Chiral Squaramides as Highly Enantioselective Catalysts for Michael Addition Reactions of 4-Hydroxycoumarins and 4-Hydroxypyrone to β,γ-Unsaturated α-Keto Esters

Cinchona-Based Hydrogen-Bond Donor Organocatalyst Metal Complexes: Asymmetric Catalysis and Structure Determination

In this study, we describe the synthesis of cinchona (thio)squaramide and a novel cinchona thiourea organocatalyst. These catalysts were employed in pharmaceutically relevant catalytic asymmetric reactions, such as Michael, Friedel-Crafts, and A3 coupling reactions, in combination with Ag(I), Cu(II), and Ni(II) salts. We identified several organocatalyst-metal salt combinations that led to a significant increase in both yield and enantioselectivity. To gain insight into the active catalyst species, we prepared organocatalyst-metal complexes and characterized them using HRMS, NMR spectroscopy, and quantum chemical calculations (B3LYP-D4/def2-TZVP), which allowed us to establish a structure-activity relationship.

Enantioselective Mukaiyama-Michael Reaction of β,γ-Unsaturated α-Keto Esters with Silyl Ketene Acetals Catalyzed by a Chiral Magnesium Phosphate

We would like to describe an efficient and highly enantioselective Mukaiyama-Michael reaction of silyl ketene acetals with β,γ-unsaturated α-keto esters catalyzed by a chiral magnesium BINOL-derived phosphate. The resulting functionalized 1,5-dicarbonyl adducts are obtained in high yields (up to 96%) and with excellent enantioselectivities (up to 98%) under mild conditions. Two plausible mechanistic pathways were proposed, including a 1,4-addition and a hetero Diels-Alder [4 + 2] cycloaddition.

Efficiency and Suitability when Exploring the Conformational Space of Phase-Transfer Catalysts

In this study, a complete exploration of the conformational space of different phase-transfer catalysts by means of computational method benchmarking is presented. For this particular research work, only the most significant and relevant conformational analysis approaches have been chosen to characterize the main Cinchona alkaloid-based phase-transfer catalysts. This particular guiding study aims to rigorously compare the performance of different conformational methods, determining the strengths of each method and providing recommendations regarding suitable and efficient choices of methods for analysis.

Chiral tertiary amine-squaramide catalyzed highly enantioselective Michael addition of cyclic diketones to β,γ-unsaturated α-keto Esters

Abstract:

A novel tertiary amine squaramide has successfully developed and applied to catalyze the asymmetric Michael addition between cyclic diketones and β,γ-unsaturated α-ketoesters. The catalyst system performed well with a low catalyst loading of 1 mol% under mild reaction conditions. A series of synthetically and pharmaceutically useful chiral bicyclic compounds were obtained with both high yields (up to 97%) and excellent enantioselectivities (up to 97% ee).

Enantioselective Synthesis of Coumarins Catalyzed by Chiral Squaramide

Abstract:

A newly chiral tertiary amine-squaramide has successfully developed and applied into the asymmetric Michael addition of 4-hydroxycoumarin to β,γ-unsaturated α-ketoesters. The catalyst system performed well with a low catalyst loading of 1 mol% under mild reaction conditions. A series of coumarin derivatives were obtained with good yields and enantioselectivities

Enantioselective synthesis of coumarins catalyzed by an isosteviol-derived tertiary amine-squaramide catalyst

A newly tertiary amine-squaramide organocatalyst has been successfully developed and applied into the asymmetric Michael addition of 4-hydroxycoumarin to β,γ-unsaturated α-ketoesters. The catalyst system performed well with a low catalyst loading of 1 mol% under mild reaction conditions. A series of coumarin derivatives were obtained in good to high yields (up to 97%) with high enantioselectivities (up to 96% ee).

Construction of Chiral Isotetronic Acid-Fused Thiochromane via Doubly Annulative Strategy

A sulfa-Michael/aldol/lactonization cascade reaction has been established to construct isotetronic acid-fused thiochromanes in a highly stereoselective fashion (≥11:1 dr, 35-98% ee). The tricyclic products were obtained in 35-99% isolated yields in the presence of a bifunctional squaramide. Three reactive sites of β,γ-unsaturated α-ketoester, including the less-explored ester carbonyl group, were sequentially utilized to construct two fused heterocycles in a one-pot operation.

Catalyst-free 1,6-conjugate addition of indoles and 4-hydroxycoumarins to para-quinone methides: synthesis of unsymmetrical triarylmethanes

The catalyst-free 1,6-conjugate addition of indoles and 4-hydroxycoumarins to para-quinone methides is reported. This protocol allowed us to access a range of unsymmetrical triarylmethanes in good to excellent yields. The outlined procedure is operationally simple, efficient, atom and step economical. The synthesized heterocyclic triarylmethanes were further converted into highly substituted indoloisoquinolines and pyranochromenones via metal-catalyzed C-H activation/annulation.

Conjugate Addition of Carbon Acids to β,γ-Unsaturated α-Keto Esters: Product Tautomerism and Applications for Asymmetric Synthesis of Benzo[a]phenazin-5-ol Derivatives

A correlation between the equilibrium ratio of tautomeric products generated by the asymmetric Michael reactions of cyclic carbon acids with β,γ-unsaturated α-keto esters and the chemical shift of the α-proton in starting nucleophilic substrates was revealed which makes equilibration predictable. New tetrahydropyran-fused benzo[a]phenazins were enantioselectively (up to 99% ee) synthesized from β,γ-unsaturated α-keto esters and benzo[a]phenazin-5-ol, a powerful anti-cancer agent sAJM589. Facile recyclability of catalyst Ia in the catalytic reactions was demonstrated.

Azetidine-Derived Dinuclear Zinc-Catalyzed Asymmetric Conjugate Addition of Bioactive Heterocycles to β,γ-Unsaturated α-Keto esters

A general AzePhenol dinuclear zinc catalytic system has been successfully developed and applied into the asymmetric Michael addition of 4-hydroxyl pyrones and 4-hydroxycoumarins to β,γ-unsaturated α-keto esters. Excellent yields (up to 99%) and high enantioselectivities (up to 94% ee) are obtained for a wide range of substrates under mild conditions in the absence of additives. This bimetallic catalytic approach represents a new and effective asymmetric synthetic protocol to access a variety of bioactive compounds with pharmacological interest. The possible mechanism is proposed to explain the origin of the asymmetric induction.

Isothiourea-catalysed enantioselective Michael addition of N-heterocyclic pronucleophiles to α,β-unsaturated aryl esters

The isothiourea-catalysed enantioselective Michael addition of 3-aryloxindole and 4-substituted-dihydropyrazol-3-one pronucleophiles to α,β-unsaturated p-nitrophenyl esters is reported. This process generates products containing two contiguous stereocentres, one quaternary, in good yields and excellent enantioselectivities (>30 examples, up to > 95 : 5 dr and 99 : 1 er). This protocol harnesses the multifunctional ability of p-nitrophenoxide to promote effective catalysis. In contrast to previous methodologies using tertiary amine Lewis bases, in which the pronucleophile was used as the solvent, this work allows bespoke pronucleophiles to be used in stoichiometric quantities.

The isothiourea-catalysed enantioselective Michael addition of 3-aryloxindole and 4-substituted-dihydropyrazol-3-one pronucleophiles to α,β-unsaturated p-nitrophenyl esters is reported.

Bifunctional Amine-Squaramides as Organocatalysts in Michael/Hemiketalization Reactions of β,γ-Unsaturated α-Ketoesters and α,β-Unsaturated Ketones with 4-Hydroxycoumarins

The catalytic efficiency of various amine-squaramides was tested in Michael/hemiketalization reactions of 4-hydroxycoumarines with two types of enones. Tertiary amine-squaramide organocatalysts afforded the best results regarding both activity and enantioselectivity when β,γ-unsaturated α-ketoesters were used as the Michael acceptors (yields up to 98%, enantioselectivities up to 90% ee). On the other hand, the primary amine-squaramides are the best choice for related reactions of 4-hydroxycoumarins with enones. The corresponding pyrano[3,2- c]chromen-5-on products were obtained in high enantiomeric purities (up to 96%). The Michael addition of 4-hydroxycoumarin to 4-phenylbut-3-en-2-on directly produced chiral anticoagulant drug ( S)-warfarin in 92% ee when green solvent 2-MeTHF and catalyst ( S, S)-C8 were used. Moreover, an enantiomeric catalyst ( R, R)-C8 gave ( R)-warfarin in >99% ee.

Highly enantioselective Michael addition of α-nitroacetate to α,β-unsaturated pyrazolamide catalyzed by a bifunctional squaramide

A highly enantioselective (91- > 99% ee) Michael addition of α-nitroacetate to α,β-unsaturated pyrazolamide was developed in the presence of a bifunctional squaramide. Satisfactory isolated yields (42%-99%) have been achieved with a wide range of α,β-unsaturated pyrazolamides, albeit acceptor of this type displayed poor reactivity in the precedent reports. The adducts resulting from this protocol are useful synthetic intermediates for further synthetic modification.

An efficient approach to the synthesis of coumarin-fused dihydropyridinones

3,4-Dihydro-2

The squaramide-catalyzed asymmetric Michael/cyclization tandem reaction for the synthesis of chiral trifluoromethylated hydroxyimino tetrahydrobenzofuranones

An enantioselective synthesis of trifluoromethylated hydroxyimino tetrahydrobenzofuranones has been developed. 1,3-Dicarbonyl carbocyclic compounds react with β-CF3-β-disubstituted nitroalkenes in the presence of a chiral squaramide catalyst, providing efficient access to diverse hydroxyimino tetrahydrobenzofuranones featuring a trifluoromethyl group at the C3-position of an all-carbon quaternary stereocenter with good yields (up to 81%) and enantioselectivities (up to 89% ee).

Asymmetric Mannich reaction: highly enantioselective synthesis of 3-amino-oxindoles via chiral squaramide based H-bond donor catalysis

A facile formation of 3-aminooxindole derivatives from the reaction of 1,3-diketones with isatin (N-Boc) via an asymmetric Mannich reaction catalyzed by chiral cinchona alkaloid based squaramide containing H-bond donor catalysts.

A New Strategy for Enantioselective Construction of Multisubstituted Five-Membered Oxygen Heterocycles via a Domino Michael/Hemiketalization Reaction

A new highly enantioselective domino Michael/hemiketalization reaction of α-hydroxyacetophenone with β,γ-unsaturated α-keto esters for the synthesis of 2,2,4,5-tetrasubstituted chiral tetrahydrofurans is reported. With 2 mol % intramolecular dinuclear zinc-AzePhenol complex prepared in situ from the reaction of multidentate semi-azacrown ether ligand with ZnEt2 , the corresponding anti-multisubstituted tetrahydrofuran products were obtained in up to 90 % yields, and 98 % enantiomeric excess (ee) at 0 °C for 45 min. Moreover, the products were easily converted to 2,3,5-trisubstituted 2,3-dihydrofurans without any loss in optical activity.

Highly enantioselective synthesis of naphthoquinones and pyranonaphthoquinones catalyzed by bifunctional chiral bis-squaramides

A variety of enantioenriched naphthoquinones has been synthesized in high yields and excellent enantioselectivities (up to >99% ee) using a bifunctional chiral bis-squaramide catalyzed conjugate addition of 2-hydroxy-1,4-naphthoquinone to 2-enoylpyridines.

Combining silver catalysis and organocatalysis: a sequential Michael addition/hydroalkoxylation one-pot approach to annulated coumarins

A highly stereoselective one-pot procedure for the synthesis of five-membered annulated hydroxycoumarins has been developed. By merging primary amine catalysis with silver catalysis, a series of functionalized coumarin derivatives were obtained in good yields (up to 91%) and good to excellent enantioselectivities (up to 99% ee) via a Michael addition/hydroalkoxylation reaction. Depending on the substituents on the enynone, the synthesis of annulated six-membered rings is also feasible.

Synthesis of functionalized coumarins and quinolinones by NHC-catalyzed annulation of modified enals with heterocyclic C-H acids

N-Heterocyclic carbene (NHC) catalyzed lactonization and lactamization of 2-bromoenals with heterocyclic C-H acids proceeding via the α,β-unsaturated acyl azolium intermediates is reported. The reaction furnished coumarin- or quinolinone-fused lactone/lactam derivatives. In addition, results of the enantioselective version of this reaction using chiral NHC are presented.