Enol Esters: Versatile Substrates for Mannich-Type Multicomponent Reactions

Atom-economic and stereoselective catalytic synthesis of fully substituted enol esters/carbonates of amides in acyclic systems enabled by boron Lewis acid catalysis

Carboacyloxylation of internal alkynes is emerging as a powerful and straightforward strategy for enol ester synthesis. However, the reported examples come with limitations, including the utilization of noble metal catalysts, the control of regio- and Z/E selectivity, and an application in the synthesis of enol carbonates. Herein, a boron Lewis acid-catalyzed intermolecular carboacyloxylation of ynamides with esters to access fully substituted acyclic enol esters in high yield with generally high Z/E selectivity (up to >96 : 4) is reported. Most importantly, readily available allylic carbonates are also compatible with this difunctionalization reaction, representing an atom-economic, catalytic and stereoselective protocol for the construction of acyclic β,β-disubstituted enol carbonates of amides for the first time. The application of the carboacyloxylation products to decarboxylative allylations provided a ready access to enantioenriched α-quaternary amides. Moreover, experimental studies and theoretical calculations were performed to illustrate the reaction mechanism and rationalize the stereochemistry.

Synthesis of Amide Enol 2-Iodobenzoates by the Regio- and Stereoselective Gold-Catalyzed Acyloxyalkynylation of Ynamides with Hypervalent Iodine Reagents

Multisubstituted alkenes are accessible by a gold-catalyzed acyloxyalkynylation of ynamides with ethynylbenziodoxolones (EBXs) with perfect atom-economy. The EBX reagents transfer both the carboxylate as well as the alkynyl entity. Overall, this cascade comprises the in situ generation of an alkynyl gold(III) species, a stereoselective C(sp)-C(sp²) bond formation, and a C-O coupling at the alkynyl position of the ynamides. This reaction proceeds under mild conditions and accepts a wide range of substrates. A number of tetrasubstituted amide enol 2-iodobenzoates bearing different functional groups were obtained in good to excellent yields. DFT calculations explain the observed regioselectivity. The synthetic potential of the reaction was further demonstrated by a number of selected follow-up transformations.

Multicomponent Reactions for the Synthesis of Active Pharmaceutical Ingredients

Multicomponent reactions 9i.e., those that engage three or more starting materials to form a product that contains significant fragments of all of them), have been widely employed in the construction of compound libraries, especially in the context of diversity-oriented synthesis. While relatively less exploited, their use in target-oriented synthesis offers significant advantages in terms of synthetic efficiency. This review provides a critical summary of the use of multicomponent reactions for the preparation of active pharmaceutical principles.

Interplay between the Directing Group and Multifunctional Acetate Ligand in Pd-Catalyzed anti-Acetoxylation of Unsymmetrical Dialkyl-Substituted Alkynes

The cooperative action of the acetate ligand, the 2-pyridyl sulfonyl (SO₂Py) directing group on the alkyne substrate, and the palladium catalyst has been shown to be crucial for controlling reactivity, regioselectivity, and stereoselectivity in the acetoxylation of unsymmetrical internal alkynes under mild reaction conditions. The corresponding alkenyl acetates were obtained in good yields with complete levels of β-regioselectivity and anti-acetoxypalladation stereocontrol. Experimental and computational analyses provide insight into the reasons behind this delicate interplay between the ligand, directing group, and the metal in the reaction mechanism. In fact, these studies unveil the multiple important roles of the acetate ligand in the coordination sphere at the Pd center: (i) it brings the acetic acid reagent into close proximity to the metal to allow the simultaneous activation of the alkyne and the acetic acid, (ii) it serves as an inner-sphere base while enhancing the nucleophilicity of the acid, and (iii) it acts as an intramolecular acid to facilitate protodemetalation and regeneration of the catalyst. Further insight into the origin of the observed regiocontrol is provided by the mapping of potential energy profiles and distortion–interaction analysis.

Enantioselective catalytic Povarov reactions

Catalytic asymmetric Povarov protocols have undergone an explosive growth, especially in the last ten years, since the first example was published in 1996. The use of chiral Lewis and Brønsted acids and dual strategies based on their combination with catalysts acting by hydrogen bond formation, as well as covalent aminocatalysis, are reviewed. More recent variations such as the nitroso Povarov reaction and interrupted Povarov reactions as a route to chiral scaffolds other than tetrahydroquinolines are also discussed.

Reexamining Povarov Reaction’s Scope and Limitation in the Generation of HCV-NS4A Peptidomimetics

Chronic Hepatitis C is a global health threat and a silent killer. Regardless of the profound progress in preventing and treating this disease, research continues to discover new direct antiviral agents (DAAs), especially against novel targets. Our research has been directed to leverage the NS4A binding site to develop peptidomimetic inhibitors of the hepatitis C virus (HCV) NS3 protease. In previous reports, we could provide evidence of tunability of this site by peptide and nonpeptide NS3/4A inhibitors. In this report, we used structure-based techniques to design 1,2,3,4-tetrahydro-1,7-naphthyridine derivative as NS4A core mimics that cover the region between residues Ile-25′ to Arg-28′. The synthetic plan featured the Povarov reaction as an efficient strategy to construct the 1,7-naphthyridine core. Although this reaction has been reported in many literatures, critical assessments for its scope and limitations are scarce. In our work, we found that Povarov was extremely sensitive to alkene and aldehyde reactants. Moreover, using pyridine amines was not as successful as anilines. The most striking results were the lack of stability of compounds during purification and storage. The four compounds that survived the stability problems (1a-1d) did not show significant binding potency with NS3, because their structures were too simple to resemble the originally planned compounds.

Development of Heterocyclic Multicomponent Reactions through Guided Exploration: Direct, Reasonable and Unpredictable Processes

This account summarizes the research of the group on the Multicomponent Reactions arena with fundamental heterocycles as substrates, using mechanistic considerations to hypothesize new processes and to rationalize results. Biomedical applications of the ensuing adducts were also envisaged, which brought about interesting discoveries.

Interrupted reactions in chemical synthesis

Interrupted reactions reroute established processes to new and often unanticipated end points. Of particular interest are the cases in which a known reactive intermediate takes on a new reaction pathway, either because this pathway is lower in energy or because the conventional pathway is no longer available. Through analysis of documented cases, we aim to dissect the known interrupted reactions and trace their mechanistic origins. As new chemical processes are being discovered at a seemingly ever-increasing pace, it is likely that new interrupted reactions will continue to emerge. Our hope is that the cases considered in this Review will help identify new classes of these fascinating transformations.

Phosphine-Free Ru-Catalyzed Regio- and Stereoselective Addition of Benzoic Acids to Trifluoromethylated Alkynes toward Facile Access to Trifluoromethyl Group-Substituted (E)-Enol Esters

A combination of ruthenium catalyst with silver salt and copper salt was proved to be a highly efficient protocol for the direct addition reaction of benzoic acids with unsymmetrical trifluoromethylated internal alkynes. Diverse trifluoromethyl group-substituted (E)-enol esters were readily obtained for a broad substrate scope in moderate to good yields with excellent regio- and stereoselectivities under mild reaction conditions.

Regioselective β-Arylation of α-Angelica Lactone through Isomerization/Addition under Mild Conditions

The conversion of biomass-based platform molecules into various high-value chemicals greatly promotes the utilization of renewable biomass resources. Herein, an example of Rh-catalyzed β-arylation of levulinic-acid-derived α-angelica lactone was reported, providing the γ-lactone-structure products with high regioselectivity. Both arylboronic and alkenylboronic acids could be applied in this transformation. This reaction tolerated a variety of synthetically important functional groups. Moreover, the obtained γ-lactone products could be readily converted to high-value products such as 1,4-diols and γ-methoxy-carboxylates.

Multicomponent synthesis of pyroglutamic acid derivatives via Knoevenagel-Michael-hydrolysis-lactamization-decarboxylation (KMHL-D) sequence

A novel and practical method for the synthesis of 3-substituted pyroglutamic acid derivatives is described. One pot multicomponent reaction of Meldrum's acid, aldehyde and Schiff's base followed an unprecedented chemoselective Knoevenagel-Michael-hydrolysis-lactamization domino sequence to afford 4-carboxy 3-substituted pyroglutamic acid derivatives under mild conditions. A carboxy intermediate formed appears to accelerate its own formation. The generality of the synthesis is exemplified by the use of a wide variety of aldehydes including enolizable aliphatic aldehydes, while substrates are stable under reaction conditions.

Angelica Lactones: From Biomass-Derived Platform Chemicals to Value-Added Products

The upgrading of biomass-derived compounds has arisen in recent years as a very promising research field in both academia and industry. In this sense, a lot of new processes and products have been developed, often involving levulinic acid as a starting material or intermediate. In the last few years, though, other scaffolds have been receiving growing attention, especially, angelica lactones. Considering these facts and the emergent applications of said molecules, in this review we will discuss their preparation and applications; the use of these frameworks as starting materials in organic synthesis to produce potential bioactive compounds will be covered, as will their use as a foundation to highly regarded compounds such as liquid alkanes with prospective use as fuels and polymers.

Visible light-induced aerobic oxidative cross-coupling of glycine esters with α-angelicalactone: a facile pathway to γ-lactams

A rapid synthesis of substituted γ-lactams is achieved via metal-free photocatalyzed aerobic oxidative dehydrogenative formal [2 + 3] cyclization of glycine esters with α-angelicalactone.

Cu(i)-Catalyzed stereoselective synthesis of trisubstituted Z-enol esters via interrupting the 1,3-O-transposition reaction

An efficient Cu(i)-catalyzed stereoselective synthesis of trisubstituted Z-enol esters via interrupting the 1,3-O-transposition process is reported, which provided a convenient approach to highly functionalized Z-enol esters.

Gold(i)-catalyzed addition of carboxylic acids to internal alkynes in aqueous medium

A broad scope catalytic system for the intermolecular addition of carboxylic acids to internal alkynes, in water under mild conditions, has been developed.

Copper(II)-Promoted Cyclization/Difunctionalization of Allenols and Allenylsulfonamides: Synthesis of Heterocycle-Functionalized Vinyl Carboxylate Esters

A unique method to affect intramolecular aminooxygenation and dioxygenation of allenols and allenylsulfonamides is described. These operationally simple reactions occur under neutral or basic conditions where copper(II) carboxylates serve as reaction promoter, oxidant, and carboxylate source. Moderate to high yields of heterocycle-functionalized vinyl carboxylate esters are formed with moderate to high levels of diastereoselectivity. Such vinyl carboxylate esters could serve as precursors to α-amino and α-oxy ketones and derivatives thereof.

Stereoselective one-pot synthesis of β-alkylsulfide enol esters. Base-triggered rearrangement under mild conditions

A stereoselective one-pot procedure was developed to prepare S-substituted (Z)-enol esters through a base-triggered rearrangement. This transition metal-free multicomponent approach can be performed under an air atmosphere at room temperature, tolerates a wide set of chemical functionalities and generally affords high isolated yields. The (Z)-selectivity arises from the [1,4]-S- to O-acyl migration.

Chiral phosphoric acid-catalyzed enantioselective three-component Povarov reaction using cyclic enethioureas as dienophiles: stereocontrolled access to enantioenriched hexahydropyrroloquinolines

Three become one: Phosphoric acid-catalyzed enantioselective three-component Povarov reactions of aldehydes, anilines, and endocyclic enethioureas have been developed (see scheme). This process afforded hexahydropyrroloquinolines in high yields with excellent diastereo- and enantioselectivities. The presence of the thiourea functionality is crucial for the enantioselectivity of the reaction.

Unsaturated lactams: new inputs for povarov-type multicomponent reactions

Unsaturated lactams with endo- or exocyclic C-C double bonds constitute a set of reactive inputs that serve as the electron-rich olefin component in Povarov reactions. These substrates afford the multicomponent adducts in convenient yields and offer a wide range of structural diversity. Postcondensation transformations allow direct access to a variety of lactam-fused and amide-substituted quinoline derivatives.