Green and sustainable approaches for the Friedel–Crafts reaction between aldehydes and indoles

• Periklis X. Kolagkis,
• Eirini M. Galathri and
• Christoforos G. Kokotos

Beilstein J. Org. Chem. 2024, 20, 379–426, doi:10.3762/bjoc.20.36

Experimental and theoretical studies on the synthesis of 1,4,5-trisubstituted pyrrolidine-2,3-diones

• Nguyen Tran Nguyen,
• Vo Viet Dai,
• Nguyen Ngoc Tri,
• Luc Van Meervelt,
• Nguyen Tien Trung and
• Wim Dehaen

Beilstein J. Org. Chem. 2022, 18, 1140–1153, doi:10.3762/bjoc.18.118

• derivative 7 in acidic media, the latter playing the role of nucleophilic reagent which attacks to iminium salt 6 to give intermediate 8. Subsequently, intramolecular nucleophilic attack of the amino moiety to the carboxylate group in the intermediate 8 leads to the formation of the target products 4a–c

• increased to 0.75 M as compared to 0.5 M of aldehyde 1a, the equilibrium would favor the side of iminium salt 6. However, the excess amount of the nucleophilic reactant 2 reacted with intermediate 6 and as a consequence, the yield of the product 4a decreased to 67% (Table 1, Scheme 2). The 1H NMR spectra of

Reactions of 3-aryl-1-(trifluoromethyl)prop-2-yn-1-iminium salts with 1,3-dienes and styrenes

• Thomas Schneider,
• Michael Keim,
• Bianca Seitz and
• Gerhard Maas

Beilstein J. Org. Chem. 2020, 16, 2064–2072, doi:10.3762/bjoc.16.173

• –Alder reaction of 1-CF3-substituted propyn-1-iminium salt 1a with cyclopentadiene was carried out in order to assess the dienophilic reactivity of the cation. High conversion into cycloaddition product 2 was observed already within one hour at 0 °C. Because of its high hydrolytic lability, adduct 2 was

• unpurified 1,4-cyclohexadien-1-iminium salt 4-Ch and 1,4-cyclohexadien-1-yl ketone 5-Ch indicated the presence of a minor byproduct. In the case of 5-Ch, obtained as an oil, the two components could not be separated by column chromatography; however, the 1H NMR spectrum suggested the cyclobutene structure 5

• °C, 19F NMR monitoring of the reaction’s progress indicated the appearance of a second product beside the 1,4-cyclohexadien-1-iminium salt 4-Ch. Further investigations revealed that the new product was the dihydrofluorene 7, resulting from 4-Ch by an intramolecular electrophilic aromatic substitution

Clickable azide-functionalized bromoarylaldehydes – synthesis and photophysical characterization

• Dominik Göbel,
• Marius Friedrich,
• Enno Lork and
• Boris J. Nachtsheim

Beilstein J. Org. Chem. 2020, 16, 1683–1692, doi:10.3762/bjoc.16.139

• three-step methylation/reduction/hydrolysis sequence. Methylation of the oxazoline nitrogen afforded iminium salt 26, which was reduced to oxazolidine 27. Again, similar to 25, cyclization took place and ammonium triflate 28 was isolated in 56% yield (starting from 24). In contrast to the cyclization of

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

• oxidant required for this process was initially generated through anodic oxidation of aqueous Na2CO3 in an undivided cell under constant current electrolysis. A catalytic amount of chiral iminium salt 103 and olefinic substrate 71' were added to the same reaction mixture, and chiral epoxides 73' were

Recent advances on the transition-metal-catalyzed synthesis of imidazopyridines: an updated coverage

• Gagandeep Kour Reen,
• Ashok Kumar and
• Pratibha Sharma

Beilstein J. Org. Chem. 2019, 15, 1612–1704, doi:10.3762/bjoc.15.165

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

• excesses, which result, according to the authors, from a dynamic kinetic resolution of the initially formed Ugi adduct. Indeed, the plausible mechanism for the reaction implies the condensation of amine 2 and aldehyde 33 to form iminium salt intermediate 47 (Scheme 14). Next, addition of isocyanate 42

NHC-catalyzed cleavage of vicinal diketones and triketones followed by insertion of enones and ynones

• Ken Takaki,
• Makoto Hino,
• Akira Ohno,
• Kimihiro Komeyama,
• Hiroto Yoshida and
• Hiroshi Fukuoka

Beilstein J. Org. Chem. 2017, 13, 1816–1822, doi:10.3762/bjoc.13.176

• . In 13C NMR spectra, one unknown signal appeared in the neighborhood of the ester carbon (δ 163.1 and 167.1). Moreover, an unusual signal appeared at δ 71.5. These data indicated definitely that the isolated product 12 was different from the expected intermediate C. Instead, the iminium salt 12, that

• salts. ORTEP drawing of Z-4ai. ORTEP drawing of 10. Reaction process. Reaction mechanism. Isomerization of the stereochemistry of 4ai. Reaction of cycloalkane-1,2-diones with phenyl vinyl ketone (6a). Preparation and reactivity of the bisacylated Breslow intermediate 10. Preparation of the iminium salt

Effect of the ortho-hydroxy group of salicylaldehyde in the A³ coupling reaction: A metal-catalyst-free synthesis of propargylamine

• Sujit Ghosh,
• Kinkar Biswas,
• Suchandra Bhattacharya,
• Pranab Ghosh and
• Basudeb Basu

Beilstein J. Org. Chem. 2017, 13, 552–557, doi:10.3762/bjoc.13.53

• by this mechanism. The p-hydroxybenzaldehyde might give rise to the corresponding p-quinonoid species but not activates the sp carbon (C–H) of the alkyne. Again, in the case of o-hydroxyacetophenone, the corresponding iminium salt might form with difficulty as well as the iminium carbon would be less

Copper-catalyzed asymmetric sp³ C–H arylation of tetrahydroisoquinoline mediated by a visible light photoredox catalyst

• Pierre Querard,
• Inna Perepichka,
• Eli Zysman-Colman and
• Chao-Jun Li

Beilstein J. Org. Chem. 2016, 12, 2636–2643, doi:10.3762/bjoc.12.260

• (SET), reducing the iridium complex to Ir(II) III and oxidizing the the nitrogen of THIQ IV to its radical cation V, which then undergoes a hydride abstraction to form the iminium salt form VI, of the THIQ. The pre-formed chiral PhCu–PyBox complex [38], coordinates to the iminium cation VI, followed by

Diastereoselective synthesis of 3,4-dihydro-2H-pyran-4-carboxamides through an unusual regiospecific quasi-hydrolysis of a cyano group

• Mikhail Yu. Ievlev,
• Oleg V. Ershov,
• Mikhail Yu. Belikov,
• Angelina G. Milovidova,
• Viktor A. Tafeenko and
• Oleg E. Nasakin

Beilstein J. Org. Chem. 2016, 12, 2093–2098, doi:10.3762/bjoc.12.198

• pathway involves the protonation of the imino group nitrogen atom to form an iminium salt (D) and subsequent decyclization of iminolactone ring (E) without participation of water (Scheme 3, path II). To establish the actual mechanism and to prove the intermediate formation of bicyclo[3.2.1]octane

Rh-Catalyzed reductive Mannich-type reaction and its application towards the synthesis of (±)-ezetimibe

• Motoyuki Isoda,
• Kazuyuki Sato,
• Yurika Kunugi,
• Satsuki Tokonishi,
• Atsushi Tarui,
• Masaaki Omote,
• Hideki Minami and
• Akira Ando

Beilstein J. Org. Chem. 2016, 12, 1608–1615, doi:10.3762/bjoc.12.157

• to assist in the formation of the iminium salt in an effort to improve the reactivity. It was clear that Et2Zn was only working as a hydride source but was ineffective as the Lewis acid. Instead when zinc chloride (ZnCl2) was added into the mixture, the yield was improved to 30% (Table 2, entry 5

• expected to have the interaction between the lone pair of the imine and the enolate metal. It seems the model B would be more stable than others to give the syn product. On the other hand, the addition of Lewis acid to the reaction mixture gave the iminium salt immediately and it would obstruct the

Multicomponent reactions: A simple and efficient route to heterocyclic phosphonates

• Mohammad Haji

Beilstein J. Org. Chem. 2016, 12, 1269–1301, doi:10.3762/bjoc.12.121

• under optimized conditions were 53–89%. This process involved a Mannich-type addition of a silver-activated α-isocyanophosphonate anion 208 to an iminium salt 207, resulting from reaction of amine 204 and ketone 205, to give intermediate 209 which cyclizes to afford (2-imidazolin-4-yl)phosphonates 210

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

• . Keywords: iminium salt; organometallic; oxidative functionalisation; photoredox catalysis; tetrahydroisoquinoline; Introduction Tetrahydroisoquinolines (THIQs) are structural motifs prominent within biologically active natural products and pharmaceutical compounds [1][2]. From (−)-carnegine (1, a

• oxidation of N-phenyl THIQ (4a) (1 mmol) to its corresponding iminium salt (5a, Table 1) in anhydrous MeCN. As observed by Stephenson [37], photoredox activation of 4a under these conditions required long reaction times (14–16 h) to reach full conversion. As reactions progressed, we observed precipitation

• and were able to collect half (by mass) of the crude iminium salt 5a by filtration. Precipitation acts to stall reactions by shielding the photocatalyst from the light. Addition of vinylmagnesium bromide directly to the reaction mixture led to a complex mixture of products by HPLC. We found that

Exploration of C–H and N–H-bond functionalization towards 1-(1,2-diarylindol-3-yl)tetrahydroisoquinolines

• Michael Ghobrial,
• Marko D. Mihovilovic and
• Michael Schnürch

Beilstein J. Org. Chem. 2014, 10, 2186–2199, doi:10.3762/bjoc.10.226

• to display certain pharmaceutical properties. Several synthetic methods for the preparation of 1-(indol-3-yl)-THIQs have been reported: In the year 2004, Venkov et al. described a metal-free procedure for the formation of 1-(indol-3-yl)-THIQs by addition of indole to a dihydroisoquinoline iminium

• salt [11]. This synthesis has been streamlined by cross dehydrogenative coupling (CDC) – a powerful method for C–C-bond formation via the C–H bonds of a pro-nucleophile and a pro-electrophile [12][13][14]. A landmark contribution published by Li and co-workers reported the successful introduction of

Silver and gold-catalyzed multicomponent reactions

• Giorgio Abbiati and
• Elisabetta Rossi

Beilstein J. Org. Chem. 2014, 10, 481–513, doi:10.3762/bjoc.10.46

• (Scheme 33) [90][91]. The reaction occurs via a Mannich-type addition of the deprotonated isocyanide intermediate 64 to an in situ generated iminium salt, a subsequent intramolecular cyclization and proton shift results in dihydroimidazole 65 showing predominantly cis-arrangment around the C4–C5 bond

Establishing the concept of aza-[3 + 3] annulations using enones as a key expansion of this unified strategy in alkaloid synthesis

• Aleksey I. Gerasyuto,
• Zhi-Xiong Ma,
• Grant S. Buchanan and
• Richard P. Hsung

Beilstein J. Org. Chem. 2013, 9, 1170–1178, doi:10.3762/bjoc.9.131

• chloride and acetate salts proved to be ineffective (Table 1, entry 3). We then explored salts prepared from stronger acids such as (+)-camphorsulfonic acid (CSA) and trifluoroacetic acid, which should tend to dissociate easier in the reaction medium and lead to a more reactive vinyl iminium salt

• . It is conceivable that propyleine (12) and isopropyleine (14) could be interconvertable under acidic conditions via intermediate iminium salt 13. To investigate this matter, Mueller and Thompson carried out the first and the only synthesis of propyleine known to date [48]. They were able to take a

• mixture of propyleine and isopropyleine with 1:3 ratio as determined by 1H NMR and watch two sets of proton resonances collapse into one that corresponds to the iminium salt 13 after addition of TFA. This experiment strongly suggested that propyleine (12) and isopropyleine (14) could equilibrate under

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

N-Heterocyclic carbene/Brønsted acid cooperative catalysis as a powerful tool in organic synthesis

• Rob De Vreese and
• Matthias D’hooghe

Beilstein J. Org. Chem. 2012, 8, 398–402, doi:10.3762/bjoc.8.43

• hypothesis that the conjugate acid of the base used to generate a carbene could (partially) convert an imine 11 to the corresponding highly electrophilic iminium salt 12, while the deployment of a chiral carbene 5 could lead to a means of controlling the stereochemistry during the reaction of the in situ

• formed homoenolate 10 with the activated iminium salt 12 to provide a new entry into the class of pyrrolidin-2-ones 13 (Scheme 4), which is potentially complementary to other methods in terms of stereochemistry. However, the formation of lactones instead of lactams through the intervention of aldehydes

Syntheses and applications of furanyl-functionalised 2,2’:6’,2’’-terpyridines

• Jérôme Husson and
• Michael Knorr

Beilstein J. Org. Chem. 2012, 8, 379–389, doi:10.3762/bjoc.8.41

• . Preparation of 4,4′,4′′-trisubstituted terpyridine containing carboxylate moieties. Synthetic pathway for the preparation of a furanyl-functionalised quinquepyridine. Utilization of an iminium salt in the preparation of a furanyl-substituted tpy. Preparation of an asymmetric furanyl-substituted terpyridine

Amines as key building blocks in Pd-assisted multicomponent processes

• Didier Bouyssi,
• Nuno Monteiro and
• Geneviève Balme

Beilstein J. Org. Chem. 2011, 7, 1387–1406, doi:10.3762/bjoc.7.163

• boiling MeCN. Acid-promoted formation of a ring-opened iminium salt intermediate 47, followed by hydrolysis and subsequent intramolecular attack of the released secondary amine onto the ester group, would account for the formation of the γ-lactams [23]. This unexpected transformation was observed during

Mitomycins syntheses: a recent update

• Jean-Christophe Andrez

Beilstein J. Org. Chem. 2009, 5, No. 33, doi:10.3762/bjoc.5.33

• oxidation and an intramolecular Heck reaction [149]. Exposure of the quinone 207 to sunlight triggered the formation of benzoxazole 208, which cleaved to form an intermediate iminium salt. Subsequent proton transfer gave the vinylogous carbamate 209 (Scheme 57). After oxidation of the hydroquinone to the

Expedient syntheses of the N-heterocyclic carbene precursor imidazolium salts IPr·HCl, IMes·HCl and IXy·HCl

• Lukas Hintermann

Beilstein J. Org. Chem. 2007, 3, No. 22, doi:10.1186/1860-5397-3-22

• behind such differences of the reaction outcome? To our knowledge, the mechanism of the Arduengo imidazolium salt synthesis has not been discussed in the literature. We propose that alkylation of the DADs A by formaldehyde and HCl (or TMSCl) leads to an iminium salt B (Scheme 2); analogous alkylations