Desymmetrisation of meso-diols mediated by non-enzymatic acyl transfer catalysts

Ionic Hydrogen Bond-Assisted Catalytic Construction of Nitrogen Stereogenic Center via Formal Desymmetrization of Remote Diols

The control of noncarbon stereogenic centers is of profound importance owing to their enormous interest in bioactive compounds and chiral catalyst or ligand design for enantioselective synthesis. Despite various elegant approaches have been achieved for construction of S-, P-, Si- and B-stereocenters over the past decades, the catalyst-controlled strategies to govern the formation of N-stereogenic compounds have garnered less attention. Here, we disclose the first organocatalytic approach for efficient access to a wide range of nitrogen-stereogenic compounds through a desymmetrization approach. Intriguingly, the pro-chiral remote diols, which are previously not well addressed with enantiocontrol, are well differentiated by potent chiral carbene-bound acyl azolium intermediates. Preliminary studies shed insights on the critical importance of the ionic hydrogen bond (IHB) formed between the dimer aggregate of diols to afford the chiral N-oxide products that feature a tetrahedral nitrogen as the sole stereogenic element with good yields and excellent enantioselectivities. Notably, the chiral N-oxide products could offer an attractive strategy for chiral ligand design and discovery of potential antibacterial agrochemicals.

Silver-Catalyzed Asymmetric Double Desymmetrization via Vinylogous Michael Addition of Prochiral α,α-Dicyanoalkenes to Cyclopentendiones

An asymmetric double desymmetrization methodology has been developed for synthesizing densely functionalized chiral cyclopentylcyclohexane scaffolds. We have constructed four chiral centers, including an all-carbon quaternary stereocenter in a single C-C bond formation event. The methodology has high functional-group tolerance and delivers a broad range of enantioenriched products. This vinylogous Michael addition reaction of prochiral α,α-dicyanocyclohexane to 2,2-disubstituted cyclopentene-1,3-dione is catalyzed by a chiral Ag-(R)-DTBM-SEGPHOS catalyst.

Brønsted-Acid-Catalyzed Enantioselective Desymmetrization of 1,3-Diols: Access to Chiral β-Amino Alcohol Derivatives

Herein, we describe a Brønsted-acid-catalyzed enantioselective desymmetrization of 1,3-diols with alkynes through a hydroalkoxylation/hydrolysis process. The reaction leads to the atom-economical synthesis of valuable chiral β-amino alcohols under mild reaction conditions. Further synthetic transformations based on the β-amino alcohol moiety provide divergent approaches toward chiral N-containing heterocycles.

Reversal of Enantioselectivity for the Desymmetrization of meso-1,2-Diols Catalyzed by Pyridine-N-oxides

The desymmetrization of meso-vic-diols with a reversal of enantioselectivity catalyzed by chiral pyridine-N-oxides with l-proline as a single source of chirality is reported. With chiral 3-substituted ArPNO C2c and 2-substituted 4-(dimethylamino)pyridine-N-oxide C3b as catalysts, a wide range of monoesters were obtained with satisfactory results with a complete and controlled switch in stereoselectivity (up to 97:3 and 1:99 er). Chiral six-membered carbocyclic uracil nucleosides were generated with excellent enantioselectivities after derivatization. A series of control experiments and density functional theory (DFT) calculations supported that the reaction proceeded in a bifunctional activated manner, where the N-oxide groups and N-H proton of the amides were vital for catalytic reactivity and stereocontrol. The DFT calculation also supported the distance-directed switching of enantioselectivity, in which the l-prolinamide moiety moved from the C3 to C2 position on the pyridine ring, resulting in the H-bond interaction between the amide N-H and OH group of meso-vic-diol also shifted from one hydroxyl group to another.

Rh-Catalyzed Enantioselective Desymmetric Hydrogenation of α-Acetamido-1,3-indanediones Using Ether-Bridged Biphenyl Diphosphine Ligands

Novel axially chiral biphenyl diphosphine ligands Enm-BridgePhos, bearing an ether chain bridge at the 5,5'-position of the biphenyl backbone, have been developed and successfully applied in the Rh-catalyzed enantioselective desymmetric hydrogenation of α-acetamido-1,3-indanediones, providing chiral α-acetamido-β-hydroxybenzocyclic pentones in high yields (up to 97%) and with excellent enantioselectivities (up to 99% ee). The reaction could be carried out on a gram scale, and the corresponding products were used as vital intermediates for the synthesis of analogues of chiral spirobenzylisoquinoline alkaloids. Both the crystal structure analysis and the DFT calculations revealed that the large dihedral angle of the Enm-BridgePhos-Rh complexes is highly related to the excellent enantioselectivities.

Dinuclear Zinc-Catalyzed Asymmetric Desymmetrization of Cyclopentendiones: Access to Functional Cyclopentanediones Bearing an All-carbon Quaternary Stereocenter

The success in the identification of the two enantioisomeric surfaces of electrophiles by dinuclear zinc catalysts is disclosed. This protocol realizes a dinuclear zinc-cocatalyzed desymmetrization of cyclopentendiones using α-hydroxy aryl ketones as nucleophiles through Michael addition reaction. Under mild conditions, a series of functional cyclopentanediones bearing multiple stereogenic centers including an all-carbon quaternary stereocenter, were obtained in moderate to good yields with excellent stereoselectivities.

Kinetics and Mechanism of Azole n-π*-Catalyzed Amine Acylation

Azole anions are highly competent in the activation of weak acyl donors, but, unlike neutral (aprotic) Lewis bases, are not yet widely applied as acylation catalysts. Using a combination of in situ and stopped-flow 1H/19F NMR spectroscopy, kinetics, isotopic labeling, 1H DOSY, and electronic structure calculations, we have investigated azole-catalyzed aminolysis of p-fluorophenyl acetate. The global kinetics have been elucidated under four sets of conditions, and the key elementary steps underpinning catalysis deconvoluted using a range of intermediates and transition state probes. While all evidence points to an overarching mechanism involving n-π* catalysis via N-acylated azole intermediates, a diverse array of kinetic regimes emerges from this framework. Even seemingly minor changes to the solvent, auxiliary base, or azole catalyst can elicit profound changes in the temporal evolution, thermal sensitivity, and progressive inhibition of catalysis. These observations can only be rationalized by taking a holistic view of the mechanism and a set of limiting regimes for the kinetics. Overall, the analysis of 18 azole catalysts spanning nearly 10 orders of magnitude in acidity highlights the pitfall of pursuing ever more nucleophilic catalysts without regard for catalyst speciation.

Electrostatically Enhanced 3- and 4-Pyridyl Borate Salt Nucleophiles and Bases

A variety of electrostatically enhanced 3- and 4-pyridylborate salt catalysts are reported and show significant improvement over an activated noncharged neutral control compound. Their nucleophilicity in a stoichiometric SN2 reaction and catalytic performance in a urethane synthesis are evaluated along with three methods for rapidly evaluating the basicity of these species. That is, qualitative titrations in CH2Cl2 and CHCl3 were carried out, two separate solution-state IR studies in CCl4 and CDCl3 are reported, and the proton affinities of the anionic components of the salts were computed. Charge differences between the anion and its protonated zwitterionic conjugate acid are evaluated along with the highest occupied molecular orbitals of the anions in relationship to some of the surprising reactivity findings that were observed in the two kinetic studies.

Diastereoselective Formal 1,3-Dipolar Cycloaddition of Trifluoroethyl Amine-Derived Ketimines Enables the Desymmetrization of Cyclopentenediones

In this research, a metal-free diastereoselective formal 1,3-dipolar cycloaddition of N-2,2,2-trifluoroethylisatin ketimines and cyclopentene-1,3-diones which can efficiently lead to the desymmetrization of cyclopentene-1,3-diones is developed. With the developed protocol, a series of tetracyclic spirooxindoles containing pyrrolidine and cyclopentane subunits can be smoothly obtained with good results (up to 99% yield and 91:9 dr). Furthermore, the methodology can be extended to trifluoromethyl-substituted iminomalonate, and the corresponding formal [3+2] cycloaddition reaction affords bicyclic heterocycles containing fused pyrrolidine and cyclopentane moieties in moderate yields with >20:1 dr. The synthetic potential of the methodology is demonstrated by the scale-up experiment and by versatile transformations of the products.

Copper-Catalyzed Enantioselective Desymmetric Protosilylation of Prochiral Diynes: Access to Optically Functionalized Tertiary Alcohols

In this protocol, a copper-catalyzed desymmetric protosilylation of prochiral diynes was developed. The corresponding products were obtained in moderate to high yields and enantiomeric ratios. This approach provides a simple method for synthesizing functionalized chiral tertiary alcohols in the presence of a chiral pyridine-bisimidazoline (Pybim) ligand.

Copper-Catalyzed Asymmetric Sulfonylative Desymmetrization of Glycerol

Glycerol is the main side product in the biodiesel manufacturing process, and the development of glycerol valorization methods would indirectly contribute the sustainable biodiesel production and decarbonization. Transformation of glycerol to optically active C3 units would be one of the attractive routes for glycerol valorization. We herein present the asymmetric sulfonylative desymmetrization of glycerol by using a CuCN/(R,R)-PhBOX catalyst system to provide an optically active monosulfonylated glycerol in high efficiency. A high degree of enantioselectivity was achieved with a commercially available chiral ligand and an inexpensive carbonate base. The optically active monosulfonylated glycerol was successfully transformed into a C3 unit attached with differentially protected three hydroxy moieties. In addition, the synthetic utility of the present reaction was also demonstrated by the transformation of the monosulfonylated glycerol into an optically active synthetic ceramide, sphingolipid E.

Organocatalytic Access to Tetrasubstituted Chiral Carbons Integrating Functional Groups

Three-dimensional organic structures containing sp³ carbons bearing four non-hydrogen substituents can provide drug-like molecules. Although such complex structures are challenging targets in synthetic organic chemistry, efficient synthetic approaches will open a new chemical space for pharmaceutical candidates. This review provides an account of our recent achievements in developing organocatalytic approaches to attractive molecular platforms based on optically active sp³ carbons integrating four different functional groups. These methodologies include asymmetric cycloetherification and cyanation of multifunctional ketones, both of which take advantage of the mild characteristics of organocatalytic activation. Enzyme-like but non-enzymatic organocatalytic systems can be used to precisely manufacture molecules containing complex chiral structures without substrate specificity problems. In addition, these catalytic systems control not only stereoselectivity but also site-selectivity and do not induce side reactions even from substrates with rich functionality.

Catalytic Asymmetric β‐Oxygen Elimination**

A catalytic enantioselective β‐O‐elimination reaction is reported in the form of a zirconium‐catalyzed asymmetric opening of meso‐ketene acetals. Furthermore, a regiodivergent β‐O‐elimination is demonstrated. The reaction proceeds under mild conditions, at low catalyst loadings, and produces chiral monoprotected cis‐1,2‐diols in good yield and enantiomeric excess. The combination with a Mitsunobu reaction or a one‐pot hydroboration/Suzuki reaction sequence then gives access to additional diol and aminoalcohol building blocks. A stereochemical analysis supported by DFT calculations reveals that a high selectivity in the hydrozirconation step is also important for achieving high enantioselectivity, although it does not constitute the asymmetric step. This insight is crucial for the future development of related asymmetric β‐elimination reactions.

Chiral Symmetry Breaking of Monoacylated Anhydroerythritols and meso-1,2-Diols through Crystallization-Induced Deracemization

We developed a chiral symmetry breaking method for monoacylated meso diols. The X-ray crystal structure analysis of monoacylated 1,4-anhydroerythritols, meso cyclic diols with a cis configuration, revealed that the O-(p-anisoyl) derivative crystallized as a racemic conglomerate of the P2₁ 2₁ 2₁ crystal system. It was confirmed that the substrate racemized by intramolecular transfer of the acyl group in the presence of a catalytic amount of base. Evaporating the solvent gradually from the solution or Viedma ripening to promote crystallization-induced deracemization efficiently led to enantiomer crystals. These results provide the first successful example of asymmetric expression and amplification by deracemization of sugar derivatives without an external chemical chiral source. Furthermore, we applied this methodology to acyclic meso-1,2-diols. Three O-monoacylated substrates were successfully deracemized to 99 % ee by Viedma ripening. We also developed asymmetric desymmetrization of meso-1,2-diols by combining acylation and crystallization-induced deracemization.

Desymmetrization of Diols by Phosphorylation with a Titanium-BINOLate Catalyst

The desymmetrization of ten prochiral diols by phosphoryl transfer with a titanium-BINOLate complex is discussed. The phosphorylation of nine 1,3-propane diols is achieved in yields of 50-98%. Enantiomeric ratios as high as 92:8 are achieved with diols containing a quaternary C-2 center incorporating a protected amine. The chiral ligand, base, solvent, and stoichiometry are evaluated along with a nonlinear effect study to support an active catalyst species that is oligomeric in chiral ligand. The use of pyrophosphates as the phosphorylating agent in the desymmetrization facilitates a user-friendly method for enantioselective phosphorylation with desirable protecting groups (benzyl, o-nitrobenzyl) on the phosphate product.

Non-Enzymatic Desymmetrization Reactions in Aqueous Media

Symmetric organic compounds are generally obtained inexpensively, and therefore they can be attractive building blocks for the total synthesis of various pharmaceuticals and natural products. The drawback is that discriminating the identical functional groups in the symmetric compounds is difficult. Water is the most environmentally benign and inexpensive solvent. However, successful organic reactions in water are rather limited due to the hydrophobicity of organic compounds in general. Therefore, desymmetrization reactions in aqueous media are expected to offer versatile strategies for the synthesis of a variety of significant organic compounds. This review focuses on the recent progress of desymmetrization reactions of symmetric organic compounds in aqueous media without utilizing enzymes.

Hydrogen-Bonding Assisted Catalytic Kinetic Resolution of Acyclic β-Hydroxy Amides

Enantioenriched acyclic α-substituted β-hydroxy amides are valuable compounds in chemical, material and medicinal sciences, but their enantioselective synthesis remains challenging. A catalytic kinetic resolution (KR) of such amides with selectivity factor(s) up to >200 is developed via enantioselective acylation of primary alcohol with N-heterocyclic carbene. An enhanced selectivity for the catalytic KR process is realized using cyclic tertiary amine as base additive. Diastereomeric transition state models for the process are proposed to rationalize the origin of enantioselectivity.

Desymmetrization of gem-diols via water-assisted organocatalytic enantio- and diastereoselective cycloetherification

The first desymmetrization of gem-diols forming chiral hemiketal carbons was accomplished via organocatalytic enantio- and diastereoselective cycloetherification, which afforded optically active tetrahydropyrans containing a chiral hemiketal carbon and tetrasubstituted stereocenters bearing synthetically versatile fluorinated groups. The desymmetrization of silanediols was also demonstrated as an asymmetric route to chiral silicon centers.

Diastereoselective desymmetric 1,2-cis-glycosylation of meso-diols via chirality transfer from a glycosyl donor

Chemical desymmetrization reactions of meso-diols are highly effective for the precise and efficient synthesis of chiral molecules. However, even though enzyme-catalyzed desymmetric glycosylations are frequently found in nature, there is no method for highly diastereoselective desymmetric chemical glycosylation of meso-diols. Herein, we report a highly diastereoselective desymmetric 1,2-cis-glycosylation of meso-diols found in myo-inositol 1,3,5-orthoesters using a boronic acid catalyst based on predictions of regioselectivity by density functional theory (DFT) calculations. The enantiotopic hydroxyl groups of the meso-diols are clearly differentiated by the stereochemistry at the C2 position of the glycosyl donor with excellent regioselectivities. In addition, the present method is successfully applied to the synthesis of core structures of phosphatidylinositolmannosides (PIMs) and glycosylphosphatidylinositol (GPI) anchors, and common β-mannoside structures of the LLBM-782 series of antibiotics.

Enzyme-catalyzed desymmetric glycosylations are often found in nature, however the corresponding chemical methods are lacking. Here, the authors report a highly diastereoselective desymmetric 1,2-cis-glycosylation of meso-diols found in myo-inositol 1,3,5-orthoesters using a boronic acid catalyst.

Computational Design of Enhanced Enantioselectivity in Chiral Phosphoric Acid-Catalyzed Oxidative Desymmetrization of 1,3-Diol Acetals

A general method for the highly enantioselective desymmetrization of 2-alkyl-substituted 1,3-diols is presented. A combination of computational and experimental studies has been utilized to understand the origin of the stereocontrol of oxidative desymmetrization of 1,3-diol benzylideneacetals. DFT calculations demonstrate that the acetal protecting group is highly influential for high enantioselectivity, and a simple but effective new protecting group has been designed. The desymmetrization reactions proceed with high enantioselectivity for a variety of substrates. Moreover, the reaction conditions are also shown to be effective for desymmetrization of 2,2-dialkyl-substituted 1,3-diols, which provides chiral products bearing acyclic all-carbon quaternary stereocenters. The method has been applied to the formal synthesis of indoline alkaloids.

Selective Monoacylation of Diols and Asymmetric Desymmetrization of Dialkyl meso-Tartrates Using 2-Pyridyl Esters as Acylating Agents and Metal Carboxylates as Catalysts

With 2-pyridyl benzoates as acylating agents and Zn(OAc)₂ as a catalyst, 1,2-diols, 1,3-diols, and catechol were selectively monoacylated. Furthermore, the highly enantioselective desymmetrization of meso-tartrates was achieved for the first time, utilizing 2-pyridyl esters and NiBr₂/AgOPiv/Ph-BOX in CH₃CN or CuCl₂/AgOPiv/Ph-BOX in EtOAc catalyst systems (up to 96% ee). The latter catalyst system was also effective for the kinetic resolution of dibenzyl dl-tartrate.

Asymmetric Cycloetherification of in Situ Generated Cyanohydrins through the Concomitant Construction of Three Chiral Carbon Centers

The organocatalytic enantio- and diastereoselective cycloetherification of in situ generated cyanohydrins through the concomitant construction of three chiral carbon centers is reported. This protocol facilitates the concise synthesis of optically active tetrahydropyran derivatives, which are ubiquitous scaffolds found in various bioactive compounds, through the simultaneous construction of multiple bonds and stereogenic centers, including tetrasubstituted chiral carbons. The resulting products also contain multiple synthetically important functional groups, which expand their possible usefulness as chiral building blocks.

Binuclear Double-Stranded Helicates and Their Catalytic Applications in Desymmetrization of Mesodiols

The ligand L₁ of 4-methyl-2,6-diformylphenol and L₂ of 4- tert-butyl-2,6-diformylphenol are synthesized through Schiff base condensation with rac-, ( R)-(+), or ( S)-(-)-1,1'-binaphthyl-2,2'-diamine (BNDA). As a result, the racemic L₁^rac, L₂^rac, and enantiopure L₁^RR, L₁^SS, L₂^RR, and L₂^SS ligands are obtained incorporating Cu(II) and Zn(II) salts by a simple one-pot metal template method. The series of dinuclear complexes of [M₂LX₂] (here, M = Cu²⁺, Zn²⁺; X = acetate ion, chloride ion; L = L₁^RR, L₁^SS, L₁^rac, L₂^RR, L₂^SS, L₂^rac) formulas are obtained in common. Among them, the single crystal X-ray structures for [Zn₂L₁^rac(OAc)₂] and [Zn₂L₁^SSCl₂] complexes are obtained. The detailed crystal structure and the chiroptical studies performed on these complexes dictates a self-sorting behavior in their self-assembly process and illustrate a chirality transfer from the ligand to the metal center on the complexes. The enantiopure dinuclear complexes [M₂L^RRX₂] and [M₂L^SSX₂] generate enantiopure ΛΛ and ΔΔ isomers, respectively, but the racemic complexes produce only homochiral ΛΛ and ΔΔ assemblies. The detailed studies based on UFLC (Ultra Fast Liquid Chromatography), CD, and single crystal X-ray structure together show the absence of heterochiral ΛΔ mesocate. All these complexes are adapted as catalysts for desymmetrization of various mesodiols, and the enantiopure complexes are found to give efficient enantioselectivity in desymmetrization of mesodiols with benzoyl chloride to monobenzoylated ester providing 98% yield and 92% ee.

Chiral DMAP-N-oxides as Acyl Transfer Catalysts: Design, Synthesis, and Application in Asymmetric Steglich Rearrangement

A DMAP-N-oxide, featuring an α-amino acid as the chiral source, was developed, synthesized and applied in asymmetric Steglich rearrangement. A series of O-acylated azlactones afforded C-acylated azlactones possessing a quaternary stereocenter in high yields (up to 97 % yield) and excellent enantioselectivities (up to 97 % ee). Compared to the widespread use of pyridine nitrogen, which serves as the nucleophilic site in the asymmetric acyl transfer reaction, we discovered that chiral DMAP-N-oxides, in which the oxygen now acts as the nucleophilic site, are efficient acyl transfer catalysts. Our finding might open a new door for the development of chiral DMAP-N-oxides for asymmetric acyl transfer reactions.

Synthesis and Desymmetrization of meso Tricyclic Systems Derived from Benzene Oxide

Ozonolysis of the Diels-Alder adducts derived from benzene oxides and N-alkylmaleimides resulted in fully substituted, meso bicyclic systems bearing six contiguous stereocenters, isolated as diols upon reductive workup with NaBH₄. Variation in the workup allowed for isolation of two different diastereoisomers, through double epimerization of the imide stereocenters. Desymmetrization of the resulting meso diols via asymmetric nucleophilic epoxide opening and acylation reactions provided access to highly substituted, enantioenriched fused rings.

Selenide-catalyzed enantioselective synthesis of trifluoromethylthiolated tetrahydronaphthalenes by merging desymmetrization and trifluoromethylthiolation

Trifluoromethylthiolated molecules are an important class of biologically active compounds and potential drug candidates. Because of the lack of efficient synthetic methods, catalytic enantioselective construction of these molecules is rare and remains a challenge. To expand this field, we herein disclose a bifunctional selenide-catalyzed approach for the synthesis of various chiral trifluoromethylthiolated tetrahydronaphthalenes bearing an all-carbon quaternary stereocenter with gem-diaryl-tethered alkenes and alkynes by merging desymmetrization and trifluoromethylthiolation strategy. The products are obtained in high yields with excellent enantio- and diastereo-selectivities. This method can be applied to the desymmetrization and sulfenylation of diols as well. Computational studies reveal that selenide can activate the electrophilic reagent better than sulfide, confirming the higher efficiency of selenide catalysis in these reactions. On the basis of the theoretical calculations, an acid-derived anion-binding interaction is suggested to exist in the whole pathway and accounts for the observed high selectivities.

Catalytic enantioselective synthesis of trifluoromethylthiolated molecules remains a challenge. Here, the authors report a bifunctional selenide-catalyzed approach for the synthesis of structurally complex chiral trifluoromethylthiolated tetrahydronaphthalenes by merging desymmetrization and trifluoromethylthiolation.

Enzymatic monoesterification of symmetric diols: restriction of molecular conformations influences selectivity

We have experimentally demonstrated that by 'locking' the molecular conformation through the introduction of a double or triple bond in the center of a symmetric diol, enzymatic monoesterification can be achieved selectively. The enzyme Candida antarctica lipase B, generally used for the transesterification of diols, can be effectively used for the monoesterification of symmetrical diols in an unbuffered system also. By varying the chain length of a carboxylic acid moiety, we have established that optimum selectivity and efficiency can be achieved in the range of 4.8 to 5.0 pK_a values. Selectivity can be improved up to 98.75% for a monoester in an overall 73% yield (mixture of a monoester and a diester) when but-2-yne-1,4-diol reacted with hexanoic acid. Water, a by-product, provides an interfacial environment for the enzyme to work in the organic reaction medium. The uniqueness of the reported monoesterification protocol is that it involves only the mechanical stirring of the reaction mixture at room temperature in the presence of the enzyme for 24 h. High percentage yield with selectivity for a monoester, easier product isolation and overall, environmental sustainability are added advantages. The synthesized monoesters are characterized by using HNMR and high resolution mass spectrometry (HRMS).