed by the dimerisation of 2a) was formed in higher amounts (Table 1, entry 3). From the tested catalysts, Pd

Dimerisation, rhodium complex formation and rearrangements of N-heterocyclic carbenes of indazoles

• Zong Guan,
• Jan C. Namyslo,
• Martin H. H. Drafz,
• Martin Nieger and
• Andreas Schmidt

Beilstein J. Org. Chem. 2014, 10, 832–840, doi:10.3762/bjoc.10.79

• groups. Heating of the carbene dimer results in the formation of novel quinazolines. Experimental General considerations: All reactions for the dimerisation and the rearrangement were carried out under an atmosphere of nitrogen in oven-dried glassware. Flash-chromatography was performed with silica gel

• , displacement parameters are drawn at 50% probability level. Generation of indazol-3-ylidenes. Reaction products of indazol-3-ylidenes in heterocycle synthesis. Syntheses of acridines from indazol-3-ylidenes. Dimerisation of indazol-3-ylidenes to spiro compounds. Rhodium complex formation. Rearrangement of the

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

The rapid generation of isothiocyanates in flow

• Marcus Baumann and
• Ian R. Baxendale

Beilstein J. Org. Chem. 2013, 9, 1613–1619, doi:10.3762/bjoc.9.184

• is prone to undergo fast dimerisation leading to furoxan byproducts (see Table 1) [39]. In fact this facile dimerisation reaction is regularly reported as troublesome in many reactions that progress through nitrile oxide intermediates. As a consequence a number of strategies to minimise this side

Anionic cascade reactions. One-pot assembly of (Z)-chloro-exo-methylenetetrahydrofurans from β-hydroxyketones

• István E. Markó and
• Florian T. Schevenels

Beilstein J. Org. Chem. 2013, 9, 1319–1325, doi:10.3762/bjoc.9.148

• , possessing unique structural features. Keywords: acetylene addition; dichloroethylene; dimerisation; dioxanes; tetrahydrofurans; Introduction Recently, we have shown that simple ketones reacted with 1,1-dichloroethylene, in the presence of potassium tert-butoxide, to afford rare (Z)-chloro-exo

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

• isomerisation reaction, 22 kcal mol−1, should be high enough for an experimental observation in solution. However, in solution the dimerisation of 3 is computed to have a very low barrier (3 kcal mol−1), and thus 3 is expected to be a short-lived reactive intermediate. Keywords: ab initio; azaborine; BN

• solution. Dimerisation of 3 Alternative pathways for disappearance of 3 may be provided by intermolecular reactions that are in principle feasible in solution. Such pathways are of particular importance, as the BN unit in 3 is an aminoborane (RHB=NHR) derivative. Aminoboranes with small substituents are

• unstable with respect to dimerisation or oligomerisation. In 1,2-dihydro-1,2-azaborine, such a dimerisation is not observed, probably due to the aromatic character of the six-membered ring. In 3, however, this aromatic stabilisation is no longer available. For the sake of simplicity, we only considered

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

• be irradiated effectively. • Large volumes of very low concentration solutions can be irradiated. This is particularly useful for reactions with competing intermolecular side reactions, e.g., dimerisation and polymerisation. • The photolysate can be concentrated by continuous evaporation and the

Intramolecular carbenoid ylide forming reactions of 2-diazo-3-keto-4-phthalimidocarboxylic esters derived from methionine and cysteine

• Marc Enßle,
• Stefan Buck,
• Roland Werz and
• Gerhard Maas

Beilstein J. Org. Chem. 2012, 8, 433–440, doi:10.3762/bjoc.8.49

• , reflux, 2 h; 2. N2=CHCO2Et (2 equiv), rt, 4 days. Rh(II)-catalysed carbenoid reactions of diazoesters 8a,b. Endo transition state for [3 + 3]-dimerisation of carbonyl ylide 14. Rh(II)-catalysed carbenoid reactions of diazoester 8c. Tandem cyclisation/intermolecular cycloaddition of diazoester 8a

Meta-metallation of N,N-dimethylaniline: Contrasting direct sodium-mediated zincation with indirect sodiation-dialkylzinc co-complexation

• David R. Armstrong,
• Liam Balloch,
• Eva Hevia,
• Alan R. Kennedy,
• Robert E. Mulvey,
• Charles T. O'Hara and
• Stuart D. Robertson

Beilstein J. Org. Chem. 2011, 7, 1234–1248, doi:10.3762/bjoc.7.144

• the overall energy of reaction for these isomers is exothermic by −5.81 and −5.59 kcal mol−1, by way of comparison the product of ortho-deprotonation is significantly more exothermic by −12.74 kcal mol−1. With respect to dimerisation the most disfavoured is the methyl isomer (relative energy of 8.60

• kcal mol−1) but in contrast the meta (at 4.65 kcal mol−1) and para (at 5.48 kcal mol−1) regioisomers benefit in relation to the ortho-product (Figure 7). The −22.08 kcal mol−1 energy of dimerisation gained by the ortho-isomer is relatively low (compared to −31.29 kcal mol−1 for the meta-isomer) due to

• a combination of increased steric repulsions and loss of stabilising interactions. On dimerisation, the Na–N interaction in the ortho-product increases in length from 2.501 to 2.662 Å with the Na atom now involved in bonding with two ortho-deprotonated C atoms, while there is also a noticeable

Asymmetric synthesis of tertiary thiols and thioethers

• Jonathan Clayden and
• Paul MacLellan

Beilstein J. Org. Chem. 2011, 7, 582–595, doi:10.3762/bjoc.7.68

• epoxide ring opening of 18 with potassium hydrogen sulfide (Scheme 7) [18]. Epoxide ring opening by a sulfur nucleophile was also employed as the key step in the synthesis of (+)-BE-52440A (22) [22] (Scheme 8). Dimerisation of nanaomycin derivative 21 through a bridging sulfide involves a double

Halide exchanged Hoveyda-type complexes in olefin metathesis

• Julia Wappel,
• César A. Urbina-Blanco,
• Mudassar Abbas,
• Jörg H. Albering,
• Robert Saf,
• Steven P. Nolan and
• Christian Slugovc

Beilstein J. Org. Chem. 2010, 6, 1091–1098, doi:10.3762/bjoc.6.125

• exists as only 1 produced small amounts of the homodimer of the acrylate. Finally, the homo-dimerisation of an acrylate was our last test reaction. Diiodo-complex 3 catalyses the dimerisation of 2-hydroxyethyl acrylate, but compared to 1, catalyst 3 is considerably slower (Table 3, Entry 8). Conclusion

Self-association of an indole based guanidinium-carboxylate-zwitterion: formation of stable dimers in solution and the solid state

• Carolin Rether,
• Wilhelm Sicking,
• Roland Boese and
• Carsten Schmuck

Beilstein J. Org. Chem. 2010, 6, No. 3, doi:10.3762/bjoc.6.3

• , 45141 Essen, Germany 10.3762/bjoc.6.3 Abstract The indole based zwitterion 2 forms stable dimers held together by H-bond assisted ion pairs. Dimerisation was confirmed in the solid state and studied in solution using dilution NMR experiments. Even though zwitterion 2 forms very stable dimers even in

• planarity from the rest of the molecule forcing the two monomers in dimer 2·2 to interact in a non-ideal orientation. Furthermore, the acidity of the NHs is lower than in 1 (as determined by UV-pH-titration) also leading to less efficient binding interactions. Keywords: dimerisation; molecular recognition

• stability is too large to evaluate with an estimated association constant of Kass> 1010 M−1. Even in water dimerisation still takes place (Kass = 170 M−1) [13]. The stability of the dimer 1·1 is significantly larger than the simple Coulomb-interactions of point charges, suggesting that indeed the formation