Catalytic Asymmetric Hydroxylation of Oxindoles by Molecular Oxygen Using a Phase-Transfer Catalyst

Organo-cation catalyzed enantioselective α-hydroxylation of pyridinone-fused lactones: asymmetric synthesis of SN-38 and irinotecan

A catalytic asymmetric α-hydroxylation of pyridinone-fused lactones, containing the core structure of camptothecin, is described. Development of a novel spiropyrrolidine amide (SPA) derived triazolium bromide organo-cation catalyst is crucial for a highly enantioselective oxidation, which also accommodates a wide array of lactones with various substituents. The resulting tricyclic tertiary alcohol with an oxa-quaternary carbon center can be further applied in the synthesis of SN-38 and irinotecan, two anti-cancer drugs derived from camptothecin.

Construction of Bridged Benzazepines via Photo-Induced Dearomatization

Bridged benzazepine scaffolds, possessing unique structural and physicochemical activities, are widespread in various natural products and drugs. The construction of these skeletons often requires elaborate synthetic effort with low efficiency. Herein, we develop a simple and divergent approach for constructing various bridged benzazepines by a photocatalytic intermolecular dearomatization of naphthalene derivatives with readily available α-amino acids. The bridged motif is created via a cascade sequence involving photocatalytic 1,4-hydroaminoalkylation, alkene isomerization and cyclization. Interestingly, the diastereoselectivity can be regulated through different reaction modes in the cyclization step. Moreover, aminohydroxylation and its further bromination have also been demonstrated to access highly functionalized bridged benzazepines. Preliminary mechanistic studies have been performed to get insights into the mechanism. This method provides a divergent synthetic approach for construction of highly functionalized bridged benzazepines, which have been otherwise difficult to access.

K2S2O8-mediated direct C-H heteroarylation/hydroxylation of indolin-2-ones with quinoxalin-2(1H)-ones

Herein, a K2S2O8-mediated direct heteroarylation and hydroxylation reaction between quinoxalin-2(1H)-ones with a C(sp2)-H bond and indolin-2-ones with a C(sp3)-H bond via an oxidative cross-coupling reaction has been reported. We have successfully established a feasible and concise reaction system that represents the first example of free-radical-promoted heteroarylation and hydroxylation reaction on the C-3 position of oxindole. A series of 3-substituted 3-hydroxyoxindoles are obtained in 0-83% yield using this methodology.

Asymmetric Rubottom-Type Oxidation Catalyzed by Chiral Calcium Phosphates

A highly efficient catalytic asymmetric Rubottom-type oxidation is described. Using meta-chloroperbenzoic acid (m-CPBA) as the oxidant and chiral calcium phosphate as the catalyst, the facile transformation enables direct hydroxylation of N-Boc oxindoles and β-ketoesters in high yields (up to 99 %) and in a highly enantioenriched fashion (up to >99 % ee). The application of the established method was demonstrated by the synthesis of a pharmaceutically important 3-hydroxyoxindole with excellent enantiocontrol.

Ion Pair Catalyst - Pentanidinium

In this account, we further describe our already developed N-sp² hybrid guanidinium as an efficient phase-transfer catalyst and ion pair catalysis based on N-sp² hybrid pentanidinium and its application in some new reactions. The sp³ hybrid quaternary ammonium salt has a tetrahedral structure, which means that three sides of it can be effectively steric, allowing the remaining side to be close to the substrate. However, the sp² hybrid ammonium salt allows the substrate to form ion pairs from both directions respectively, so it is a greater challenge to control the stereoselectivity of the reaction. Van der Waals forces, such as hydrogen bonds and π - π ${\pi -\pi }$ interactions, have been used to make electrophiles approach from a certain direction, leading to a higher enantioselectivity. Based on the above idea, we designed an N-sp² hybrid phase-transfer catalyst, pentanidinium. Pentanidinium has five conjugated nitrogen atoms, one of which has a formal positive charge, which is necessary for it to become an ion pair catalyst. We have confirmed that pentanidinium can catalyze α-hydroxylation of 3-substituted-2-oxindoles, Michael addition of 3-alkyloxindoles with vinyl sulfone, and alkylation reactions of sulfenate anions and dihydrocoumarins, desymmetrization of pro-chiral sulfinate to afford enantioenriched sulfinate esters. Pentanidinium with side chain structure changes can also be catalyzed efficiently with enantioconvergent halogenophilic nucleophilic substitution, including azidation and thioesterification. In the reaction catalyzed by pentanidinium, it always attracts us with the advantages of low catalytic load and good enantioselectivity.

Direct Aerobic α-Hydroxylation of Arylacetates for the Synthesis of Mandelates

Aerobic α-hydroxylation of α-methylene esters has proven challenging due to overoxidation and hydrolysis of the materials. In this article, KO^tBu-promoted TBAB-catalyzed α-hydroxylation of α-methylene aryl esters using O₂ as the oxygen source has been developed. Both low reaction temperature and catalyst TBAB are keys to success. This reaction provides an environmentally friendly and low-cost approach to mandelates, which are valuable building blocks and widely present in pharmaceuticals.

One-pot synthesis of 3-substituted-3-hydroxyindolin-2-ones: three component coupling of N-protected isatin, aryne precursor and 1,3-cyclodione under metal-free conditions

A transition-metal free synthetic strategy for the direct synthesis of 3-substituted-3-hydroxy-indolin-2-ones via three component coupling of N-protected isatin, aryne precursor, and 1,3-cyclodione.

α-Hydroxylation of α,α-Disubstituted N-tert-Butanesulfinyl Ketimines with Molecular Oxygen: Stereoselective Synthesis of α-Tertiary Hydroxyimines

α-Tertiary hydroxyimines were stereoselectively synthesized from enantioenriched N-tert-butanesulfinyl ketimines using potassium tert-butoxide, molecular oxygen, and trimethyl phosphite. The stereoselective hydroxylation of acyclic ketimines bearing two sterically similar α-substituents was achieved by controlling the geometry of the metalloenamine intermediates and the facial selectivity of hydroxylation. The synthetic utility of the resulting α-tertiary hydroxyimines was demonstrated through the successful diastereoselective synthesis of highly substituted β-amino alcohols.

Tandem Photoredox-Chiral Phosphoric Acid Catalyzed Radical-Radical Cross-Coupling for Enantioselective Synthesis of 3-Hydroxyoxindoles

A photochemical protocol that couples diarylamines and α-ketoesters to afford the chiral 3-hydroxyoxindoles through tandem photoredox and chiral phosphoric acid catalysis is developed. The reaction involves an enantioselective photochemical radical-radical cross-coupling process. The chiral phosphoric acid is discovered to play crucial roles by decreasing the reductive potentials of α-ketoesters and stereocontrolling the downstream asymmetric radical-radical cross-coupling via the formation of pentacoordinate complex.

A Unified Catalytic Asymmetric (4+1) and (5+1) Annulation Strategy to Access Chiral Spirooxindole-Fused Oxacycles

A unified catalytic asymmetric (N+1) (N=4, 5) annulation reaction of oxindoles with bifunctional peroxides has been achieved in the presence of a chiral phase-transfer catalyst (PTC). This general strategy utilizes peroxides as unique bielectrophilic four- or five-atom synthons to participate in the C-C and the subsequent umpolung C-O bond-forming reactions with one-carbon unit nucleophiles, thus providing a distinct method to access the valuable chiral spirooxindole-tetrahydrofurans and -tetrahydropyrans with good yields and high enantioselectivities under mild conditions. DFT calculations were performed to rationalize the origin of high enantioselectivity. The gram-scale syntheses and synthetic utility of the resultant products were also demonstrated.

Solvent-Switched Oxidation Selectivities with O2 : Controlled Synthesis of α-Difluoro(thio)methylated Alcohols and Ketones

The solvent-switched hydroxylation and oxygenation of α-difluoro(thio)methylated carbanions with molecular oxygen under mild conditions are reported. This strategy tames the redox reactions of the in situ generated hydroperoxy difluoromethylsulfides, in which solvent-bonding can alter their reactivity and switch the oxidation selectivities. These controllable three-component reactions of gem-difluoroalkenes, thiols and molecular oxygen afford various useful α-difluoro(thio)methylated alcohols and ketones in high yields. Significantly, this protocol has been applied in the synthesis different bioactive molecules. Mechanism studies enable the detection of the hydroperoxy difluoromethylsulfide intermediates and exclude the thiol-based radical pathway.

Aerobic Oxidations in Asymmetric Synthesis: Catalytic Strategies and Recent Developments

Despite the remarkable advances in the area of asymmetric catalytic oxidations over the past decades, the development of sustainable and environmentally benign enantioselective oxidation techniques, especially with the efficiency level similar to natural enzymes, still represents a challenge. The growing demand for enantiopure compounds and high interest to industry-relevant green technological advances continue to encourage the research pursuits in this field. Among various oxidants, molecular oxygen is ubiquitous, being available at low cost, environmentally benign and easy-to-handle material. This review highlights recent achievements in catalytic enantioselective oxidations utilizing molecular oxygen as the sole oxidant, with focus on the mechanisms of dioxygen activation and chirogenesis in these transformations.

Catalytic Asymmetric Addition of Diorganozinc Reagents to Pyrazole-4,5-Diones and Indoline-2,3-Diones

The catalytic enantioselective diorganozinc additions to cyclic diketones including pyrazolin-4,5-diones and isatins have been developed. In the presence of morpholine-containing chiral amino alcohol ligand, the corresponding chiral cyclic tertiary alcohols were produced in good to excellent yields (up to 97 %) and enantioselectivities (up to 95 % ee). The notable feature of this protocol includes its mild reaction conditions, Lewis acid additives free and broad functional group tolerance.

Asymmetric α-Hydroxylation of α-Aryl-δ-lactams with Molecular Oxygen under Phase-Transfer Conditions

An asymmetric synthesis of α-aryl-α-hydroxy-δ-lactams via phase-transfer-catalyzed hydroxylation with molecular oxygen is described. High yields and high enantioselectivities were achieved using 2,2-diarylvinyl group as an achiral auxiliary. This strategy allows facile access to α-aryl-α-hydroxy-δ-lactam derivatives containing a chiral quaternary center.

Visible light mediated aerobic oxidative hydroxylation of 2-oxindole-3-carboxylate esters: an alternative approach to 3-hydroxy-2-oxindoles

A convenient aerobic oxidative hydroxylation of 3-substituted oxindoles under mild reaction conditions is described herein. This process was accomplished by the activation of molecular oxygen in the air in the presence of a photocatalyst under the irradiation of visible light. The desired product was delivered in up to 89% yield without the addition of base or stoichiometric oxidant.

BINOL derivatives-catalysed enantioselective allylboration of isatins: application to the synthesis of (R)-chimonamidine

The asymmetric synthesis of the 3-allyl-3-hydroxyoxindole skeleton was accomplished in yields up to 99% via a metal-free and enantioselective allylation of isatins (90-96% ee) using BINOL derivatives as catalysts and an optimized allylboronate. This methodology was applied at a gram-scale to the synthesis of the natural product (R)-chimonamidine.

Recent developments in stereoselective organocatalytic oxyfunctionalizations

In this chapter, asymmetric at carbon oxidations using organocatalytic systems reported from 2012 up to 2018 have been illustrated. Asymmetric epoxidations and oxidation of heteroatom-containing molecules were not included. The processes selected encopass alpha-hydroxylation of carbonyl compounds, dihydroxylation and dioxygenation of alkenes, Baeyer-Villiger and oxidative desymmetrization reactions.

Enantioselective nickel-catalyzed arylative and alkenylative intramolecular 1,2-allylations of tethered allene-ketones

The enantioselective nickel-catalyzed reaction of tethered allene-ketones with (hetero)arylboronic acids or potassium vinyltrifluoroborate is described. Carbonickelation of the allene gives allylnickel species, which undergo cyclization by 1,2-allylation to produce chiral tertiary-alcohol-containing aza- and carbocycles in high diastereo- and enantioselectivities.

Bi(OAc)3/chiral phosphoric acid catalyzed enantioselective allylation of isatins

Herein, we disclosed an efficient protocol for the construction of chiral 3-allyl-3-hydroxyoxindoles via the enantioselective allylation reaction of isatins and allylboronates catalyzed by a simple binary acid Bi(OAc)₃/CPA system under mild conditions.

Addition-Rearrangement of Ketenes with Lithium N- tert-Butanesulfinamides: Enantioselective Synthesis of α,α-Disubstituted α-Hydroxycarboxylic Acid Derivatives

Addition of the lithium salts of chiral N-substituted tert-butanesulfinamides to ketenes and subsequent silylation initiates stereoselective [2,3]-rearrangement, which affords enantioenriched α,α-disubstituted α-sulfenyloxy carboxamides through a reaction that faithfully transfers the absolute stereochemistry of the lithiated sulfinylamides to the α-carbon of the amide products. This addition-rearrangement can be performed together with ketene formation from acyl chloride in a single flask, providing a new and practical synthetic route to α-hydroxycarboxylic acid derivatives.

Catalytic Enantioselective Ynamide Additions to Isatins: Concise Access to Oxindole Alkaloids

The highly enantioselective addition of terminal ynamides to a variety of isatins, catalyzed by a bisoxazolidine copper complex under mild, base-free reaction conditions, is described. The reaction is broad in scope, scalable, applicable to unprotected isatins, and provides efficient access to 3-hydroxyoxindoles carrying a tetrasubstituted chiral center with excellent yields and enantioselectivities. Synthetically versatile, multifunctional 3-hydroxyindolinones are obtained by hydration, partial hydrogenation, or hydroxyacyloxylation of the ynamide moiety at room temperature and exhaustive hydrogenation followed by reductive detosylation and spontaneous cyclization affords cinchonamidine alkaloids.

Catalyst-Free Ring Opening of Spiroaziridine Oxindoles by Heteronucleophiles: An Approach to the Synthesis of Enantiopure 3-Substituted Oxindoles

A simple catalyst-free method was developed for the ring opening of spiroaziridine oxindoles by three different nucleophiles, namely, amines, thiols, and methanol, to produce enantiopure (up to 99%) vicinal diaminooxindoles, β-aminosulfides, and β-amino-3-methoxyoxindole, respectively, in good to excellent yields. In contrast to the spiroepoxides, spiroaziridines are opened regio- and stereospecifically through the pseudobenzylic spirocenter under catalyst-free conditions. Moreover, unlike simple 2-substituted aziridines, these spiroaziridines are opened up with retention in configuration at the C3-spirocenter.

Diastereoselective α-Hydroxylation of N-tert-Butanesulfinyl Imidates and N'-tert-Butanesulfinyl Amidines with Molecular Oxygen

Diastereoselective α-hydroxylation using molecular oxygen has been achieved with chiral α-alkyl N-tert-butanesulfinyl imidates and α-aryl N'-tert-butanesulfinyl amidines. The aza-enolates generated from deprotonation of imidates/amidines can be intercepted by O₂ with excellent diastereocontrol and subsequently transformed into α-hydroxylation products in the presence of the reductant trimethyl phosphite.

Chiral phase-transfer catalysis in the asymmetric α-heterofunctionalization of prochiral nucleophiles

Chiral phase-transfer catalysis is one of the major catalytic principles in asymmetric catalysis. A broad variety of different catalysts and their use for challenging applications have been reported over the last decades. Besides asymmetric C–C bond forming reactions the use of chiral phase-transfer catalysts for enantioselective α-heterofunctionalization reactions of prochiral nucleophiles became one of the most important field of application of this catalytic principle. Based on several highly spectacular recent reports, we thus wish to discuss some of the most important achievements in this field within the context of this review.