Isothiourea-Catalyzed Asymmetric Synthesis of β-Lactams and β-Amino Esters from Arylacetic Acid Derivatives and N-Sulfonylaldimines

Synthesis of functionalized 3-aryl-3H-benzofuranone derivatives from aryl acetate via [3 + 2] annulation of 1,4-dihydroxy-2-naphthoic acid ester

A novel [3 + 2] cascade reaction of aryl acetate with 1,4-dihydroxy-2-naphthoic acid ester by cooperative catalysis of isothiourea and vanadium is developed. The key step of the reaction is that the C1-ammonium enolates generated from aryl acetate will cycloadduct to 1,4-naphthoquinone intermediates. 3-Aryl-3H-benzofuranone derivatives can be prepared in medium to good yields under mild conditions.

Isothiourea-Catalyzed [2 + 2] Cycloaddition of C(1)-Ammonium Enolates and N-Alkyl Isatins

Enantioselective [2 + 2] cycloaddition of C(1)-ammonium enolates generated catalytically using the isothiourea HyperBTM with N-alkyl isatins gives spirocyclic β-lactones. In situ ring opening with an amine nucleophile generates isolable highly enantioenriched products in up to 92:8 dr and in >99:1 er.

Solvent directed chemically divergent synthesis of β-lactams and α-amino acid derivatives with chiral isothiourea

A protocol for the chemically divergent synthesis of β-lactams and α-amino acid derivatives with isothiourea (ITU) catalysis by switching solvents was developed. The stereospecific Mannich reaction occurring between imine and C(1)-ammonium enolate generated zwitterionic intermediates, which underwent intramolecular lactamization and afforded β-lactam derivatives when DCM and CH₃CN were used as solvents. However, when EtOH was used as the solvent, the intermediates underwent an intermolecular esterification reaction, and α-amino acid derivatives were produced. Detailed mechanistic experiments were conducted to prove that these two kinds of products came from the same intermediates. Furthermore, chemically diversified transformations of β-lactam and α-amino acid derivatives were achieved.

A protocol for the solvent directed chemically divergent synthesis of β-lactam and α-amino acid derivatives with chiral isothiourea was reported.

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).

cis-Diastereoselective Synthesis of Spirooxindolo-β-Lactams by Staudinger Cycloaddition with TsCl as Activating Co-reagent

A convenient and versatile one-pot method for synthesis of 1,3-bis-aryl spirooxindolo-β-lactams from isatin Schiff bases and substituted phenylacetic acids using ketene-imine cycloaddition reaction with TsCl for a ketene generation has been developed. The reaction procedure does not require absolute solvents and unstable starting reagents. The studied reactions lead to cis-diastereoselective β-lactam formation for all tested phenylacetic acids except 4-MeOC6H4CH2COOH. An increase of trans-diastereomers yields with increasing temperature and solvent polarity was demonstrated.

Visible-Light-Induced Cycloaddition of α-Ketoacylsilanes with Imines: Facile Access to β-Lactams

We report the synthesis of β-lactams from α-ketoacylsilanes and imines, which proceeds via a formal [2+2] photochemical cycloaddition with in situ generation of siloxyketene. This mild and operationally simple reaction proceeds in an atom-economic fashion with broad substrate scope, including aldimines, ketimines, hydrazones, and fused nitrogen heterocycles, affording a variety of important β-lactams with satisfactory diastereoselectivities in most cases. This reaction also features good functional-group tolerance, facile scalability and product diversification. Experimental and computational studies suggest that α-ketoacylsilanes can serve as photochemical precursors by engaging in a 1,3 silicon shift to the distal carbonyl group.

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.

Insight into Isothiourea-Catalyzed Enantioselective Addition of Saturated Esters to Iminium Ions

The possible mechanisms and origin of the selectivities of isothiourea-catalyzed addition of saturated esters to iminium ions have been investigated by density functional theory. The favorable reaction pathway includes three stages: formation of an ammonium enolate intermediate, enantioselective addition of the ammonium enolate intermediate to the iminium ion, and dissociation of the catalyst to form the product. The enantioselective addition process is the stereoselectivity-determining step, while the chemoselectivity-determining step is included in the formation of the final product. The calculated energy barriers show that the chemoselectivity is thermodynamically controlled, and it depends on the polarities of the products and the nucleophilicities of the N atoms of the enamine reactant moieties of the intermediates. The origin of the stereoselectivity was investigated by non-covalent interaction analysis of the key transition states. Hydrogen bonding interactions were identified as the determining factor for controlling the stereoselectivity. The obtained insight will be valuable for rational design of novel Lewis base organocatalyst-promoted enantioselective addition reactions with special chemoselectivities.

Frontiers in Halogen and Chalcogen-Bond Donor Organocatalysis

Non-covalent molecular interactions on the basis of halogen and chalcogen bonding represent a promising, powerful catalytic activation mode. However, these "unusual" non-covalent interactions are typically employed in the solid state and scarcely exploited in catalysis. In recent years, an increased interest in halogen and chalcogen bonding has been awaken, as they provide profound characteristics that make them an appealing alternative to the well-explored hydrogen bonding. Being particularly relevant in the binding of "soft" substrates, the similar strength to hydrogen bonding interactions and its higher directionality allows for solution-phase applications with halogen and chalcogen bonding as the key interaction. In this mini-review, the special features, state-of-the-art and key examples of these so-called σ-hole interactions in the field of organocatalysis are presented.

Catalytic enantioselective synthesis of perfluoroalkyl-substituted β-lactones via a concerted asynchronous [2 + 2] cycloaddition: a synthetic and computational study

The enantioselective preparation of a range of perfluoroalkyl-substituted β-lactones through an isothiourea (HyperBTM) catalysed reaction using symmetric anhydrides as ammonium enolate precursors and perfluoroalkylketones (R_F = CF₃, C₂F₅, C₄F₉) is reported. Following optimisation, high diastereo- and enantioselectivity was observed for β-lactone formation using C₂F₅- and C₄F₉-substituted ketones at room temperature (26 examples, up to >95 : 5 dr and >99 : 1 er), whilst -78 °C was necessary for optimal dr and er with CF₃-substituted ketones (11 examples, up to >95 : 5 dr and >99 : 1 er). Derivatisation of the β-lactones through ring-opening, as well as a two-step conversion to give perfluoroalkyl-substituted oxetanes, is demonstrated without loss of stereochemical integrity. Density functional theory computations, alongside ¹³C natural abundance KIE studies, have been used to probe the reaction mechanism with a concerted asynchronous [2 + 2]-cycloaddition pathway favoured over a stepwise aldol-lactonisation process.

Evaluating aryl esters as bench-stable C(1)-ammonium enolate precursors in catalytic, enantioselective Michael addition–lactonisations

An evaluation of a range of aryl, alkyl and vinyl esters as prospective C(1)-ammonium enolate precursors in enantioselective Michael addition–lactonisation processes with (E)-trifluoromethylenones using isothiourea catalysis is reported.

Chalcogen Bonding: An Overview

In the last few decades, "unusual" noncovalent interactions like anion-π and halogen bonding have emerged as interesting alternatives to the ubiquitous hydrogen bonding in many research areas. This is also true, to a somewhat lesser extent, for chalcogen bonding, the noncovalent interaction involving Lewis acidic chalcogen centers. Herein, we aim to provide an overview on the use of chalcogen bonding in crystal engineering and in solution, with a focus on the recent developments concerning intermolecular chalcogen bonding in solution-phase applications. In the solid phase, chalcogen bonding has been used for the construction of nano-sized structures and the self-assembly of sophisticated self-complementary arrays. In solution, until very recently applications mostly focused on intramolecular interactions which stabilized the conformation of intermediates or reagents. In the last few years, intermolecular chalcogen bonding has increasingly also been exploited in solution, most notably in anion recognition and transport as well as in organic synthesis and organocatalysis.

Organocatalytic Enantioselective Synthesis of α-Fluoro-β-amino Acid Derivatives

Asymmetric cyclocondensation of N-sulfonylimines with fluoroacetic acid promoted by isothiourea catalyst HBTM-2 generates 3-fluoro-β-lactams with high enantio- and diastereoselectivity. These reactive compounds are opened with alcohols or amines to produce the corresponding α-fluoro-β-amino acid derivatives in moderate yields.

Chalcogen bonding in synthesis, catalysis and design of materials

Chalcogen bonding is a type of noncovalent interaction in which a covalently bonded chalcogen atom (O, S, Se or Te) acts as an electrophilic species towards a nucleophilic (negative) region(s) in another or in the same molecule. In general, this interaction is strengthened by the presence of an electron-withdrawing group on the electron-acceptor chalcogen atom and upon moving down in the periodic table of elements, from O to Te. Following a short discussion of the phenomenon of chalcogen bonding, this Perspective presents some demonstrative experimental observations in which this bonding is crucial for synthetic transformations, crystal engineering, catalysis and design of materials as synthons/tectons.

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.

Difference in Energy between Two Distinct Types of Chalcogen Bonds Drives Regioisomerization of Binuclear (Diaminocarbene)PdII Complexes

The reaction of cis-[PdCl₂(CNXyl)₂] (Xyl = 2,6-Me₂C₆H₃) with various 1,3-thiazol- and 1,3,4-thiadiazol-2-amines in chloroform gives a mixture of two regioisomeric binuclear diaminocarbene complexes. For 1,3-thiazol-2-amines the isomeric ratio depends on the reaction conditions and kinetically (KRs) or thermodynamically (TRs) controlled regioisomers were obtained at room temperature and on heating, respectively. In CHCl₃ solutions, the isomers are subject to reversible isomerization accompanied by the cleavage of Pd-N and C-N bonds in the carbene fragment XylNCN(R)Xyl. Results of DFT calculations followed by the topological analysis of the electron density distribution within the formalism of Bader's theory (AIM method) reveal that in CHCl₃ solution the relative stability of the regioisomers (ΔG_exp = 1.2 kcal/mol; ΔG_calcd = 3.2 kcal/mol) is determined by the energy difference between two types of the intramolecular chalcogen bonds, viz. S···Cl in KRs (2.8-3.0 kcal/mol) and S···N in TRs (4.6-5.3 kcal/mol). In the case of the 1,3,4-thiadiazol-2-amines, the regioisomers are formed in approximately equal amounts and, accordingly, the energy difference between these species is only 0.1 kcal/mol in terms of ΔG_exp (ΔG_calcd = 2.1 kcal/mol). The regioisomers were characterized by elemental analyses (C, H, N), HRESI⁺-MS and FTIR, 1D (¹H, ¹³C{¹H}) and 2D (¹H,¹H-COSY, ¹H,¹H-NOESY, ¹H,¹³C-HSQC, ¹H,¹³C-HMBC) NMR spectroscopies, and structures of six complexes (three KRs and three TRs) were elucidated by single-crystal X-ray diffraction.

Isothiourea-catalysed enantioselective pyrrolizine synthesis: synthetic and computational studies

The catalytic enantioselective synthesis of a range of cis-pyrrolizine carboxylate derivatives with outstanding stereocontrol (14 examples, >95 : 5 dr, >98 : 2 er) through an isothiourea-catalyzed intramolecular Michael addition-lactonisation and ring-opening approach from the corresponding enone acid is reported. An optimised and straightforward three-step synthetic route to the enone acid starting materials from readily available pyrrole-2-carboxaldehydes is delineated, with benzotetramisole (5 mol%) proving the optimal catalyst for the enantioselective process. Ring-opening of the pyrrolizine dihydropyranone products with either MeOH or a range of amines leads to the desired products in excellent yield and enantioselectivity. Computation has been used to probe the factors leading to high stereocontrol, with the formation of the observed cis-steroisomer predicted to be kinetically and thermodynamically favoured.

Enantioselective isothiourea-catalysed trans-dihydropyridinone synthesis using saccharin-derived ketimines: scope and limitations

The catalytic enantioselective synthesis of a range of trans-dihydropyridinones from aryl-, heteroaryl- and alkenylacetic acids and saccharin-derived ketimines with good to excellent stereocontrol (15 examples, up to >95 : 5 dr, up to >99 : 1 er) is reported. After extensive optimisation, HyperBTM proved the optimal isothiourea catalyst for this transformation at -78 °C, giving trans-dihydropyridones with generally excellent levels of diastereo- and enantioselectivity.

Development and Pharmacological Characterization of Selective Blockers of 2-Arachidonoyl Glycerol Degradation with Efficacy in Rodent Models of Multiple Sclerosis and Pain

We report the discovery of compound 4a, a potent β-lactam-based monoacylglycerol lipase (MGL) inhibitor characterized by an irreversible and stereoselective mechanism of action, high membrane permeability, high brain penetration evaluated using a human in vitro blood-brain barrier model, high selectivity in binding and affinity-based proteomic profiling assays, and low in vitro toxicity. Mode-of-action studies demonstrate that 4a, by blocking MGL, increases 2-arachidonoylglycerol and behaves as a cannabinoid (CB1/CB2) receptor indirect agonist. Administration of 4a in mice suffering from experimental autoimmune encephalitis ameliorates the severity of the clinical symptoms in a CB1/CB2-dependent manner. Moreover, 4a produced analgesic effects in a rodent model of acute inflammatory pain, which was antagonized by CB1 and CB2 receptor antagonists/inverse agonists. 4a also relieves the neuropathic hypersensitivity induced by oxaliplatin. Given these evidence, 4a, as MGL selective inhibitor, could represent a valuable lead for the future development of therapeutic options for multiple sclerosis and chronic pain.

Organocatalytic Synthesis of Fused Bicyclic 2,3-Dihydro-1,3,4-oxadiazoles through an Intramolecular Cascade Cyclization

Hydrazone-carboxylic acids undergo intramolecular cyclization in the presence of pivaloyl chloride, iPr(2)NEt, and catalytic DABCO to form a range of substituted fused tricyclic 2,3-dihydro-1,3,4-oxadiazoles in high yields.

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.

Organocatalytic Lewis base functionalisation of carboxylic acids, esters and anhydrides via C1-ammonium or azolium enolates

This tutorial review highlights the organocatalytic Lewis base functionalisation of carboxylic acids, esters and anhydrides via C1-ammonium/azolium enolates. The generation and synthetic utility of these powerful intermediates is highlighted through their application in various methodologies including aldol-lactonisations, Michael-lactonisations/lactamisations and [2,3]-rearrangements.

Crystal structure of 2-[(3S,4S)-4-(anthracen-9-yl)-1-(4-meth-oxy-phen-yl)-2-oxoazetidin-3-yl]-2-aza-2H-phenalene-1,3-dione unknown solvate

The central β-lactam ring of the title compound, C36H24N2O4, is almost planar (r.m.s. deviation = 0.003 Å) and makes dihedral angles of 17.17 (19), 89.76 (17) and 78.44 (17)° with the benzene ring, the anthracene ring (r.m.s. deviation = 0.003 Å) and the 1H-benzo[de]iso-quinoline-1,3(2H)-dione moiety, which is nearly planar [maximum deviation = 0.098 (2) Å], respectively. The mol-ecular structure is stabilized by an intra-molecular C-H⋯N hydrogen bond. In the crystal, mol-ecules are linked via C-H⋯π and π-π stacking inter-actions [centroid-centroid distances = 3.5270 (19) and 3.779 (2) Å], forming a three-dimensional structure. A region of disordered electron density, probably disordered solvent mol-ecules, was treated with the SQUEEZE procedure in PLATON [Spek (2015 ▶). Acta Cryst. C71, 9-18], which indicated a solvent cavity of 322 Å(3) containing approximately 91 electrons. Their formula mass and unit-cell characteristics were not taken into account during the refinement.

An NHC-catalyzed in situ activation strategy to β-functionalize saturated carboxylic acid: an enantioselective formal [3+2] annulation for spirocyclic oxindolo-γ-butyrolactones

An in situ NHC-catalyzed activation strategy to β-functionalize saturated carboxylic acid was developed. This asymmetric formal [3+2] annulation could deliver spirocyclic oxindolo-γ-butyrolactones from saturated carboxylic acid and isatin in good yields with high to excellent enantioselectivities. The easy availability of the starting materials, direct installation of functional units at unreactive carbon atom and the convergent assembly make this protocol attractive in the field of organic synthesis.

Enantioselective cis-β-lactam synthesis by intramolecular C-H functionalization from enoldiazoacetamides and derivative donor-acceptor cyclopropenes

β-Lactam derivatives are produced through donor–acceptor cyclopropene intermediates in high yield with exclusive cis-diastereoselectivity, and high enantiocontrol.

β-Lactam derivatives are produced through intermediate donor–acceptor cyclopropene intermediates in high yield, exclusive cis-diastereoselectivity, and high enantiocontrol in a chiral dirhodium carboxylate catalyzed intramolecular C–H functionalization reaction of enoldiazoacetamides.

Regiodivergent Lewis base-promoted O- to C-carboxyl transfer of furanyl carbonates

Triazolinylidenes promote γ-selective C-carboxylation (up to 99 : 1 regioselectivity) in the O- to C-carboxyl transfer of furanyl carbonates in contrast to DMAP that promotes preferential α-C-carboxylation with moderate regiocontrol (typically 60 : 40 regioselectivity).