Batch and Flow Synthesis of Pyrrolo[1,2-a]-quinolines via an Allene-Based Reaction Cascade

Pd/Brønsted Acid Co-catalyzed Dehydrative Coupling of Propargylic Alcohols with Diarylphosphine Oxides

An efficient dehydrative coupling of propargylic alcohols with diarylphosphine oxides to construct tetrasubstituted allenylphosphoryl compounds in the presence of a Pd/Brønsted acid co-catalyst has been developed. As a benefit from the use of a Brønsted acid, this reaction could perform under mild conditions with excellent yields, accommodating a wide range of functional groups. The potential utility of this method has also been demonstrated.

Diastereoselective Double Annulation of Allenoates with Povarov Adducts: Modular Synthesis of Multisubstituted Pyrroloquinolines

Herein, we report an atom-economical, one-pot, four-component, diastereoselective double-annulation reaction to construct polyfused pyrroloquinolines. This reaction highlights the cyclization of in situ-formed Povarov adducts with allenoates for the first time.

Pyrazole-promoted synthesis of pyrrolo[3,4-c] quinoline-1,3-diones in a novel diketene-based reaction

We describe the first classic example of green synthesis of pyrrolo[3,4-c]quinolones scaffolds by catalyst-free unusual reaction of diketene, isatin, and primary amines in ethanol in the presence of pyrazole as a promoter for 4 h. The whole structure of the new product was confirmed by X-ray analysis. The overall transformation involves the cleavage and generation of multiple carbon-nitrogen and carbon-carbon bonds. This report represents a simple and straightforward approach for the synthesis of pyrrolo[3,4-c]quinoline-1,3-diones, which has significant advantages like readily available precursors, non-use of toxic solvent, operational simplicity, mild conditions, good atom economy, and excellent yields; therefore it provides a green and sustainable strategy for access to a range of interesting N-containing heterocyclic compounds in medicinal and organic chemistry.

Synthesis of Multifused Pyrrolo[1,2-a]quinoline Systems by Tandem Aza-Michael-Aldol Reactions and Their Application to Molecular Sensing Studies

Herein, we have presented a weak acid-promoted tandem aza-Michael-aldol strategy for the synthesis of diversely fused pyrrolo[1,2-a]quinoline (tricyclic to pentacyclic scaffolds) by the construction of both pyrrole and quinoline ring in one pot. The described protocol fabricated two C-N bonds and one C-C bond in the pyrrole-quinoline rings which have been sequentially formed under transition-metal-free conditions by the extrusion of eco-friendly water molecules. A ketorolac drug analogue has been synthesized following the current protocol, and one of the synthesized tricyclic pyrrolo[1,2-a]quinoline fluorophores has been used to detect highly toxic picric acid via the fluorescence quenching effect.

Rhodium(III)-Catalyzed Intramolecular Annulation and Aromatization for the Synthesis of Pyrrolo[1,2-a]quinolines

A rhodium(III)-catalyzed protocol for the synthesis of pyrrolo[1,2-a]quinolines through intramolecular annulation of o-alkynyl amino aromatic ketones and subsequent aromatization is reported. This transformation builds the pyrrole and quinoline moieties of the pyrrolo[1,2-a]quinoline in one pot and achieves a flexible introduction of different substituent groups at 4- and 5-positions on products that were difficult to prepare by other means. The reaction proceeds smoothly on a gram scale, and the products are amenable to downstream synthetic manipulations.

Continuous Flow Inter- and Intramolecular Macrolactonization under High Dilution Conditions

An efficient continuous flow process for the macrolactonization of seco acids and diacids using diols in the presence of Mukaiyama reagent (N-methyl-2-chloropyridinium iodide) has been developed for medium to large sized macrocyclic lactones. In comparison with other methods, the continuous flow process provided good to high yield in a short reaction time. By using this methodology, a wide range of macrocyclic lactones (11 compounds), dilactones (15 compounds), and tetralactone derivatives (2 compounds) with various ring sizes (12-26 atoms in the core) were synthesized in just 35 min of residence time. Advantageously, macrolactonization under the flow process is very elegant to handle the high dilution of reactants with a defined perfluoroalkoxy alkanes (PFA) tube reactor volume (7 mL).

Synthesis of 7-Chloroquinoline Derivatives Using Mixed Lithium-Magnesium Reagents

We have prepared a library of functionalized quinolines through the magnesiation of 7-chloroquinolines under mild conditions, employing both batch and continuous flow conditions. The preparation involved the generation of mixed lithium-magnesium intermediates, which were reacted with different electrophiles. Mixed lithium-zinc reagents allowed the synthesis of halogenated and arylated derivatives. Some of the synthesized 4-carbinol quinolines have shown interesting antiproliferative properties, their hydroxyl group being a suitable amino group bioisostere. We also report a two-step approach for optically active derivatives.

Heteroarene-tethered Functionalized Alkyne Metamorphosis

Heteroarene-tethered functionalized alkynes are multipotent synthons in organic chemistry. This detailed Review described herein offers a thorough discussion of the metamorphosis of heteroarene-tethered functionalized alkynes, an area which has earned much attention over the past decade in the straightforward synthesis of architecturally complex heterocyclic scaffolds in atom and step economic manner. Depending upon the variety of functionalized alkynes, this Review is divided into multiple sections. Amongst the vast array of synthetic transformations covered, dearomatizing spirocyclizations and cascade spirocyclization/rearrangement are of great interest. Synthetic transformations involving the heteroarene-tethered functionalized alkynes with scope, challenges, limitations, mechanism, their application in the total synthesis of natural products and future perceptions are surveyed.

Continuous flow synthesis of lipophilic cations derived from benzoic acid as new cytotoxic chemical entities in human head and neck carcinoma cell lines

Continuous flow chemistry was used for the synthesis of a series of delocalized lipophilic triphenylphosphonium cations (DLCs) linked by means of an ester functional group to several hydroxylated benzoic acid derivatives and evaluated in terms of both reaction time and selectivity. The synthesized compounds showed cytotoxic activity and selectivity in head and neck tumor cell lines. The mechanism of action of the molecules involved a mitochondrial uncoupling effect and a decrease in both intracellular ATP production and apoptosis induction.

Calcium-Catalyzed Stereoselective Tandem [4+2] and [3+2] Annulation Reaction for the Synthesis of Dihydropyrrolo[1,2-a]quinolines

Calcium-catalyzed highly facile one-pot, A⁴ annulation of aldehyde, amine, alkene, and alkyne to form fused 4,5-dihydropyrrolo[1,2-a]quinolines with exclusive syn diastereoselectivity is reported. This selectivity arises from an inverse electron demand [4+2] aza-Diels-Alder cycloaddition, and the adduct further undergoes a formal [3+2] cyclization with activated alkynes. This diversity-oriented protocol is highly general and furnishes the dihydropyrrolo[1,2-a]quinoline derivatives with a broad substrate scope in good to excellent yields.

Synthesis of Tetracyclic Fused Quinolines via a Friedel–Crafts and Beckmann Ring Expansion Sequence

An efficient protocol for the construction of tetracyclic fused quinolines (pyrazole-fused azepino-, azocino-, and azonino[3,2-b]quinolinones) via consecutive Friedel–Crafts and Beckmann reactions has been developed. The key steps in the syntheses of these new molecular scaffolds involve acid-mediated cyclization of 2-(pyrazol-3-yl)quinoline based carboxylic acids 6a–c, 8, and 12 to ketones 13a–e, followed by Beckmann rearrangements of the corresponding oximes 14a–e to provide the tetracyclic-fused quinoline skeletons 15a–e. Structures of synthesised compounds without stereochemical implication were confirmed by NMR and elemental analyses.

Synthesis of pyrrolo[3,4-c]quinoline-1,3-diones: a sequential oxidative annulation followed by dehydrogenation and N-demethylation strategy

A synthetic strategy for pyrrolo[3,4-c]quinoline-1,3-diones via oxidative annulation followed by DDQ mediated dehydrogenation and N-demethylation.

Synthesis of Polycyclic Benzo[b]indolo[3,2,1-de]acridines via Sequential Allenylation, Diels-Alder Cyclization, and Hydrogen Migration Reaction

A novel methodology for stereoselective synthesis of benzo[b]indolo[3,2,1-de]acridines through the tandem reaction of propargylic compounds with organoboron is described, and only one diastereoisomer was obtained. The sequential procedure was triggered by Pd(0)-catalyzed allenylation of propargyl carbonate. Then, Diels-Alder cyclization and hydrogen migration processes proceeded successively to furnish the polycyclic target molecules. Control reactions suggested the base (Cs₂CO₃) was indispensable for the hydrogen migration.

Exploring allene chemistry using phosphorus-based allenes as scaffolds

In this paper, we review some of our results on cycloaddition and cyclization reactions of allenylphosphonates/and allenyl phosphine oxides. Thus nitro-substituted propargylic alcohols react with P(III)–Cl substrates to lead to unprecedented phosphono-benzazepines or -hydroxyindolinones. A similar reaction using a higher stoichiometry of P(III)–Cl precursor has led to the first observation of spontaneous resolution by crystallization in allene chemistry. In the reaction of these phosphorus based allenes with diphenyl isobenzofuran (DPBF), depending on the substituents, both [α, β] and [β, γ]-cycloaddition products are formed. Extension of this work to sulfur based allenes and leading to a new [4+2] cycloadduct with DPBF is reported. A novel cyclization reaction of a functionalized allenylphosphine oxide with diethylamine leading to 3-diethylamino-4-diphenylphosphinoyl-1-naphthol is discovered. Base-catalyzed reactions of allenylphosphonates with 2-substituted benzaldehdes lead to novel thiochromans, chromenes, and quinolines. Allenylphosphonates undergo phosphonylation in the presence of tetrabutyl ammonium fluoride (TBAF). Vinyl azides derived from allenylphosphonates undergo thermolysis to form bis-phosphonopyrazines. [Pd]-catalyzed reactions of allenes with 2-iodophenols, 2-iodobenzyl alcohol and 3-iodo-indole-2-carboxylic acids lead to a variety of benzofurans, benzopyrans and benzopyranones, respectively.

Alves L, Brocksom TJ, de Oliveira KT. Combining batch and continuous flow setups in the end-to-end synthesis of naturally occurring curcuminoids

A successful end-to-end continuous flow synthesis of pure curcumin (1) and two other natural derivatives present in turmeric is described.

Efficient one-pot synthesis of new 1-amino substituted pyrrolo[1,2-a]quinoline-4-carboxylate esters via copper-free Sonogashira coupling reactions

The reactions of several 2-chloroquinoline-3-carboxylate esters with propargyl alcohol and a secondary amine in the presence of palladium catalyst leads to the formation of new alkyl 1-amino substituted pyrrolo[1,2-a]quinoline-4-carboxylate derivatives. This one-pot process, carried out in the absence of any copper salt, provides an efficient method for the synthesis of functionalized pyrrolo[1,2-a]quinolines in good-to-high yields.

One-Pot Synthesis of Benzo[4,5]imidazo[2,1-a]isoquinolines and Isoquinolino[3,4-b]quinoxalines via Tandem Cyclization Strategies

Two operationally simple one-pot protocols have been developed for the synthesis of amino-functionalized benzo[4,5]imidazo[2,1-a]isoquinolines and isoquinolino[3,4-b]quinoxalines. Optimization data and substrate scope for these atom-economical transformations, which engage commercially available o-phenylenediamines and o-cyanobenzaldehydes, are discussed.

Base catalysed intermolecular cyclisation of N-protected o-amino benzaldehyde/acetophenone with phosphorus/sulphur based allenes: facile synthesis of substituted quinolines

Base catalyzed reaction of N-Bz protected o-aminobenzaldehyde with allenylphosphonates affords O-phosphorylated quinolines via a route involving a thermal rearrangement.

A theoretical study of DABCO and PPh3 catalyzed annulations of allenoates with azodicarboxylate

Previous experiments have shown that DABCO-catalyzed annulation of 2,3-butadienoate and diethylazodicarboxylate leads to 1,2-diazetidine (reaction (1)), whereas PPh3-catalyzed 2-benzyl-2,3-butadienoate and diethylazodicarboxylate gives pyrazoline (reaction (2)).

Synthesis of pyrrolo[1,2-a]quinolines and ullazines by visible light mediated one- and twofold annulation of N-arylpyrroles with arylalkynes

Fused nitrogen heterocycles and ullazines are synthesised by one and twofold annulation of N-arylpyrroles with arylalkynes using metal free visible light photocatalysis.

Regioselective carboannulation of electron-deficient allenes with dialkyl (2-formylphenyl)malonates leading to multisubstituted naphthalenes

A regioselective carboannulation of allenoates (or allenylphosphonates) with dialkyl 2-(2-formylphenyl)malonates leads to multi-substituted naphthalenes via Michael addition, cyclisation, dealkoxycarboxylation and tautomerisation.