One-Pot Synthesis of Multi-Substituted 4-Pyrones by a Gold- Catalyzed Cascade of Wolff Rearrangement/[4+2] Cycloaddition/ Elimination Reactions

Synthesis and anticancer evaluation of new lupane triterpenoid derivatives containing various substituent at the 2 or 3 position

Betulonic acid benzyl ester 1 has been subjected to a series of structural modifications for the purpose of new triterpenoid synthesis and evaluating for anticancer activity. The one-pot two step synthesis of 2α-(aminomethyl)betulinic acid benzyl ester derivatives 3a-f (yield 46-69 %) was achieved by the Mannich reaction of compound 1 with methyleneiminium salts, generated in situ from N,N-disubstituted bis(amino)methanes 2a-f by the action of acetyl chloride in dichloromethane, and subsequent reduction of aminomethylation products with sodium borohydride. Minor 2β-(aminomethyl) triterpenoids 4c,d,f were also isolated (yield 6-15 %). We found, that the stereoselective reaction of triterpenoid 1 with acetylides, generated at -78 °C from alkynes in the presence of n-BuLi, has been useful and noteworthy as the key step in providing of new alkyne substituted triterpenoids - benzyl 3-alkynyl-3-deoxy-2(3),20(29)-lupadiene-28-oates or 3-deoxy-2(3)-dehydro-28-oxoallobetulin derivative. The new compounds were examined for anticancer activity against the human cell lines (MTT assay). All tested derivatives were non-toxic on human fibroblasts. The 3-(phenylethynyl)lupa-2(3),20(29)-diene 9 showed selective cytotoxicity on cervical cancer cell lines. Tumor cells death trigged by the most active compound 4f resulted from apoptotic processes. These data make the series of synthesized 2 or 3 substituted lupane derivatives as promising compounds with anticancer potential.

Advancements in Gold-Catalyzed Cascade Reactions to Access Carbocycles and Heterocycles: An Overview

This review summarizes recent developments (from 2006 to 2022) in numerous important and efficient carbo- and heterocycle generations using gold-catalyzed cascade protocols. Herein, methodologies involve selectivity, cost-effectiveness, and ease of product formation being controlled by the ligand as well as the counter anion, catalyst, substrate, and reaction conditions. Gold-catalyzed cascade reactions covered different strategies through the compilation of various approaches such as cyclization, hydroarylation, intermolecular and intramolecular cascade reactions, etc. This entitled reaction is also useful for the synthesis of spiro, fused, bridged carbo- and heterocycles.

Synthesis of γ-Pyrones and N-Methyl-4-pyridones via the Au Nanoparticle-Catalyzed Cyclization of Skipped Diynones in the Presence of Water or Aqueous Methylamine

Supported Au nanoparticles on TiO₂ catalyze the hydration/6-endo cyclization of skipped diynones to γ-pyrones in aqueous dioxane, via triple bond activation. The isomeric 3(2H)-furanones which could be formed through a competing and often prevailing 5-exo cyclization pathway using homogeneous ionic Au(I) catalysts were not seen. The reaction does not proceed via the initial 1,3-transposition of the skipped diynones to their corresponding conjugated 1,3-diynone isomers. If aqueous methylamine is added, N-methyl-4-pyridones are exclusively formed in 69-79% yields via an analogous hydroamination/Au-catalyzed 6-endo cyclization pathway.

Catalyst-Controlled Chemodivergent Reactions of 2-Pyrrolyl-α-diazo-β-ketoesters and Enol Ethers: Synthesis of 1,2-Dihydrofuran Acetals and Highly Substituted Indoles

A catalyst-controlled, chemodivergent reaction of pyrrolyl-α-diazo-β-ketoesters with enol ethers is reported. While Cu(II) catalysts selectively promoted a [3 + 2] cycloaddition to provide pyrrolyl-substituted 2,3-dihydrofuran (DHF) acetals, dimeric Rh(II) catalysts afforded 6-hydroxyindole-7-carboxylates via an unreported [4 + 2] benzannulation. The choice of enol ether proved to be crucial in determining both regioselectivity and yield of the respective products (up to 91% yield for Cu(II) and 82% for Rh(II) catalysis). Furthermore, the DHF acetals were shown to serve as precursors to 7-hydroxyindole-6-carboxylates (isomeric to the indoles formed from Rh) and highly substituted furans in the presence of Lewis acids. Thus, from a common pyrrolyl-α-diazo-β-ketoester, up to three unique heterocyclic scaffolds can be achieved based on catalyst selection.

"One-Pot" Synthesis of γ-Pyrones from Aromatic Ketones/Heteroarenes and Carboxylic Acids

Despite the various attractive properties of γ-pyrones, there are still some deficiencies in their synthetic approaches such as lower atom economy, multistep processes, and prefunctionalization of the reagents. In this work, an efficient and simple (CF₃CO)₂O/CF₃SO₃H-mediated "one-pot" approach was realized to produce γ-pyrones by applying aromatic ketones/heteroarenes and carboxylic acids as the starting materials. The target products were isolated in moderate to excellent yields. The reaction mechanism was studied by density functional theory calculational methods. The results of experimental and theoretical investigations not only helped us explain the reason of high selectivity formation of β-diketones but also proved that 1,3,5-ketones might be important intermediates for the cyclization to afford γ-pyrones.

Normal, Abnormal, and Cascade Wittig Olefinations of α-Oxoketenes

α-Oxoketenes generated in situ by a thermal Wolff rearrangement have been found to participate as 1,2- and 1,4-ambident C-electrophilic/O-nucleophilic reagents towards donor/acceptor carbonyl-stabilized Wittig ylides. This resulted in the very direct and practical syntheses of functionalized allenes by a normal Wittig olefination, 4H-pyran-4-ones by an abnormal Wittig olefination, or 4H-pyranylidenes following a Wittig/abnormal Wittig cascade sequence as a function of the substrates combination employed. Mechanistic experimental and computational studies provided a full rational for these reactivity switches. Some unusual mechanistic features for Lewis acid-free Wittig olefinations were identified in this series such as the involvement of betaine intermediates and some degree of reversibility in the normal Wittig olefination. The abnormal Wittig olefination was fully uncovered.

Regiospecific construction of diverse and polyfunctionalized γ-pyrone cores by indium(iii)-catalyzed annulation of diazodicarbonyls with active methylenes, 4-hydroxycoumarins, or 4-hydroxyquinolinone

An efficient and novel annulation protocol for the construction of diverse and polyfunctionalized γ-pyrones via an InBr₃-catalyzed reaction is described.

Regioselective Reversal in the Cyclization of 2-Diazo-3,5-dioxo-6-ynoates (Ynones, Ynamide): Construction of γ-Pyrones and 3(2H)-Furanones Starting from Identical Materials

The AgSbF₆-catalyzed cyclization of 2-diazo-3,5-dioxo-6-ynoates (ynones, ynamide) in alcoholic solvents affords γ-pyrones, whereas the AgOAc-catalyzed cyclization in 1,2-dichloroethane (DCE) produces 3(2H)-furanones. The cyclization reactions proceeded cleanly under mild reaction conditions, and the desired γ-pyrones or 3(2H)-furanones were obtained in excellent yield. It was observed for the first time that both the catalyst and solvent play key roles in the selective formation. This unique method for the reversal of regioselectivity proved to be highly efficient except for substrates with aliphatic and Me₃Si groups at the triple bond position.

Gold-catalyzed transformations of α-diazocarbonyl compounds: selectivity and diversity

Metal carbenes, generated from the decomposition of diazo compounds by transition-metals, have a broad range of applications in organic synthesis. The progress of organic transformation via transition metal (such as Rh, Cu, Fe, Ag) carbenes has been well summarized in a few excellent reviews. This review will summarize gold-catalyzed transformations of α-diazocarbonyl compounds by highlighting the specificity and applicability of these diverse transformations such as X-H insertion, C-H functionalization, cyclopropanation, cycloaddition, and coupling reactions. In order to understand these reactions, the mechanistic rationale for selected examples is also provided.

Synthesis of diverse isatins via ring contraction of 3-diazoquinoline-2,4-diones

An efficient protocol for the construction of diverse isatin derivatives was developed starting from 3-diazoquinoline-2,4-diones via ring contraction reaction.

Rhodium(ii)-catalyzed direct sulfenylation of diazooxindoles with disulfides

Rhodium(ii)-catalyzed reactions for direct sulfenylation of diazooxindoles with disulfides via C–S bond formation was developed.

Regioselective synthesis of 3-anthracenyloxindoles and 3-carbazolyloxindoles by indium(iii)-catalyzed direct arylation and their fluorescent chemosensor properties

An efficient protocol for diverse 3-anthracenyloxindoles and 3-carbazolyloxindoles has been developed by In(OTf)₃-catalyzed direct arylation of 3-diazooxindoles with anthracenes or carbazoles.

Addition of silylated nucleophiles to α-oxoketenes

A general evaluation of silylated nucleophiles to intercept transient α-oxoketenes generated by microwave-assisted Wolff rearrangement of 2-diazo-1,3-dicarbonyl compounds is presented.

Gold and diazo reagents: a fruitful tool for developing molecular complexity

After a decade since its discovery and being yet at its sunrise, the use of gold catalysis for the transfer of carbene groups from diazo compounds is becoming a powerful synthetic tool. The main advances in the area and future directions are provided in this Feature Article.