Quadri‐Synergetic Effect for Highly Effective Carbon Dioxide Fixation and Its Application to Indoloquinolinone

Base-Promoted 5-exo-dig Cyclization of o-Alkynylamides or 2-En-4-ynamides with CO2 toward Fully Substituted Acrylates

A base-promoted sequential cyclization and carboxylation of o-alkynylamides or 2-en-4-ynamides with CO₂ has been achieved with high efficiency, stereoselectivity, and regioselectivity. This approach begins with 5-exo-dig cyclization followed by trapping the resulting vinyl anion with CO₂ and MeI, which provides a convenient access to diverse cyclic and fully substituted acrylates with CO₂ as the carboxylic source.

Electrochemical Dearomative Dicarboxylation of Heterocycles with Highly Negative Reduction Potentials

The dearomative dicarboxylation of stable heteroaromatics using CO₂ is highly challenging but represents a very powerful method for producing synthetically useful dicarboxylic acids, which can potentially be employed as intermediates of biologically active molecules such as natural products and drug leads. However, these types of transformations are still underdeveloped, and concise methodologies with high efficiency (e.g., high yield and high selectivity for dicarboxylations) have not been reported. We herein describe a new electrochemical protocol using the CO₂ radical anion (E_1/2 of CO₂ = -2.2 V in DMF and -2.3 V in CH₃CN vs SCE) that produces unprecedented trans-oriented 2,3-dicarboxylic acids from N-Ac-, Boc-, and Ph-protected indoles that exhibit highly negative reduction potentials (-2.50 to -2.94 V). On the basis of the calculated reduction potentials, N-protected indoles with reduction potentials up to -3 V smoothly undergo the desired dicarboxylation. Other heteroaromatics, including benzofuran, benzothiophene, electron-deficient furans, thiophenes, 1,3-diphenylisobenzofuran, and N-Boc-pyrazole, also exhibit reduction potentials more positive than -3 V and served as effective substrates for such dicarboxylations. The dicarboxylated products thus obtained can be derivatized into useful synthetic intermediates for biologically active compounds in few steps. We also show how the dearomative monocarboxylation can be achieved selectively by choice of the electrolyte, solvent, and protic additive; this strategy was then applied to the synthesis of an octahydroindole-2-carboxylic acid (Oic) derivative, which is a useful proline analogue.

How the Lewis Base F- Catalyzes the 1,3-Dipolar Cycloaddition between Carbon Dioxide and Nitrilimines

The mechanism of the Lewis base F^– catalyzed 1,3-dipolar cycloaddition between CO₂ and nitrilimines is interrogated using DFT calculations. F^– activates the nitrilimine, not CO₂ as proposed in the literature, and imparts a significant rate enhancement for the cycloaddition. The origin of this catalysis is in the strength of the primary orbital interactions between the reactants. The Lewis base activated nitrilimine–F^– has high-lying filled FMOs. The smaller FMO-LUMO gap promotes a rapid nucleophilic attack and overall cycloaddition with CO₂.

Copper(I)-Catalyzed Nitrile-Addition/N-Arylation Ring-Closure Cascade: Synthesis of 5,11-Dihydro-6H-indolo[3,2-c]quinolin-6-ones as Potent Topoisomerase-I Inhibitors

In this paper, we present a copper(I)-catalyzed nitrile-addition/N-arylation ring-closure cascade for the synthesis of 5,11-dihydro-6H-indolo[3,2-c]quinolin-6-ones from 2-(2-bromophenyl)-N-(2-cyanophenyl)acetamides. Using CuBr and t-BuONa in dimethylformamide (DMF) as the optimal reaction conditions, the cascade reaction gave the target products, in high yields, with a good substrate scope. Application of the cascade reaction was demonstrated on the concise total syntheses of alkaloid isocryptolepine. Further optimization of the products from the cascade reaction led to 3-chloro-5,12-bis[2-(dimethylamino)ethyl]-5,12-dihydro-6H-[1,3]dioxolo[4',5':5,6]indolo[3,2-c]quinolin-6-one (2k), which exhibited the characteristic DNA topoisomerase-I inhibitory mechanism of action with potent in vitro anticancer activity. Compound 2k actively inhibited ARC-111- and SN-38-resistant HCT-116 cells and showed in vivo activity in mice bearing human HCT-116 and SJCRH30 xenografts. The interaction of 2k with the Top-DNA cleavable complex was revealed by docking simulations to guide the future optimization of 5,11-dihydro-6H-indolo[3,2-c]quinolin-6-ones as topoisomerase-I inhibitors.

Synthesis of benzimidazolones via CO2 fixation and N-phenyl formamides using formic acid in presence of zinc embedded polymer complex

A merrifield supported heterogeneous zinc catalyst [Zn(Meri-Ald-Py)] was synthesized and applied for benzimidazolone synthesis through the fixation of carbon dioxide (CO₂) and for different N-formylated products synthesis under mild reaction conditions.

A four-component coupling reaction of carbon dioxide, amines, cyclic ethers and 3-triflyloxybenzynes for the synthesis of functionalized carbamates

A novel four-component coupling reaction of carbon dioxide, amines, cyclic ethers and 3-triflyloxybenzynes for the synthesis of functionalized carbamates has been developed for the first time.

Domino cyclization/trifluoromethylation of 2-alkynylanilines using fluoroform-derived CuCF3: synthesis of 3-(trifluoromethyl)indoles

A domino cyclization/trifluoromethylation strategy for the construction of indole cores with concomitant installation of a CF₃ group is described.

PhI(OCOCF3)2-Mediated Construction of a 2-Spiropseudoindoxyl Skeleton via Cascade Annulation of 2-Sulfonamido-N-phenylpropiolamide Derivatives

A cascade annulation of 2-sulfonamido-N-phenylpropiolamide derivatives leading to the construction of the 2-spiropseudoindoxyl skeleton was realized under mild conditions with phenyliodine(III) bis(trifluoroacetate) (PIFA) as the sole oxidant. This metal-free spirocyclization process is suggested to encompass a sequential C(sp²)-C(sp) and C(sp²)-N bond formation with the concomitant introduction of a carbonyl oxygen.

Synthesis of Carboxylic Acids and Esters from CO2

The achievements in the synthesis of carboxylic acids and esters from CO₂ have been summarized and discussed.

Regioselective Copper-Catalyzed Boracarboxylation of Vinyl Arenes

Regioselective copper-catalyzed boracarboxylation of vinyl arenes with bis(pinacolato)diboron and carbon dioxide has been achieved. New boron-functionalized α-aryl carboxylic acids, including nonsteroidal anti-inflammatory drugs (NSAIDs), are obtained in moderate to excellent yields. The synthetic utility of the transformation was shown through subsequent derivatization of the carbon-boron bond yielding formal hydroxy- and fluorocarboxylation products as well as anionic difluoroboralactones.

Double carboxylation of o-alkynyl acetophenone with carbon dioxide

A copper-catalyzed double carboxylation of o-alkynyl acetophenone using carbon dioxide via a carboxylation/intramolecular cyclization/second carboxylation sequence to afford 1(3H)-isobenzofuranylidene dicarboxylic esters in good yields is developed.

Upgrading carbon dioxide by incorporation into heterocycles

Carbon dioxide is commonly regarded as the primary greenhouse gas, but from a synthetic standpoint can be utilized as an alternative and sustainable C1 synthon in organic synthesis rather than a waste. This results in the production of organic carbonates, carboxylic acids, and derivatives. Recently, CO2 has emerged as an appealing tool for heterocycle synthesis under mild conditions without using stoichiometric amounts of organometallic reducing reagents. This Minireview summarizes recent advances on methodologies for CO2 incorporation into N-, O-, and C-nucleophiles to provide various heterocycles, including cyclic carbamates, benzoxazine-2-one, 4-hydroxyquinolin-2-one, quinazoline-2,4(1 H,3 H)-diones, benzimidazolones, α-alkylidene cyclic carbonate.

Pd(ii)-catalyzed ligand controlled synthesis of methyl 1-benzyl-1H-indole-3-carboxylates and bis(1-benzyl-1H-indol-3-yl)methanones

A simple change of ligand and solvent allows controlled, effective switching between cyclization–carbonylation and cyclization–carbonylation–cyclization-coupling (CCC-coupling) reactions.