Direct Access to 9-Chloro-1H-benzo[b]furo[3,4-e]azepin-1-ones via Palladium(II)-Catalyzed Intramolecular syn-Oxypalladation/Olefin Insertion/sp2-C–H Bond Activation Cascade

Pd(0)-Catalyzed Asymmetric Cyclization/Coupling Cascade of Alkyne-Tethered Unsaturated Carbonyls: Development and Mechanism Elucidation

While the Pd(0)-catalyzed cyclization of alkyne-tethered unsaturated carbonyl substrates has been reported, the mechanism has not been well elucidated, and the potential asymmetric version remains to be developed. Here, we disclose that a chiral Pd(0) complex can efficiently promote the desymmetrizative cyclization of alkyne-tethered cyclohexadienones in CH3OH, and the resultant Pd(II) intermediates further undergo an array of tandem coupling reactions, including Suzuki, Sonogashira, and even chemoselective reduction by CH3OH in the absence of additional coupling partners. As a result, a broad spectrum of hydrobenzofuran derivatives, having a tetra- or trisubstituted exo-alkene motif, is constructed with moderate to outstanding enantioselectivity in an exclusive cis-difunctionalization pattern. In addition, this enantioselective protocol can be well expanded to linear alkyne-tethered unsaturated carbonyls, and a new desymmetrizative and asymmetric cyclization/coupling cascade of bis-alkyne-tethered enones is further realized efficiently, furnishing diversely structured frameworks with high stereoselectivity. Moreover, kinetic transformation for various racemic alkyne-tethered enones can be accomplished under similar catalytic conditions, and unusual kinetic reactions by chemoselectively undertaking Suzuki or Sonogashira coupling, or reduction by CH3OH, occur sequentially, finally yielding two types of chiral products, both with high enantioselectivity via either ligand- or substrate-based control. The experimental results demonstrate that the current Pd(0)-based strategy is superior to the classical Pd(II)-catalyzed carbopalladation/cyclization process of the identical substrates with regard to enantioselectivity and synthetic versatility. Moreover, density functional theory calculations are conducted to rationalize the Pd(0)-catalyzed oxidative cyclometalation pathway in the key cyclization step, which leads to the observed cis-difunctionalized products exclusively.

Synthesis of Azonia Aromatic Heterocycles Bearing 6-6-6-5-6 Pentacyclic Core via Intramolecular [4 + 2]-Cycloaddition and Oxidative Aromatization Reaction Sequence in One Pot

Cationic aza-heterocycle-fused compounds have gained wide applications in materials science, biological applications, and synthetic organic chemistry. In this report, synthesis of benzothiazolochromenopyridinium tetrafluoroborates, a novel molecular scaffold, bearing 6-6-6-5-6 pentacyclic core is described that proceeds via (i) piperidine-catalyzed Knoevenagel condensation between 2-propargyloxyarylaldehydes bearing internal alkynes and 2-benzothiazoleacetonitrile, (ii) intramolecular formal [4 + 2]-cycloaddition, and (iii) crucial molecular oxygen-mediated oxidative aromatization reaction sequence in one pot. These quaternary pyridinium salts are obtained at ambient temperature in good to high yields.

Rapid Assembly of Functionalized 2H-Chromenes and 1,2-Dihydroquinolines via Microwave-Assisted Secondary Amine-Catalyzed Cascade Annulation of 2-O/N-Propargylarylaldehydes with 2,6-Dialkylphenols

An efficient, secondary amine-catalyzed cascade annulation of 2-O/N-propargylarylaldehydes with 2,6-dialkylphenols was established to access biologically relevant functionalized 2H-chromenes and 1,2-dihydroquinolines tethered with a synthetically useful p-quinone methide scaffold in high yields under microwave irradiation and conventional heating conditions. The microwave-assisted strategy was convenient, clean, rapid, and high yielding in which the reactions were completed in just 15 min, and the yields obtained were up to 95%. This highly atom-economical domino process constructed two new C-C double bonds and a six-membered O/N-heterocyclic ring in a single synthetic operation. Its mechanism process was rationalized as involving sequential iminium ion formation, nucleophilic addition, and intramolecular annulation steps. Furthermore, the synthesized 2H-chromene derivatives were transformed into valuable indeno[2,1-c]chromenes, 5H-indeno[2,1-c]quinolines, and oxireno[2,3-c]chromene via a palladium-catalyzed double C-H bond activation process and epoxidation, respectively.

Diversity-Oriented Synthesis of Benzo[f][1,4]oxazepine-, 2H-Chromene-, and 1,2-Dihydroquinoline-Fused Polycyclic Nitrogen Heterocycles under Microwave-Assisted Conditions

An efficient, diversity-oriented synthesis of oxazepino[5,4-b]quinazolin-9-ones, 6H-chromeno[4,3-b]quinolines, and dibenzo[b,h][1,6]naphthyridines was established involving a substrate-based approach under microwave-assisted and conventional heating conditions in high yields (up to 88%). The CuBr₂-catalyzed, chemoselective cascade annulation of O-propargylated 2-hydroxybenzaldehydes and 2-aminobenzamides delivered oxazepino[5,4-b]quinazolin-9-ones involving a 6-exo-trig cyclization-air oxidation-1,3-proton shift-7-exo-dig cyclization sequence. This one-pot process showed excellent atom economy (-H₂O) and constructed two new heterocyclic rings (six- and seven-membered) and three new C-N bonds in a single synthetic operation. On the other side of diversification, the reaction between O/N-propargylated 2-hydroxy/aminobenzaldehydes and 2-aminobenzyl alcohols delivered 6H-chromeno[4,3-b]quinolines and dibenzo[b,h][1,6]naphthyridines involving sequential imine formation-[4 + 2] hetero-Diels-Alder reaction-aromatization steps. The influence of microwave assistance was superior to conventional heating, where the reactions were clean, rapid, and completed in 15 min, and the conventional heating required a longer reaction time at a relatively elevated temperature.

Microwave-assisted one-pot two-step imine formation-hetero-Diels-Alder-detosylation/aromatization sequence: direct access to dibenzo[b,h][1,6]naphthyridines

A microwave-assisted, copper-catalyzed, one-pot, two-step reaction is established to access functionalized [1,6]naphthyridines in high yields (up to 96%) starting from 2-(N-propargylamino)benzaldehydes and arylamines. This rapid and operationally simple sequential reaction allowed the construction of two new heterocyclic rings and three new (2 C-C and 1 C-N) bonds in a single synthetic operation. This reaction tolerated various electron-donating and electron-withdrawing substituents well and delivered the desired products in a shorter reaction time under microwave irradiation. This reaction proceeds through a sequential imine formation, intramolecular [4 + 2] hetero-Diels-Alder reaction, and air oxidation, followed by detosylation-aromatization steps.

Biomacromolecule-Assisted Screening for Reaction Discovery and Catalyst Optimization

Reaction discovery and catalyst screening lie at the heart of synthetic organic chemistry. While there are efforts at de novo catalyst design using computation/artificial intelligence, at its core, synthetic chemistry is an experimental science. This review overviews biomacromolecule-assisted screening methods and the follow-on elaboration of chemistry so discovered. All three types of biomacromolecules discussed─enzymes, antibodies, and nucleic acids─have been used as "sensors" to provide a readout on product chirality exploiting their native chirality. Enzymatic sensing methods yield both UV-spectrophotometric and visible, colorimetric readouts. Antibody sensors provide direct fluorescent readout upon analyte binding in some cases or provide for cat-ELISA (Enzyme-Linked ImmunoSorbent Assay)-type readouts. DNA biomacromolecule-assisted screening allows for templation to facilitate reaction discovery, driving bimolecular reactions into a pseudo-unimolecular format. In addition, the ability to use DNA-encoded libraries permits the barcoding of reactants. All three types of biomacromolecule-based screens afford high sensitivity and selectivity. Among the chemical transformations discovered by enzymatic screening methods are the first Ni(0)-mediated asymmetric allylic amination and a new thiocyanopalladation/carbocyclization transformation in which both C-SCN and C-C bonds are fashioned sequentially. Cat-ELISA screening has identified new classes of sydnone-alkyne cycloadditions, and DNA-encoded screening has been exploited to uncover interesting oxidative Pd-mediated amido-alkyne/alkene coupling reactions.

Microwave-Assisted Copper(II)-Catalyzed Cascade Cyclization of 2-Propargylamino/Oxy-Arylaldehydes and O-Phenylenediamines: Access to Densely Functionalized Benzo[f]Imidazo[1,2-d][1,4]Oxazepines and Benzo[f]Imidazo[1,2-d][1,4]Diazepines

A highly efficient microwave-assisted copper(II)-catalyzed cyclization cascade was established starting from readily accessible O/N-propargylated 2-hydroxy or 2-aminobenzaldehydes and o-phenylenediamines to synthesize densely functionalized imidazo[1,2-d][1,4]oxazepines and imidazo[1,2-d][1,4]diazepines in high yields (up to 93%). This one-pot two-step process was found to be highly atom economical (-H₂O, -H₂) and operationally simple and enabled the generation of two new heterocycle rings (seven- and five-membered) and three new C-N bonds in a single synthetic operation. These reactions well tolerated a variety of substituents including electron-donating and electron-withdrawing groups and furnished the desired fused heterocycles in high yields under microwave irradiation in a very short reaction time. The mechanism of the established protocol involves sequential imine formation-intramolecular cyclization-air oxidation followed by 7-exo-dig cyclization steps. A comparative study between the microwave-assisted approach and conventional heating was also performed to demonstrate the advantages of the microwave-assisted protocol in terms of high yield and shorter reaction time.

The synthesis of seven- and eight-membered rings by radical strategies

Radical strategies for preparation of seven- or eight-membered rings.

Direct Access to Bridged Tetrahydroquinolines and Chromanes via an InCl3-Catalyzed Sequential Three-Component Cascade

A sequential three-component cascade process was developed for the synthesis of bridged tetrahydroquinolines and chromanes bearing 2,6-methanobenzo[d][1,3]diazocine and 2,6-methanobenzo[g][1,3]oxazocine scaffolds, respectively, in good yields from readily available materials. The InCl₃-catalyzed reaction progressed via enamine formation, Michael addition, intramolecular cyclization, and intramolecular iminium ion cyclization steps. Notably, this high atom economic approach (-2H₂O) allowed the generation of four new bonds (1 C-C and 3 C-N or 1 C-C, 1 C-O and 2 C-N) and two heterocyclic rings in a single operation.

Chemodivergent synthesis of functionalized methanodibenzo[b,f][1,5]diazocin-13-ylmethanones and tetrahydroquinolines via solvent-dependent AB2 and A2B2 multicomponent annulation reactions

A solvent-dependent chemodivergent approach was developed for the synthesis of 6,12-methanodibenzo[b,f][1,5]diazocin-13-ylmethanones and 2,3,4-trisubstituted 1,2,3,4-tetrahydroquinolines involving two distinct AB₂ and A₂B₂ multicomponent processes.

Regioselective synthesis of tetrahydroquinolines via syn- and anti-nucleopalladation-initiated cascade processes

Regioselective synthesis of tetrahydroquinolines via nucleopalladation-initiated cascade sequences was established.

The synthetic versatility of the Tiffeneau–Demjanov chemistry in homologation tactics

Abstract

The Tiffeneau–Demjanov rearrangement can be regarded as an interesting alternative to the more common semi-pinacol transposition. It is usually employed for ring extension but, under specific conditions, it can also be used for ring contraction. Compared to other techniques, such as the Demjanov rearrangement or homologations with diazo compounds, the Tiffeneau–Demjanov pathway presents attractive features including high yielding and selective processes. Ring enlargements follow very strict and simple rules, such as the movement of the less substituted carbon and retention of the configuration. The rearrangement process is mainly affected by steric factors, due to presence of neighbouring groups, rather than electronic ones. The ring contraction may be achieved positioning the amine within the ring, thus achieving a high level of control. Unfortunately, applications of the reaction in modern homologation chemistry are rare; therefore, the aim of the review is re-proposing to the synthetic community the versatility of this venerable reaction and thus, spurring its employment for tackling challenging homologations processes.

Graphic abstract