A versatile route towards 6-arylpipecolic acids

• Erich Gebel,
• Cornelia Göcke,
• Carolin Gruner and
• Norbert Sewald

Beilstein J. Org. Chem. 2025, 21, 1104–1115, doi:10.3762/bjoc.21.88

• to investigate how the configuration of the stereocenter in C2 position influences diastereoselectivity. In the first approach, NaBH3CN was used under acidic conditions to reduce the acyliminium intermediate formed from the N-acyl enamine upon protonation at C5, while in the second approach

• , heterogeneous catalytic hydrogenation of the enamine with palladium on carbon was chosen. While the hydride reduction of the acyliminium intermediate gave a nearly 1:1 diastereomer ratio, a 9:1 ratio was obtained for the catalytic hydrogenation (Scheme 4). While the hydride reduction of the N-acyliminium

• product 4 (Scheme 5a). The transformation of the enamine had to be carried out by reduction of the N-acyliminium ion by NaBH3CN in the presence of TFA, as Pd/C with H2 would lead also to the reduction of the triple bond. The products (2R,6S)-13 and (2R,6R)-13 were obtained in a 1:1 ratio (Scheme 5b

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

• CuBr promoted hydrogen atom abstraction from the amide 94 resulting in the benzylic radical species 95, followed by oxidation to give acyliminium species 96. Luque and co-workers (2020) developed a biogenic carbonate of CuO–CaCO3 to catalyze solvent- and additive-free amidation reactions in air

Recent total synthesis of natural products leveraging a strategy of enamide cyclization

• Chun-Yu Mi,
• Jia-Yuan Zhai and
• Xiao-Ming Zhang

Beilstein J. Org. Chem. 2025, 21, 999–1009, doi:10.3762/bjoc.21.81

• of the resulting acyliminium [30]. Unlike the monocyclization, which involves deprotonation of the acyliminium ion, the success of this polycyclization relies on the interception of the acyliminium ion by an aryl nucleophile, resulting in the formation of N-heterocyclic fused[6,6,5]tricycles

• attack of the electron-rich arene on the acyliminium ion occurs from the Si-face. This stereochemical outcome is attributed to the steric discrepancy of the phenyl or tert-butyl group and the hydroxy group. The resulting tricyclic products could be further elaborated by elimination or amide reduction to

• tertiary enamides is employed. Inspired by Wang’s earlier work, Zhang and Tu’s group developed a tandem cyclization/Mannich reaction to construct this architecture [35]. However, unlike the electron-rich arenes, the use of silyl enol ethers to terminate the second cyclization of the acyliminium

Entry to 2-aminoprolines via electrochemical decarboxylative amidation of N‑acetylamino malonic acid monoesters

• Olesja Koleda,
• Janis Sadauskis,
• Darja Antonenko,
• Edvards Janis Treijs,
• Raivis Davis Steberis and
• Edgars Suna

Beilstein J. Org. Chem. 2025, 21, 630–638, doi:10.3762/bjoc.21.50

• quench of a transient N-acyliminium species was found to be a major side-reaction as evidenced by the formation of an open-chain hemiaminal 10a (the hemiaminal could not be isolated due to the instability on silica gel). Screening of other supporting electrolytes revealed that basic salts (K2CO3, Na2CO3

• , and afforded trace amounts of 6h together with the corresponding acyclic hemiaminal 10h as the major product. Such an outcome can be attributed to a slower formation of a 6-membered ring [15] from transient N-acyliminium species. Gratifyingly, the addition of KOH (1 equiv) to the electrolysis mixture

Synthesis of tetrahydrofuro[3,2-c]pyridines via Pictet–Spengler reaction

• Elena Y. Mendogralo and
• Maxim G. Uchuskin

Beilstein J. Org. Chem. 2023, 19, 991–997, doi:10.3762/bjoc.19.74

• ][18] was described (Scheme 1c). The most studied variation of this cyclization is based on the generation of an acyliminium cation from the corresponding alcohols [19][20][21][22][23] or alkenes [24][25][26][27][28][29], subsequent attack of furan ring and the formation of annulated tetrahydrofuro[3,2

• -c]pyridines. Moreover, multistep cascade processes with the simultaneous construction of several cores were described, where the key step is the generation of an acyliminium cation and the Pictet–Spengler cyclization [30][31][32], including solid-phase synthesis [33][34][35]. Another route for the

Catalytic aza-Nazarov cyclization reactions to access α-methylene-γ-lactam heterocycles

• Bilge Banu Yagci,
• Selin Ezgi Donmez,
• Onur Şahin and
• Yunus Emre Türkmen

Beilstein J. Org. Chem. 2023, 19, 66–77, doi:10.3762/bjoc.19.6

• Research Center, Institute of Materials Science and Nanotechnology, Bilkent University, Ankara 06800, Turkey 10.3762/bjoc.19.6 Abstract We have developed a catalytic aza-Nazarov reaction of N-acyliminium salts generated in situ from the reaction of a variety of cyclic and acyclic imines with α,β

• multisubstituted pyrrole derivatives (Scheme 1a) [16][17][18][19]. Another seminal work in this field was the aza-Nazarov cyclization of N-acyliminium salts 1 reported by Klumpp and co-workers in 2007 (Scheme 1b) [20]. This reaction was promoted with the use of superstoichiometric amounts of TfOH

• (trifluoromethanesulfonic acid), and the N-acyliminium cation was proposed to be protonated with the super acidic TfOH to form a dicationic species, which was shown by computational studies to be crucial for the success of this transformation. In a later study, the same research group showed that benzamides 2 bearing an

Electrochemical Friedel–Crafts-type amidomethylation of arenes by a novel electrochemical oxidation system using a quasi-divided cell and trialkylammonium tetrafluoroborate

• Hisanori Senboku,
• Mizuki Hayama and
• Hidetoshi Matsuno

Beilstein J. Org. Chem. 2022, 18, 1040–1046, doi:10.3762/bjoc.18.105

• 1,3,5-trimethoxybenzene or indoles in DMA containing 0.1 M iPr2NHEtBF4 using an undivided cell equipped with a Pt plate cathode and a Pt wire anode (a quasi-divided cell) resulted in selective formation of N-acyliminium ions of DMA at the anode, which reacted with arenes to give the corresponding

• amidomethylated products in good to high yields. Keywords: electrochemical oxidation; Friedel–Crafts type amidomethylation; N-acyliminium ion; quasi-divided cell; trialkylammonium salt; Introduction Oxidation of amides generates useful intermediates, N-acyliminium ions, which have been widely used in organic

• synthesis [1][2][3][4]. For example, Friedel–Crafts-type amidomethylation [5][6][7][8][9][10][11][12][13][14][15] proceeds efficiently by the reaction of N-acyliminium ions with electron-rich arenes to give the corresponding amidomethylated products in good yields. Since amides are important intermediates

The ethoxycarbonyl group as both activating and protective group in N-acyl-Pictet–Spengler reactions using methoxystyrenes. A short approach to racemic 1-benzyltetrahydroisoquinoline alkaloids

• Marco Keller,
• Karl Sauvageot-Witzku,
• Franz Geisslinger,
• Nicole Urban,
• Michael Schaefer,
• Karin Bartel and
• Franz Bracher

Beilstein J. Org. Chem. 2021, 17, 2716–2725, doi:10.3762/bjoc.17.183

• highly electrophilic N-acyliminium intermediates [17]. As a special aspect, we used a carbamate unit (instead of the commonly used carboxamide), ending up with 1-benzyl-1,2,3,4-tetrahydroisoquinolines bearing an N-ethoxycarbonyl residue, which in turn was easily converted directly into an N-methyl group

Synthesis of highly substituted fluorenones via metal-free TBHP-promoted oxidative cyclization of 2-(aminomethyl)biphenyls. Application to the total synthesis of nobilone

• Ilya A. P. Jourjine,
• Lukas Zeisel,
• Jürgen Krauß and
• Franz Bracher

Beilstein J. Org. Chem. 2021, 17, 2668–2679, doi:10.3762/bjoc.17.181

• through inductive effects, and on amides, which should be more prone to engage in the cyclization owing to the high reactivity of expected N-acyliminium ions 4a (R1 = alkyl, R2 = acyl) in SEAr reactions. In the second screening round, we determined the isolated yields of the promising reactions identified

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

• studies revolved around hydrogen bond donor catalysts and their application in N-acyliminium ion reactions. At this point, the mechanistic proposal, albeit speculative, was based on the hypothesis that neutral chloroamide structures I were the reactive intermediates in the reaction. Under this premise, H

• solvent was observed and, therefore, a SN1-type mechanism was concluded. Furthermore, their studies proved that an ion pair is required for the reaction to proceed and, most importantly, that the thiourea catalyst 9 interacts with the chloride of the N-acyliminium ion as opposed to the carbonyl group

• . Based on this concept, the applicability of N-acyliminium chlorides in thiourea-catalyzed anion-binding reactions was further explored. In 2008, an intramolecular asymmetric Pictet–Spengler-type cyclization reaction with pyrrole derivatives 13 was reported. The authors were not only able to control the

Prins cyclization-mediated stereoselective synthesis of tetrahydropyrans and dihydropyrans: an inspection of twenty years

• Asha Budakoti,
• Pradip Kumar Mondal,
• Prachi Verma and
• Jagadish Khamrai

Beilstein J. Org. Chem. 2021, 17, 932–963, doi:10.3762/bjoc.17.77

• presence of the mild Lewis acid InCl3 and benzaldehyde (88), which produced all-cis-tetrahydropyran-4-one 90 in excellent yield. The transformation proceeded through cyclization of a diequatorial chair-like conformation of the oxocarbenium ion 89 to provide an N-acyliminium ion, which upon hydrolysis

A complementary approach to conjugated N-acyliminium formation through photoredox-catalyzed intermolecular radical addition to allenamides and allencarbamates

• Olusesan K. Koleoso,
• Matthew Turner,
• Felix Plasser and
• Marc C. Kimber

Beilstein J. Org. Chem. 2020, 16, 1983–1990, doi:10.3762/bjoc.16.165

• radical addition, using photoredox catalysis, to allenamides and allencarbamates is reported. This transformation synthesizes N-acyl-N’-aryl-N,N’-allylaminals, and proceeds by a conjugated N-acyliminium intermediate that previously has principally been generated by electrophilic activation methods. The

• radical adds to the central carbon of the allene giving a conjugated N-acyliminium that undergoes nucleophilic addition by arylamines and alcohols. Keywords: allenamide; allene; intermolecular; N-acyliminium; photoredox; Introduction Allenamides (Scheme 1, 1) and their congeners have attracted

• of oxidation [20][29][30][31] or through the use of a transition metal such as Au(I) [8][10][11][12][13][14][15][16][32][33][34][35][36]. The reaction of the allenamide with an electrophilic source promotes the formation of a conjugated N-acyliminium intermediate 2 [37][38][39][40] that subsequently

One-step route to tricyclic fused 1,2,3,4-tetrahydroisoquinoline systems via the Castagnoli–Cushman protocol

• Aleksandar Pashev,
• Nikola Burdzhiev and
• Elena Stanoeva

Beilstein J. Org. Chem. 2020, 16, 1456–1464, doi:10.3762/bjoc.16.121

• ][14][20][21]. The mechanism of the reaction is still under debate with two prevailing versions in the literature (Scheme 2) [16][17][22][23][24]. The first reaction pathway includes the formation of an N-acyliminium ion 15, followed by a ring closure through an enolate ion 16. The other mechanistic

A review of asymmetric synthetic organic electrochemistry and electrocatalysis: concepts, applications, recent developments and future directions

• Munmun Ghosh,
• Valmik S. Shinde and
• Magnus Rueping

Beilstein J. Org. Chem. 2019, 15, 2710–2746, doi:10.3762/bjoc.15.264

• imides and no racemization was observed during the reaction (Scheme 50). In 2000, Pilli and co-workers published a vinylogous Mannich addition of silyloxyfuran to chiral N-acyliminium ions generated in situ from 157 which had been obtained from the anodic oxidation of 156 bearing a cyclohexyl-based

• chiral auxiliary ([92]. The authors established that the Mannich addition occurred exclusively on the Si-face of the N-acyliminium ions, resulting in the threo-isomer as the major isomer (with moderate yields and good diastereomeric ratios). Upon catalytic hydrogenation followed by methanolysis, threo

• -158a, and 159a were further converted to the corresponding lactams 160a and 160b enabling efficient recovery of chiral auxiliaries (Scheme 51). Furthermore, the same group reported an identical method for the TiCl4-promoted addition of allyltrimethylsilane to N-acyliminium ions containing the same

Multicomponent reactions (MCRs): a useful access to the synthesis of benzo-fused γ-lactams

• Edorta Martínez de Marigorta,
• Jesús M. de Los Santos,
• Ana M. Ochoa de Retana,
• Javier Vicario and
• Francisco Palacios

Beilstein J. Org. Chem. 2019, 15, 1065–1085, doi:10.3762/bjoc.15.104

• -catalysed carbonylation of iodoarylimine 100 to produce acid chloride 102 (Scheme 29). Intramolecular nucleophilic attack of the imine onto the acyl chloride would furnish cyclic N-acyliminium derivative 103, which can then undergo a second palladium-catalysed carbonylation to form a stabilized ketene 104

Synthesis of pyrimido[1,6-a]quinoxalines via intermolecular trapping of thermally generated acyl(quinoxalin-2-yl)ketenes by Schiff bases

• Svetlana O. Kasatkina,
• Ekaterina E. Stepanova,
• Maksim V. Dmitriev,
• Ivan G. Mokrushin and
• Andrey N. Maslivets

Beilstein J. Org. Chem. 2018, 14, 1734–1742, doi:10.3762/bjoc.14.147

• quinoxalin-2-ylideneacetates [9], multicomponent Mannich–Ritter transformations of quinoxalin-2(1H)-ones under the action of nitriles and 3,4-dihydro-2H-pyran [10] and a microwave-assisted cascade strategy via in situ-generated N-acyliminium ion precursors and amines [11] (Figure 2). To develop a new

Stereoselective nucleophilic addition reactions to cyclic N-acyliminium ions using the indirect cation pool method: Elucidation of stereoselectivity by spectroscopic conformational analysis and DFT calculations

• Koichi Mitsudo,
• Junya Yamamoto,
• Tomoya Akagi,
• Atsuhiro Yamashita,
• Masahiro Haisa,
• Kazuki Yoshioka,
• Hiroki Mandai,
• Koji Ueoka,
• Christian Hempel,
• Jun-ichi Yoshida and
• Seiji Suga

Beilstein J. Org. Chem. 2018, 14, 1192–1202, doi:10.3762/bjoc.14.100

• , Okayama 700-8530, Japan Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan 10.3762/bjoc.14.100 Abstract In this study, six-membered N-acyliminium ions were generated by the “indirect cation pool” method and

• reacted with several nucleophiles. These reactions afforded disubstituted piperidine derivatives with high diastereoselectivities and good to excellent yields. The conformations of the obtained N-acyliminium ions were studied by low temperature NMR analyses and DFT calculations and were found to be

• consistent with the Steven’s hypothesis. Keywords: cation pool; conformation; electroorganic synthesis; N-acyliminium ion; NMR analysis; piperidine; Introduction Cyclic amines are significant key motifs in pharmaceutical and natural products because a variety of compounds bearing those moieties exhibit

1-Imidoalkylphosphonium salts with modulated C_α–P⁺ bond strength: synthesis and application as new active α-imidoalkylating agents

• Jakub Adamek,
• Roman Mazurkiewicz,
• Anna Węgrzyk and
• Karol Erfurt

Beilstein J. Org. Chem. 2017, 13, 1446–1455, doi:10.3762/bjoc.13.142

• ]. N-Acylimine 2 or the much more reactive N-acyliminium cation 3 are active reagents in α-amidoalkylation reactions, usually generated in situ from a precursor of structure 1 under basic or acidic conditions, respectively (Scheme 1) [3]. The limitations and disadvantages of the most commonly used

• InCl3) [11][17][19]. Moreover, the application of Lewis acids can diminish the activity of a reacting nucleophile, and usually requires a labor-intensive aqueous work-up procedure of the reaction mixture. The inevitable equilibrium between the N-acyliminium cation 3 and the less reactive uncharged N

• synthesis and properties of hitherto unknown 1-imidoalkylphosphonium salts 5 (Scheme 2). We expected that the 1-imidoalkylcarbenium cation generated from these salts (see Scheme 2) would display greater electrophilic reactivity in comparison with N-acyliminium cation 3, due to the strong electron

Brønsted acid-mediated cyclization–dehydrosulfonylation/reduction sequences: An easy access to pyrazinoisoquinolines and pyridopyrazines

• Ramana Sreenivasa Rao and
• Chinnasamy Ramaraj Ramanathan

Beilstein J. Org. Chem. 2017, 13, 428–440, doi:10.3762/bjoc.13.46

• for the synthesis of many alkaloids (Figure 1) [16]. To synthesize pyrazinoisoquinoline and its derivatives, various approaches such as the Ugi multicomponent reaction [17] amidoalkylation [18][19], N-acyliminium ion cyclization [20] and a radical cyclization [21] have been reported. To this end

Synthesis of structurally diverse 3,4-dihydropyrimidin-2(1H)-ones via sequential Biginelli and Passerini reactions

• Andreas C. Boukis,
• Baptiste Monney and
• Michael A. R. Meier

Beilstein J. Org. Chem. 2017, 13, 54–62, doi:10.3762/bjoc.13.7

• , aldehyde 1 is activated by a Lewis- or a Brønsted acid. In the next step, urea/thiourea 2 can serve as a nucleophile and react with the activated carbonyl carbon to form a heminal species. However, under acidic conditions heminals can eliminate water and form an N-acyliminium cation 3. This reactive cation

Biomimetic synthesis and HPLC–ECD analysis of the isomers of dracocephins A and B

• Viktor Ilkei,
• András Spaits,
• Anita Prechl,
• Áron Szigetvári,
• Zoltán Béni,
• Miklós Dékány,
• Csaba Szántay Jr,
• Judit Müller,
• Árpád Könczöl,
• Ádám Szappanos,
• Attila Mándi,
• Sándor Antus,
• Ana Martins,
• Attila Hunyadi,
• György Tibor Balogh,
• György Kalaus (†),
• Hedvig Bölcskei,
• László Hazai and
• Tibor Kurtán

Beilstein J. Org. Chem. 2016, 12, 2523–2534, doi:10.3762/bjoc.12.247

• cyclization of the Strecker aldehyde 5 yields the acylaminocarbinol intermediate 6, which readily loses water on protonation, resulting in the N-acyliminium ion 7a/b, a strong electrophilic reagent. Results and Discussion The aim of our work was to synthesize the natural flavonoid alkaloids 2a–d and 3a–d in a

• resembled more the experimental HPLC–ECD spectrum of 3c than that of 3b. A plausible mechanism for the formation of dracochepins A and B involves the electrophilic attack of the N-acyliminium ion 7a,b on the aromatic ring A of racemic naringenin ((±)-1) at C-6 and C-8 (Figure 14). The electrophilic reagent

(Thio)urea-mediated synthesis of functionalized six-membered rings with multiple chiral centers

• Giorgos Koutoulogenis,
• Nikolaos Kaplaneris and
• Christoforos G. Kokotos

Beilstein J. Org. Chem. 2016, 12, 462–495, doi:10.3762/bjoc.12.48

• polycyclization of hydroxylactams 240 leads to the corresponding polycyclized products 241, using organocatalyst 242 (Scheme 75) [94]. The authors postulated that the existence of an extended aromatic framework on the catalyst is very crucial, as the delocalized π-electron system interacts with the N-acyliminium

Enantioselective additions of copper acetylides to cyclic iminium and oxocarbenium ions

• Jixin Liu,
• Srimoyee Dasgupta and
• Mary P. Watson

Beilstein J. Org. Chem. 2015, 11, 2696–2706, doi:10.3762/bjoc.11.290

• -component coupling of pyridine, benzoyl chloride and phenylacetylene [27], the Arndtsen group tackled the challenge of developing a catalyst for the alkynylation of cyclic N-acyliminium ions with unactivated alkynes [28]. Using the reaction of N-acylquinoline and phenylacetylene as their model, they

Novel stereocontrolled syntheses of tashiromine and epitashiromine

• Loránd Kiss,
• Enikő Forró and
• Ferenc Fülöp

Beilstein J. Org. Chem. 2015, 11, 596–603, doi:10.3762/bjoc.11.66

• the indolizidine framework; access to tashiromine in racemic form can be achieved through the alkylation of succinimide, followed by ring closure via acyliminium intermediates [23][24], the reduction of cyclized pyridinium salts [25], iminium cascade cyclization [26], alkyne-mediated hydroformylation

The Shono-type electroorganic oxidation of unfunctionalised amides. Carbon–carbon bond formation via electrogenerated N-acyliminium ions

• Alan M. Jones and
• Craig E. Banks

Beilstein J. Org. Chem. 2014, 10, 3056–3072, doi:10.3762/bjoc.10.323

• Alan M. Jones Craig E. Banks Manchester Metropolitan University, Faculty of Science and Engineering, School of Science and the Environment, Division of Chemistry and Environmental Science, John Dalton Building, Chester Street, Manchester, M1 5GD, UK 10.3762/bjoc.10.323 Abstract N-acyliminium ions

• electroorganic techniques and future directions. Keywords: anodic oxidation; electrochemistry; electroorganic, electrosynthesis, N-acyliminium ions; natural products; non-Kolbe oxidation; peptidomimetics; Shono oxidation; synthesis; Review N-Acyliminium ions are synthetically versatile N-Acyliminium ions [1][2

• electrochemical oxidation of unfunctionalised amides (last comprehensively reviewed in 1984 by Prof. T. Shono) [10] to N-acyliminium ion intermediates and their application to synthetic challenges. The Shono electrooxidation route to N-acyliminium intermediates Shono and colleagues reported the first direct