Chiral Squaramide-Catalyzed Asymmetric Mannich Reactions for Synthesis of Fluorinated 3,3′-Bisoxindoles

Recent Progress in Synthesis of Alkyl Fluorinated Compounds with Multiple Contiguous Stereogenic Centers

Organic fluorides are widely used in pharmaceuticals, agrochemicals, material sciences, and other fields due to the special physical and chemical properties of fluorine atoms. The synthesis of alkyl fluorinated compounds bearing multiple contiguous stereogenic centers is the most challenging research area in synthetic chemistry and has received extensive attention from chemists. This review summarized the important research progress in the field over the past decade, including asymmetric electrophilic fluorination and the asymmetric elaboration of fluorinated substrates (such as allylic alkylation reactions, hydrofunctionalization reactions, Mannich addition reactions, Michael addition reactions, aldol addition reactions, and miscellaneous reactions), with an emphasis on synthetic methodologies, substrate scopes, and reaction mechanisms.

Organocatalyzed Asymmetric Michael Addition of 3-Fluorooxindole to Vinylidene Bisphosphonates

Both the 3-fluorooxindole and germinal bisphosphonate structural motifs are prevalent in bioactive molecules because of their associated biological activities. We describe an approach to accessing 3,3-disubstituted 3-fluorooxindoles bearing a geminal bisphosphate fragment through a highly enantioselective Michael addition reaction between 3-fluorooxindoles and vinylidene bisphosphonates. These reactions are catalyzed by a commercially available cinchona alkaloid catalyst, have a broad substrate scope concerning 3-fluorooxindoles, and provide the corresponding addition products in a yield of up to 95% with an enantiomeric excess of up to 95%. A reasonable reaction pathway to explain the observed stereochemistry is also proposed.

Asymmetric Mannich Reaction of Isatin-Derived Ketimines with α-Fluoroindanones Catalyzed by a Chiral Phase-Transfer Catalyst

A highly enantioselective Mannich reaction of α-fluoroindanones with isatin-derived N-Boc-ketimines catalyzed by a quinine-derived phase-transfer catalyst was developed. A variety of 3-substituted 3-amino-2-oxindoles bearing fluorine-containing, vicinal, tetrasubstituted stereocenters were constructed using this protocol in high yields (83-95%), with moderate to excellent enantioselectivities (66-91%) and high diastereoselectivities (up to >99:1).

Enantioseparation and racemization of 3-fluorooxindoles

Fluorinated oxindoles are frequently used building blocks in asymmetric synthesis and represent an important scaffold found in a variety of biologically relevant compounds. While it is understood that incorporation of fluorine atoms into organic molecules can improve their pharmacological properties, the impact on the configurational stability of chiral organofluorines is still underexplored. In this study, semipreparative HPLC enantioseparations of five oxindoles were carried out, and the resulting enantiomerically enriched solutions were used to investigate base promoted racemization kinetics at room temperature. It was found that incorporation of fluorine at the chiral center increases the configurational stability, while substitutions on the aromatic ring and at the lactam moiety also have significant effects on the rate of racemization, which generally follows reversible first-order reaction kinetics.

Construction of a Chiral β-Fluoroamine by Asymmetric Mannich Reaction of 2-Fluoroindanone with Pyrazolinone Ketimines

The asymmetric Mannich reaction of 2-fluoroindanone with ketimine was developed under the catalysis of a kind of chiral copper complex, affording a chiral tetrahedral center containing fluorine. A series of β-fluoroamine derivatives can be obtained in excellent yields (73-94%) with high diastereoselectivities (>99:1 dr) and enantioselectivities (89-99%). The possible transition state was supported by density functional theory calculation.

Catalytic Asymmetric Allylic Alkylation with Arylfluoroacetonitriles

A palladium(phosphinoxazoline) catalyzed method for asymmetric allylic alkylation of α-aryl-α-fluoroacetonitriles is introduced. This reaction achieves C-C bond formation and incorporation of two adjacent chirality centers with moderate to good yields, high enantioselectivities, and up to 15:1 dr.

Chiral Urea-Catalyzed Asymmetric Mannich reaction of 3-fluorooxindoles with α-amidosulfones : Synthesis of Optically Active α-fluoro-β-amino-oxindoles

The asymmetric Mannich reaction of 3-fluorooxindoles and ɑ-amidosulfones catalyzed by a chiral urea catalyst derived from quinine in presence of K3PO4 was developed. Through the asymmetric reaction, a series of ɑ-fluoro-β-amino-oxindoles, containing a four-substituted carbon continuous stereocenter, could be obtained in high yields (up to 95%) with high enantioselectivity (95%) and diastereoselectivity (>99:1). Such ɑ-fluoro-β-amino-oxindole compounds are expected to become candidates in the field of medicine.

Synthesis of oxindoles bearing a stereogenic 3-fluorinated carbon center from 3-fluorooxindoles

3,3-Disubstituted oxindoles bearing a stereogenic 3-fluorinated carbon center are privileged structural motifs present in many bioactive molecules. The straightforward functionalization of 3-fluorooxindoles constitutes a powerful method for the synthesis of 3-fully substituted 3-fluorooxindoles, taking advantage of the ease of preparation of 3-fluorooxindoles with different substitution patterns and the atom efficiency of chemical reactions. In the past decade, many papers have appeared on the synthesis of 3-fully substituted 3-fluorooxindoles from 3-fluorooxindoles. Importantly, many asymmetric catalytic methods have been developed for the enantioselective synthesis of these valuable compounds. This review summarizes the achievements in this area, and overviews synthetic opportunities that still exist.

Catalytic stereoselective Mannich-type reactions for construction of fluorinated compounds

For its unique role in developing and designing new bioactive materials and healthcare products, fluoro-organic compounds have attracted remarkable interest. Along with ever-increasing demand for a wider availability of fluorine-containing structural units, a large diversity of methods has been introduced to incorporate fluorine atoms specially in a stereoselective fashion. Among them, catalytic Mannich reaction can proceed with a broad variety of reactants and open clear paths for the synthesis of versatile amine synthons in the synthesis of natural product and pharmaceutical molecules. This review provides an overview of the employment of catalytic asymmetric Mannich reactions in the synthesis of fluorine-containing amine compounds and highlights the conceivable distinct mechanisms.

Bifunctional thiosquaramide catalyzed asymmetric reduction of dihydro-β-carbolines and enantioselective synthesis of (-)-coerulescine and (-)-horsfiline by oxidative rearrangement

Tetrahydro-β-carboline (THBC) is a tricyclic ring system that can be found in a large number of bioactive alkaloids. Herein, we report a simple and efficient method for the synthesis of enantiopure THBCs through a chiral thiosquaramide (11b) catalyzed imine reduction of dihydro-β-carbolines (17a-f). The in situ generated Pd-H employed as hydride source in the reaction of differently substituted chiral THBCs (18a-f) afforded high selectivities (R isomers, up to 96% ee) and good isolated yields (up to 88%). Moreover, the chiral thiosquaramide used also afforded exceptional catalyst activity in the syntheses of (-)-coerulescine (5) and (-)-horsfiline (6) with excellent enantioselectivities up to 98% and 93% ee, respectively, via an enantioselective oxidative rearrangement approach.

Recent advances in the catalytic synthesis of 3-aminooxindoles: an update

3-Substituted-3-aminooxindoles are versatile scaffolds and these motifs constitute the core structure of number of natural products and biologically active compounds.

Organocatalytic Asymmetric Mannich Addition of 3-Fluorooxindoles to Dibenzo[b,f][1,4]oxazepines: Highly Enantioselective Construction of Tetrasubstituted C-F Stereocenters

3-Fluorooxindoles and the dibenzo[b,f][1,4]oxazepane scaffolds are important pharmacophores that have important application in medicinal chemistry. An organocatalyzed asymmetric Mannich reaction of 3-fluorooxindoles with dibenzo[b,f][1,4]oxazepines affording various seven-member cyclic amines containing chiral tetrasubstituted C-F stereocenters was developed. These reactions which were catalyzed by a bifunctional Cinchona alkaloid-derived thiourea catalyst afforded a wide range of substrates in moderate to high yields with excellent diastereo- and enantioselectivities (up to 88% yield, >20:1 dr and >99% ee). A feasible reaction mechanism was also proposed.

Squaramide-catalyzed asymmetric Mannich reactions between 3-fluorooxindoles and pyrazolinone ketimines

An enantioselective Mannich reaction between 3-fluorooxindoles and pyrazolinone ketimines has been developed for the construction of amino-pyrazolone-oxindoles containing stereogenic C-F units. Based on this new protocol that allows for the generation of two adjacent tetrasubstituted stereocenters, a variety of structurally diverse fluorinated amino-pyrazolone-oxindoles were obtained in good to excellent yields with excellent diastereoselectivities and enantioselectivities (up to 98% yield, >20 : 1 dr and >99% ee). What's more, good yield and high stereoselectivities were obtained in the gram-scale reaction.

Enantioselective Mannich Reactions of 3-Fluorooxindoles with Cyclic N-Sulfamidate Aldimines

Both the 3-fluorooxindole and cyclic sulfamidate frameworks are important in medicinal chemistry owing to their associated biological activities. We report an approach accessing 3-fully substituted 3-fluorooxindoles, containing a benzo-fused sulfamidate subunit through highly enantioselective Mannich-type reactions between 3-fluorooxindoles and cyclic benzo-fused N-sulfamidate aldimines. These reactions are promoted by a commercially available cinchona alkaloid catalyst, accommodate a broad substrate scope, and deliver the desired products in a yield of up to 99% with an enantiomeric excess of up to 94%. A plausible reaction pathway is also presented.

Catalytic Asymmetric Mannich Reaction of α-Fluoronitriles with Ketimines: Enantioselective and Diastereodivergent Construction of Vicinal Tetrasubstituted Stereocenters

Diastereodivergent and enantioselective conversion of isatin ketimines to α-fluoro-β-aminonitriles with vicinal tetrasubstituted stereocenters is achieved by a chiral copper complex/guanidine base catalyzed Mannich reaction with proper choice of the bisphosphine ligand. The reaction is broad in scope, scalable, and provides efficient access to a series of 3-aminoindolinones exhibiting a quaternary carbon-fluorine stereocenter with high yields and stereoselectivities. Selective transformations of the Mannich reaction products into multifunctional 3-aminooxindoles without erosion of enantiomeric and diastereomeric purity highlight the synthetic utility.

Chemistry of detrifluoroacetylativelyin situgenerated fluoro-enolates

This review article provides a summary of the detrifluoroacetylativein situgeneration of fluorine-containing enolates and their related reactions.

Copper-catalyzed enantioselective Mannich reaction between N-acylpyrazoles and isatin-derived ketimines

A copper-catalyzed enantioselective Mannich reaction between N-acylpyrazoles and isatin-derived ketimines is developed.