Isothiourea-Catalyzed Enantioselective Functionalisation of Glycine Schiff Base Aryl Esters via 1,6- and 1,4-Additions

The enantioselective α-functionalisation of glycine Schiff base aryl esters through isothiourea catalysis is successfully demonstrated for 1,6-additions to para-quinone methides (21 examples, up to 95:5 dr and 96:4 er) and 1,4-additions to methylene substituted dicarbonyl or disulfonyl Michael acceptors (17 examples, up to 98:2 er). This nucleophilic organocatalysis approach gives access to a range of α-functionalised α-amino acid derivatives and further transformations of the activated aryl ester group provide a straightforward entry to advanced amino acid-based esters, amides or thioesters.

Recent Advances in the Synthesis of 3,4-Dihydropyran-2-Ones Organocatalyzed by N-Heterocyclic Carbenes

In recent years, N-heterocyclic carbenes (NHC) have gained recognition as versatile molecules capable of acting as organocatalysts in various reactions, particularly through the activation of aldehydes via Breslow-type adducts. This organocatalytic activation has enabled the production of numerous 3,4-dihydropyran-2-ones and related derivatives. In this review, we provide an overview of the production of 3,4-dihydropyran-2-ones and derivatives via organocatalytic processes involving NHCs over the past eight years. These processes involve the use of a diverse range of substrates, catalysts, and reaction conditions, which can be classified into [4+2]-and [3+3]-type cycloadditions, primarily aimed at synthesizing this skeleton due to its biological activity and multiple stereocenters. These processes are scaled up to the gram scale, and the resulting products are often directed towards epimerization and functionalization to produce more complex molecules with potential applications in the biological field. Finally, we provide a perspective and the future directions of this topic in organic synthesis.

A Desilylative Approach to Alkyl Substituted C(1)‐Ammonium Enolates: Application in Enantioselective [2+2] Cycloadditions

The catalytic generation of C(1)‐ammonium enolates from the corresponding α‐silyl‐α‐alkyl substituted carboxylic acids using the isothiourea HyperBTM is reported. This desilylative approach grants access to α‐unsubstituted and α‐alkyl substituted C(1)‐ammonium enolates, which are typically difficult to access through traditional methods reliant upon deprotonation. The scope and limitations of this process is established in enantioselective [2+2]‐cycloaddition processes with perfluoroalkylketones (31 examples, up to 96 % yield and >99 : 1 er), as well as selective [2+2]‐cycloaddition with trifluoromethyl enones (4 examples, up to 75 % yield and >99 : 1 er). Preliminary mechanistic studies indicate this process proceeds through an initial kinetic resolution of an in situ prepared (±)‐α‐silyl‐α‐alkyl substituted anhydride, while the reaction process exhibits overall pseudo zero‐order kinetics.

Catalytic enantioselective synthesis of 1,4-dihydropyridines via the addition of C(1)-ammonium enolates to pyridinium salts

The regio- and stereoselective addition of C(1)-ammonium enolates - generated in situ from aryl esters and the isothiourea catalyst (R)-BTM - to pyridinium salts bearing an electron withdrawing substituent in the 3-position allows the synthesis of a range of enantioenriched 1,4-dihydropyridines. This represents the first organocatalytic approach to pyridine dearomatisation using pronucleophiles at the carboxylic acid oxidation level. Optimisation studies revealed a significant solvent dependency upon product enantioselectivity, with only toluene providing significant asymmetric induction. Using DABCO as a base also proved beneficial for product enantioselectivity, while investigations into the nature of the counterion showed that co-ordinating bromide or chloride substrates led to higher product er than the corresponding tetrafluoroborate or hexafluorophosphate. The scope and limitations of this process are developed, with enantioselective addition to 3-cyano- or 3-sulfonylpyridinium salts giving the corresponding 1,4-dihydropyridines (15 examples, up to 95 : 5 dr and 98 : 2 er).

[2.2]Paracyclophane-Based Isothiourea-Catalyzed Highly Enantioselective α-Fluorination of Carboxylic Acids

Planar chiral [2.2]paracyclophane-based isothiourea catalysts have been prepared over a few simple steps in high yields. In the presence of these catalysts, highly efficient catalytic enantioselective fluorination of carboxylic acids has been accomplished, providing a broad range of optically active α-fluoroesters in high yield and excellent enantioselectivity.

Enantioselective Synthesis of α-Aryl-β2 -Amino-Esters by Cooperative Isothiourea and Brønsted Acid Catalysis

The synthesis of α-aryl-β2 -amino esters through enantioselective aminomethylation of an arylacetic acid ester in high yields and enantioselectivity via cooperative isothiourea and Brønsted acid catalysis is demonstrated. The scope and limitations of this process are explored (25 examples, up to 94 % yield and 96:4 er), with applications to the synthesis of (S)-Venlafaxine⋅HCl and (S)-Nakinadine B. Mechanistic studies are consistent with a C(1)-ammonium enolate pathway being followed rather than an alternative dynamic kinetic resolution process. Control studies indicate that (i) a linear effect between catalyst and product er is observed; (ii) an acyl ammonium ion can be used as a precatalyst; (iii) reversible isothiourea addition to an in situ generated iminium ion leads to an off-cycle intermediate that can be used as a productive precatalyst.

Generation and Reactivity of C(1)-Ammonium Enolates by Using Isothiourea Catalysis

C(1)-Ammonium enolates are powerful, catalytically generated synthetic intermediates applied in the enantioselective α-functionalisation of carboxylic acid derivatives. This minireview describes the recent developments in the generation and application of C(1)-ammonium enolates from various precursors (carboxylic acids, anhydrides, acyl imidazoles, aryl esters, α-diazoketones, alkyl halides) using isothiourea Lewis base organocatalysts. Their synthetic utility in intra- and intermolecular enantioselective C-C and C-X bond forming processes on reaction with various electrophiles will be showcased utilising two distinct catalyst turnover approaches.

Tandem sequential catalytic enantioselective synthesis of highly-functionalised tetrahydroindolizine derivatives

An isothiourea-catalysed enantioselective synthesis of novel tetrahydroindolizine derivatives is reported through a one-pot tandem sequential process. The application of 2-(pyrrol-1-yl)acetic acid in combination with either a trifluoromethyl enone or an α-keto-β,γ-unsaturated ester in an enantioselective Michael addition–lactonisation process, followed by in situ ring-opening and cyclisation, led to a range of 24 tetrahydroindolizine derivatives containing three stereocentres in up to >95 : 5 dr and >99 : 1 er.

The isothiourea-catalysed enantioselective synthesis of tetrahydroindolizine derivatives containing three stereocentres is reported through a one-pot tandem sequential process.

Base-free Enantioselective C(1)-Ammonium Enolate Catalysis Exploiting Aryloxides: A Synthetic and Mechanistic Study

An isothiourea-catalyzed enantioselective Michael addition of aryl ester pronucleophiles to vinyl bis-sulfones via C(1)-ammonium enolate intermediates has been developed. This operationally simple method allows the base-free functionalization of aryl esters to form α-functionalized products containing two contiguous tertiary stereogenic centres in excellent yield and stereoselectivity (all ≥99:1 er). Key to the success of this methodology is the multifunctional role of the aryloxide, which operates as a leaving group, Brønsted base, Brønsted acid and Lewis base within the catalytic cycle. Comprehensive mechanistic studies, including variable time normalization analysis (VTNA) and isotopologue competition experiments, have been carried out. These studies have identified (i) orders of all reactants; (ii) a turnover-limiting Michael addition step, (iii) product inhibition, (iv) the catalyst resting state and (v) catalyst deactivation through protonation.

An isothiourea-catalyzed asymmetric formal [4 + 2] cycloaddition of in situ generated azoalkenes with C1 ammonium enolates

An efficient isothiourea-catalyzed stereoselective formal [4 + 2] cycloaddition of α-chloro cyclic hydrazones with carboxylic acids has been developed.

Insights into the isothiourea-catalyzed asymmetric [4 + 2] annulation of phenylacetic acid with alkylidene pyrazolone

A computational study on the isothiourea-catalyzed asymmetric [4 + 2] annulation of phenylacetic acid with alkylidene pyrazolone was performed using the DFT method.

Asymmetric Synthesis of cis-3,4-Dihydrocoumarins via [4 + 2] Cycloadditions Catalyzed by Amidine Derivatives

A highly efficient chiral amidine derivative-catalyzed tandem Michael addition/lactonization of carboxylic acids and o-quinone methides (o-QMs) has been developed that enables the asymmetric synthesis of cis-3,4-dihydrocoumarins bearing contiguous tertiary stereogenic centers in high yields with excellent stereoselectivities.

Targeting flavivirus RNA dependent RNA polymerase through a pyridobenzothiazole inhibitor

RNA dependent RNA polymerases (RdRp) are essential enzymes for flavivirus replication. Starting from an in silico docking analysis we identified a pyridobenzothiazole compound, HeE1-2Tyr, able to inhibit West Nile and Dengue RdRps activity in vitro, which proved effective against different flaviviruses in cell culture. Crystallographic data show that HeE1-2Tyr binds between the fingers domain and the priming loop of Dengue virus RdRp (Site 1). Conversely, enzyme kinetics, binding studies and mutational analyses suggest that, during the catalytic cycle and assembly of the RdRp-RNA complex, HeE1-2Tyr might be hosted in a distinct binding site (Site 2). RdRp mutational studies, driven by in silico docking analysis, allowed us to locate the inhibition Site 2 in the thumb domain. Taken together, our results provide innovative concepts for optimization of a new class of anti-flavivirus compounds.

Enantioselective Synthesis of 3,5,6-Substituted Dihydropyranones and Dihydropyridinones using Isothiourea-Mediated Catalysis

The scope of dihydropyranone and dihydropyridinone products accessible by isothiourea-catalyzed processes has been expanded and explored through the use of 2-N-tosyliminoacrylates and 2-aroylacrylates in a Michael addition-lactonization/lactamization cascade reaction. Notably, to ensure reproducibility it is essential to use homoanhydrides as ammonium enolate precursors with 2-aroyl acrylates, while carboxylic acids can be used with 2-N-tosyliminoacrylates, delivering a range of 3,5,6-substituted dihydropyranones and dihydropyridinones with high enantioselectivity (typically >90 % ee). The derivatization of the heterocyclic core of a 3,5,6-substituted dihydropyranone through hydrogenation is also reported.

Asymmetric Isothiourea-Catalysed Formal [3+2] Cycloadditions of Ammonium Enolates with Oxaziridines

A highly enantioselective Lewis base-catalysed formal [3+2] cycloaddition of ammonium enolates and oxaziridines to give stereodefined oxazolidin-4-ones in high yield is described. Employing an enantioenriched oxaziridine in this process leads to a matched/mis-matched effect with the isothiourea catalyst and allowed the synthesis of either syn- or anti-stereodefined oxazolidin-4-ones in high d.r., yield and ee. Additionally, the oxazolidin-4-one products have been derivatised to afford functionalised enantioenriched building blocks.

Exploring the scope of the isothiourea-mediated synthesis of dihydropyridinones

The exploration and expansion of the scope of the isothiourea-mediated synthesis of dihydropyridinones is presented.