Direct Asymmetric Aldol Reaction of 5H-Oxazol-4-ones with Aldehydes Catalyzed by Chiral Guanidines

De novo design and synthesis of dipyridopurinone derivatives as visible-light photocatalysts in productive guanylation reactions

Described here is the de novo design and synthesis of a series of 6H-dipyrido[1,2-e:2',1'-i]purin-6-ones (DPs) as a new class of visible-light photoredox catalysts (PCs). The synthesized DP1-5 showed their λ _Abs(max) values in 433-477 nm, excited state redox potentials in 1.15-0.69 eV and -1.41 to -1.77 eV (vs. SCE), respectively. As a representative, DP4 enables the productive guanylation of various amines, including 1°, 2°, and 3°-alkyl primary amines, secondary amines, aryl and heteroaryl amines, amino-nitrile, amino acids and peptides as well as propynylamines and α-amino esters giving diversities in biologically important guanidines and cyclic guanidines. The photocatalytic efficacy of DP4 in the guanylation overmatched commonly used Ir and Ru polypyridyl complexes, and some organic PCs. Other salient merits of this method include broad substrate scope and functional group tolerance, gram-scale synthesis, and versatile late-stage derivatizations that led to a derivative 81 exhibiting 60-fold better anticancer activity against Ramos cells with the IC₅₀ of 0.086 μM than that of clinical drug ibrutinib (5.1 μM).

Varying the Directionality of Protein Catalysts for Aldol and Retro-Aldol Reactions

Natural aldolase enzymes and created retro-aldolase protein catalysts often catalyze both aldol and retro-aldol reactions depending on the concentrations of the reactants and the products. Here, we report that the directionality of protein catalysts can be altered by replacing one amino acid. The protein catalyst derived from a scaffold of a previously reported retro-aldolase catalyst, catalyzed aldol reactions more efficiently than the previously reported retro-aldolase catalyst. The retro-aldolase catalyst efficiently catalyzed the retro-aldol reaction but was less efficient in catalyzing the aldol reaction. The results indicate that protein catalysts with varying levels of directionality in usually reversibly catalyzed aldol and retro-aldol reactions can be generated from the same protein scaffold.

Conjugated ynones in catalytic enantioselective reactions

Conjugated ynones are easily accessible feedstock and the existence of an alkyne bond endows ynones with different attractive reactivities, thus making them unique substrates for catalytic asymmetric reactions. Their compatibility under organocatalytic, metal-catalyzed as well as cooperative catalytic conditions has resulted in numerous enantioselective transformations. Importantly, conjugated ynones can act as nucleophiles or electrophiles, and serve as easily accessed synthons for different cyclization pathways. This review summarizes the recent literature examples of the catalytic reactions of conjugated ynones and related compounds such as alkyne conjugated α-ketoesters, and classifies these reaction types alongside mechanistic insights whenever possible. We aim to trigger more intensive research in the future to render the asymmetric transformation of ynones as a common and reliable tool for asymmetric synthesis.

Non-hydrogen bond catalyst-mediated diastereoselective conjugate additions of 5H-oxazol-4-ones to o-hydroxyphenyl-substituted p-quinone methides

An efficient DBU-catalyzed conjugate addition of 5H-oxazol-4-ones to o-hydroxyphenyl-substituted p-quinone methides has been developed, affording the valuable diarylmethanes in high yields with excellent diastereoselectivity. This strategy demonstrates a robust access to a wide range of diarylmethane derivatives possessing biologically significant o-hydroxyphenol and p-hydroxyphenol moieties under mild reaction conditions.

Asymmetric Synthesis of Adjacent Tri- and Tetrasubstituted Carbon Stereocenters: Organocatalytic Aldol Reaction of an Hydantoin Surrogate with Azaarene 2-Carbaldehydes

A bifunctional amine/squaramide catalyst promoted direct aldol addition of an hydantoin surrogate to pyridine 2-carbaldehyde N-oxides to afford adducts bearing two vicinal tertiary/quaternary carbons in high diastereo- and enantioselectivity (d.r. up to >20:1; ee up to 98 %) is reported. Acid hydrolysis of adducts followed by reduction of the N-oxide group yields enantiopure carbinol-tethered quaternary hydantoin-azaarene conjugates with densely functionalized skeletons. DFT studies of the potential energy surface (B3LYP/6-31+G(d)+CPCM (dichloromethane)) of the reaction correlate the activity of different catalysts and support an intramolecular hydrogen-bond-assisted activation of the squaramide moiety in the transition state of the catalytic reaction.

Direct Access to β-Trifluoromethyl-β-hydroxy Thioesters by Biomimetic Organocatalytic Enantioselective Aldol Reaction

A broadly applicable biomimetic enantioselective decarboxylative catalytic aldol reaction of trifluoromethyl ketones with malonic acid half-thioesters (MAHTs) is described. Utilizing cinchona-based thioureas as highly efficient polyketide synthase-mimic catalysts, chiral tertiary aldols, β-trifluoromethyl-β-hydroxy thioesters, were obtained in up to 99% yield and 95% ee. Facile transformation of the thioester moiety of the aldol adducts showcases the synthetic utility of this biomimetic aldol protocol to deliver a range of chiral trifluoromethylated tertiary aldol pharmacophores.

Synergistic promotion by intramolecular hydrogen bonding: a bi-functionally catalyzed cascade reaction for the synthesis of enantiopure chromenopyrrolidines

An effective organocatalytic asymmetric cascade reaction of o-hydroxy aromatic aldimines and β,γ-unsaturated-α-ketoesters was developed.

Asymmetric One-Pot Construction of Three Stereogenic Elements: Chiral Carbon Center, Stereoisomeric Alkenes, and Chirality of Axial Styrenes

An organocatalytic enantioselective method for the synthesis of multiple stereoisomers bearing E, Z configurations, stereogenic carbon centers, and axially chiral styrenes is reported. The method enabled the rapid construction of a series of stereochemical complexity products with excellent E, Z selectivity, diastereoselectivity (>20:1 dr), and enantioselectivity (up to 96% ee). This method provides an efficient and concise synthesis route of multiple stereoisomers with a wide range of potential applications in organic synthesis.

Stereoconvergent 1,4-Addition Reaction of 5H-Oxazol-4-ones with E,Z Isomeric Mixture of Alkylidene β-Ketoesters Catalyzed by Chiral Guanidines

A 1,4-addition reaction of 5H-oxazol-4-ones to alkylidene β-ketoesters, which was catalyzed by using chiral guanidines via a dynamic kinetic resolution process that involved geometric isomerization of the alkylidene β-ketoesters, was developed. This method allowed using E,Z isomeric mixture of various acyclic alkylidene β-ketoesters to obtain the products stereoselectively. The relation between the geometry and the stereoselectivity of products was investigated, and the derivatization of the products to the corresponding α-acyl-γ-lactone was performed.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Catalytic asymmetric synthesis of CF3-substituted tertiary propargylic alcohols via direct aldol reaction of α-N3 amide

Organofluorine compounds are found in several important classes of chemicals, such as pharmaceuticals, agrochemicals, and functional materials. Chemists have been immensely interested in the development of methodologies for expeditious access to fluorine containing building blocks. In this study, we report a new method for the catalytic asymmetric synthesis of CF₃-substituted tertiary propargylic alcohols with two contiguous stereogenic centers via the direct aldol reaction of an α-N₃ amide to trifluoromethyl ketones. The key to the success of this method is the identification of a catalyst comprising Cu(ii)/chiral hydroxamic acid to promote the desired aldol reaction, constructing a tetrasubstituted carbon in a highly stereoselective fashion. Despite substantial prior advances in asymmetric catalysis, this class of catalysts has not been utilized for the formation of carbon-carbon bond-forming reactions. Our mechanistic study sheds light on the unique profile of this catalytic system, where the Cu(ii) complex plays a bifunctional role of serving as a Lewis acid and a Brønsted base. Furthermore, the densely functionalized aldol adducts undergo chemoselective transformations, affording a series of fluorine containing chiral building blocks with widespread application.

1H-Imidazol-4(5H)-ones and thiazol-4(5H)-ones as emerging pronucleophiles in asymmetric catalysis

Asymmetric catalysis represents a very powerful tool for the synthesis of enantiopure compounds. In this context the main focus has been directed not only to the search for new efficient chiral catalysts, but also to the development of efficient pronucleophiles. This review highlights the utility and first examples of 1H-imidazol-4(5H)-ones and thiazol-4(5H)-ones as pronucleophiles in catalytic asymmetric reactions.

Catalyst-controlled switch of regioselectivity in the asymmetric allylic alkylation of oxazolones with MBHCs

A catalyst-controlled switch of regioselectivity in asymmetric allylic alkylation of oxazolones with MBHCs was described.

Organocatalytic asymmetric conjugate addition of diaryloxazolidin-2,4-diones to nitroolefins: an efficient approach to chiral α-aryl-α-hydroxy carboxylic acids

The first asymmetric conjugate addition of diaryloxazolidin-2,4-diones to nitroolefins using an l-threonine-based tertiary amine-urea catalyst has been developed.

Chiral phosphoric acid catalyzed enantioselective 1,3-dipolar cycloaddition reaction of azlactones

The first chiral phosphoric acid catalyzed highly diastereo- and enantioselective 1,3-dipolar cycloaddition reaction of azlactones and methyleneindolinones was disclosed to yield 3,3′-pyrrolidonyl spirooxindole scaffolds.

Organocatalytic 1,4-Addition Reaction of 2-Formyl(thio)esters to Vinylketones: An Efficient Access to Acyclic Chiral Building Blocks with a Quaternary Carbon Stereocenter

2-Formyl(thio)esters were utilized as pronucleophiles to obtain less-accessible acyclic chiral building blocks bearing versatile functional groups on a quaternary carbon atom for enantioselective 1,4-addition to vinylketones. To achieve high enantioselectivity in the present 1,4-addition reaction, thiourea-tertiary amines containing a bulky chiral backbone were developed as catalysts, and several derivatizations of the products were performed to demonstrate the synthetic utility of the products.

Direct Catalytic Asymmetric Mannich-Type Reaction of α- and β-Fluorinated Amides

The last two decades have witnessed the emergence of direct enolization protocols providing atom-economical and operationally simple methods to use enolates for stereoselective C-C bond-forming reactions, eliminating the inherent drawback of the preformation of enolates using stoichiometric amounts of reagents. In its infancy, direct enolization relied heavily on the intrinsic acidity of the latent enolates, and the reaction scope was limited to readily enolizable ketones and aldehydes. Recent advances in this field enabled the exploitation of carboxylic acid derivatives for direct enolization, offering expeditious access to synthetically versatile chiral building blocks. Despite the growing demand for enantioenriched fluorine-containing small molecules, α- and β-fluorinated carbonyl compounds have been neglected in direct enolization chemistry because of the competing and dominating defluorination pathway. Herein we present a comprehensive study on direct and highly stereoselective Mannich-type reactions of α- and β-fluorine-functionalized 7-azaindoline amides that rely on a soft Lewis acid/hard Brønsted base cooperative catalytic system to guarantee an efficient enolization while suppressing undesired defluorination. This protocol contributes to provide a series of fluorinated analogs of enantioenriched β-amino acids for medicinal chemistry.

Managing the retro-pathway in direct catalytic asymmetric aldol reactions of thioamides

Thioamides are the preferred pronucleophiles for direct catalytic asymmetric aldol reactions in the context of soft Lewis acid/hard Brønsted base cooperative catalysis. In-depth investigation of this proton-transfer catalysis, which is virtually in equilibrium, revealed that the prominence of the retro-aldol reaction depended on the substrate combination. The retro-aldol reaction is a serious issue in direct aldol reactions because the product distribution, including enantiomers and diastereomers, is governed by thermodynamic parameters, and the aldol products are obtained in much lower stereoselectivity compared with the kinetically controlled process. Herein we report the beneficial effect of an additive with a functional group architecture similar to that of the aldol adduct that suppresses the retro-aldol reaction by competitively binding to the catalyst. Strategic use of the additive led to high stereoselectivity, even when the combination of substrates was prone to the retro-aldol reaction.