Palladium-Catalyzed Regioselective Syn-Chloropalladation-Olefin Insertion-Oxidative Chlorination Cascade: Synthesis of Dichlorinated Tetrahydroquinolines

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.

Intramolecular cyclization reactions of arylpropargyl amides of electron-deficient α,β-alkenyl carboxylates and related compounds

Intramolecular cyclization reactions of arylpropargyl amides of electron-deficient α,β-alkenyl carboxylates such as fumarates and ethenetricarboxylates were investigated. The reaction of the fumaramides with a base, Et₃N or DBU in xylenes at 140 °C under air gave benz[f]isoindoline derivatives in 21-63% yields. The benz[f]isoindolines may be produced via the formation of an allenic intermediate, intramolecular Diels-Alder reaction, proton transfer, and dehydrogenation by oxygen. The suitable bases and the product yields depend on the substituents on the benzene ring. On the other hand, the reaction of the amides of fumarate and ethenetricarboxylate by heating in DMSO gave aroyl-substituted pyrrolidine derivatives as major products, probably via addition of water under metal-free conditions. Furthermore, cyclization reactions of H and Me substituted alkyne derivatives were investigated for comparison. The selective formation of various types of products, such as ethyl 2-(1-benzyl-4-formyl-2-oxopyrrolidin-3-yl)acetate and diethyl 2-(1-benzyl-2-oxo-4-vinylidenepyrrolidin-3-yl)malonate, was found, depending upon the alkyne substituents and the reaction conditions. The reaction mechanisms have been discussed using density functional theory (DFT) calculations.

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.

Palladium-Catalyzed Regio- and Stereoselective Coupling of Alkynylsulfones with Alkenes: Access to Dichlorinated Vinyl Sulfones

A palladium-catalyzed coupling reaction of alkynylsulfones with alkenes has been described, which provides an efficient and practical entry to various functionalized dichlorinated vinyl sulfones. This method features excellent regio- and stereoselectivities, good functional group compatibility, as well as mild reaction conditions. Mechanistic studies suggest that the reaction goes through sequential syn-chloropalladation, alkene insertion, and C(sp³)-Cl bond formation processes, and the sulfonyl group is crucial to the stereoselectivity control of the reaction.

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.

LiCl-Mediated and Palladium-Catalyzed Oxidative Cyclization of Furan-Ynes via Dearomatizing Alkoxyalkenylation of Furan

A LiCl-mediated and palladium(II)-catalyzed oxidative cyclization of furan-ynes via dearomatizing alkoxyalkenylation of furan is reported to afford polyfunctionalized spiro-dihydrofurans with diastereospecification and high Z/E selectivity. This protocol is likely to involve chloropalladation of alkynes, subsequent intramolecular dearomatizing alkenylation of furan, and final O-allylation. The density functional theory (DFT) calculations demonstrated that the coordination of furan with dissolved Li⁺ weakens its nucleophilicity and promoted the Heck insertion, and dissolved Cl^- promoted chloropalladation of alkynes.

Regio- and diastereoselective Pd-catalyzed aminochlorocyclization of allylic carbamates: scope, derivatization, and mechanism

The regio- and diastereoselective synthesis of oxazolidinones via a Pd-catalyzed vicinal C-N/C-Cl bond-forming reaction from internal alkenes of allylic carbamates is reported. The oxazolidinones are obtained in yields of 44 to 95% with high to excellent diastereoselectivities (from 6 : 1 to >20 : 1 dr) from readily available precursors. This process is scalable, and the products are suitable for the synthesis of useful amino alcohols. A detailed theoretical and experimental mechanistic study was carried out to describe that the reaction proceeds through an anti-aminopalladation of the alkene followed by an oxidative C-Pd(ii) cleavage with retention of the carbon stereochemistry to yield the major diastereomer. The role of Cu(ii) in a C-Cl bond-forming mechanism step has also been proposed.

Synthesis of Highly Conjugated Functionalized 2-Pyridones by Palladium-Catalyzed Aerobic Oxidative Dicarbonation Reactions of N-(Furan-2-ylmethyl) Alkyne Amides and Alkenes as Coupling Partners

A mild, step-economical method for the synthesis of highly conjugated functionalized 2-pyridones from N-(furan-2-ylmethyl) alkyne amides is reported. This method involves Pd-catalyzed aerobic oxidative dicarbonation reactions of alkynes with carbon nucleophiles of a furan ring and an acrylate or styrene as coupling partners. The UV-vis absorption spectra of some of the 2-pyridones indicated that they absorbed shortwave radiation, suggesting their potential utility for filtration of such radiation.

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.

Heterogeneous Amberlyst-15-catalyzed synthesis of complex hybrid heterocycles containing [1,6]-naphthyridine under metal-free green conditions

An efficient green protocol for the synthesis of complex hybrid heterocycles containing [1,6]-naphthyridine and coumarin/pyrazole moieties was established, involving an intramolecular [4 + 2] hetero Diels-Alder reaction as the key step. The biologically significant 12,13-dihydro-6H-benzo[h]chromeno[3,4-b][1,6]naphthyridin-6-ones and 6,10-dihydro-5H-benzo[h]pyrazolo[3,4-b][1,6]naphthyridines were synthesized starting from 2-(N-propargylamino)-arylaldehydes and 3-aminocoumarins or 3-methyl-1-aryl-1H-pyrazol-5-amines in the presence of an Amberlyst-15 catalyst in PEG-200 in good yields. The easy access to diverse complex molecules in a single operation from readily available starting materials, a commercially available, transition metal-free and recyclable catalyst, the use of a green solvent, a very high atom economy and the release of water as the only side product are the highlights of this protocol.