Organocatalytic Enantioselective Decarboxylative Aldol Reaction of Malonic Acid Half Thioesters with Aldehydes

N-to-S Acyl Transfer as an Enabling Strategy in Asymmetric and Chemoenzymatic Synthesis

The observation of thioester-mediated acyl transfer processes in nature has inspired the development of novel protein synthesis and functionalization methodologies. The chemoselective transfer of an acyl group from S-to-N is the basis of several powerful ligation strategies. In this work, we sought to apply the reverse process, the transfer of an acyl group from N-to-S, as a method to convert stable chiral amides into more reactive thioesters. To this end, we developed a novel cysteine-derived oxazolidinone that serves as both a chiral imide auxiliary and an acyl transfer agent. This auxiliary combines the desirable features of rigid chiral imides as templates for asymmetric transformations with the synthetic applicability of thioesters. We demonstrate that the auxiliary can be applied in a range of highly selective asymmetric transformations. Subsequent intramolecular N-to-S acyl transfer of the chiral product and in situ trapping of the resulting thioester provides access to diverse carboxylic acid derivatives under mild conditions. The oxazolidinone thioester products can also be isolated and used in Pd-mediated transformations to furnish highly valuable chiral scaffolds, such as noncanonical amino acids, cyclic ketones, tetrahydropyrones, and dihydroquinolinones. Finally, we demonstrate that the oxazolidinone thioesters can also serve as a surrogate for SNAC-thioesters, enabling their seamless use as non-native substrates in biocatalytic transformations.

Electrochemical decarboxylative alkylation of β-ketoacids with phenol derivatives

An electrochemical method for the decarboxylative alkylation of β-ketoacids with phenol derivatives has been developed. The protocol was carried out in readily available unseparated cells at room temperature in the absence of catalysts and oxidants. The corresponding aryl ketones were obtained in satisfactory yields without additional electrolytes, and were easy to produce in gram-scale synthesis. Based on control experiments and cyclic voltammetry, a plausible reaction mechanism was proposed.

Application of Oxazaborolidine Catalysts (CBS) on Enantioselective 1,4-Addition of Diarylphosphine Oxides to α,β-Unsaturated Thioesters

Here, we report the first catalytic enantioselective 1,4-addition of diarylphosphine oxides to α,β-unsaturated thioesters. Importantly, the most common and commercial oxazaborolidine (CBS) was employed as a catalyst for its new application without being activated by strong protonic acids or Lewis acids and led to the chiral thioesters in excellent yields and enantioselectivities. Furthermore, this method features mild reaction conditions (room temperature and air-insensitive), good substrate tolerance, and easy scalability.

A catalytic enantioselective stereodivergent aldol reaction

The aldol reaction is among the most powerful and strategically important carbon-carbon bond-forming transformations in organic chemistry. The importance of the aldol reaction in constructing chiral building blocks for complex small-molecule synthesis has spurred continuous efforts toward the development of direct catalytic variants. The realization of a general catalytic aldol reaction with control over both the relative and absolute configurations of the newly formed stereogenic centers has been a longstanding goal in the field. Here, we report a decarboxylative aldol reaction that provides access to all four possible stereoisomers of the aldol product in one step from identical reactants. The mild reaction can be carried out on a large scale in an open flask, and generates CO₂ as the only by-product. The method tolerates a broad substrate scope and generates chiral β-hydroxy thioester products with substantial downstream utility.

Catalytic Asymmetric Decarboxylative Michael Addition To Construct an All-Carbon Quaternary Center with 3-Alkenyl-oxindoles

The first highly enantioselective asymmetric decarboxylative addition of β-keto acids with 3-alkenyl-oxindoles bearing an all-carbon quaternary stereocenter have been developed. The relevant products were acquired in 49-98% yields with 88-98% enantioselectivities in the presence of 0.04-1.0 mol % of chiral rhodium catalyst. The comprehensive practicability of this method was proven in the preparation of the key intermediate, which can be easily transformed into analogues of physovenine and physostigmine.

Cooperative Asymmetric Cation-Binding Catalysis

Asymmetric cation-binding catalysis in principle enables the use of (alkali) metal salts, otherwise insoluble in organic solvents, as reagents and effectors in enantioselective reactions. However, this concept has been a formidable challenge due to the difficulties associated with creating a highly organized chiral environment for cations and anions simultaneously. Over the last four decades, various chiral crown ethers have been developed as cation-binding phase-transfer catalysts and examined in asymmetric catalysis. However, the limited ability of chiral crown ethers to generate soluble reactive anions in a confined chiral cage offers a restricted reaction scope and unsatisfactory chirality induction. To address the constraints of monofunctional chiral crown ethers as cation-binding catalysts, it is therefore desirable to develop a cooperative cation-binding catalyst possessing secondary binding sites for anions, which enables the generation of a reactive anion within a chiral cage of a catalyst. This account summarizes our design, development, and applications of chiral BINOL-based oligoethylene glycols (oligoEGs) as a new type of bifunctional cation-binding catalyst. We initially found that achiral oligoEGs were efficient promoters in nucleophilic fluorination with potassium fluoride. Thereby, we hypothesized that, by breaking the closed cyclic ether unit of chiral crown ethers, the free terminal -OH groups could activate the electrophiles by hydrogen bonding whereas the ether oxygens could act as the Lewis base to coordinate metal ions, thus generating soluble anions in a confined chiral cage. This hypothesis was realized by synthesizing a series of chiral variants of oligoEGs by connecting two 3,3'-disubstituted-BINOL units with glycol linkers. Readily available BINOL-based chiral oligoEGs enabled numerous asymmetric transformations out of the reach of chiral monofunctional crown ether catalysts. We have demonstrated that this new type of bifunctional cation-binding catalysts can generate a soluble fluoride anion from alkali metal fluorides, which can be a versatile chiral promoter for diverse asymmetric catalytic reactions, kinetic resolution (selectivity factor of up to ∼2300), asymmetric protonation, Mannich reactions, tandem cyclization reactions, and the isomerization of allylic alcohols and hemithioacetals. We have also successfully utilized our chiral oligoEG catalysts along with alkali metal salts of carbon- and heteroatom-based nucleophiles, respectively, for asymmetric Strecker reactions and the asymmetric synthesis of chiral aminals. The power of our cooperative cation-binding catalysis was exemplified by kinetic resolution reactions of secondary alcohols, achieving highly enantioselective catalysis with only <1 ppm loading of an organocatalyst with high TOFs (up to ∼1300 h^-1 at 1 ppm catalyst loading). The broadness and generality of our cooperative asymmetric cation-binding catalysis can be ascribed, in a similar fashion, to active-site architectures of enzymes using allosteric interactions, highly confined chiral cages formed by the incorporation of alkali metal salts in the catalyst polyether chain backbone, and the cooperative activation of reacting partners by hydrogen-bonding and ion-ion interactions. Confining reactive components in such a chiral binding pocket leads to enhanced reactivity and efficient transfer of the stereochemical information.

π-Face Promoted Catalysis in Water: From Electron-deficient Molecular Cages to Single Aromatic Slides

Exploiting noncovalent π-interactions particularly emerging anion-π interactions to drive efficient catalysis is fascinating. Even with exciting progresses, can anion-π activation operate in water remains elusive. Here we report the design, synthesis and catalytic studies of a class of water-soluble electron-deficient molecular cages and relevant aromatic slide compounds. The prism-like cages contain three divided, long, cationic aromatic walls which constitute three highly electron-deficient V-shape cavities. They were efficiently synthesized in two steps from a parent triformyl cage in gram-scale. Crystal structure showed the π-walls bind to the counter bromide through strong anion-π interactions. Just 5 mol% of cages were effective in catalyzing decarboxylative Aldol reactions of aldehydes and malonic acid half thioesters in water but not in organic solvents, showing a pronounced hydrophobic amplification effect. Meantime, a series of single π-slides resembling the π-wall of the cage performed equally well, while those lacking an extended π-surface were ineffective, highlighting the essential role of electron-deficient π-face on promoting the conversion.

The synthesis of chiral β-naphthyl-β-sulfanyl ketones via enantioselective sulfa-Michael reaction in the presence of a bifunctional cinchona/sulfonamide organocatalyst

Cinchona alkaloid-derived organocatalysts are widely employed in various asymmetric transformations, yielding products with high enantiopurity. In this respect, a bifunctional quinine-derived sulfonamide organocatalyst was developed to catalyze the asymmetric sulfa-Michael reaction of naphthalene-1-thiol with trans-chalcone derivatives. The target sulfa-Michael adducts were obtained with up to 96% ee under mild conditions and with a low (1 mol %) catalyst loading. Selected enantiomerically enriched sulfa-Michael addition products were subjected to oxidation to obtain the corresponding sulfones.

Organocatalytic Enantioselective γ-Elimination: Applications in the Preparation of Chiral Peroxides and Epoxides

An organocatalyzed enantioselective γ-elimination process has been achieved and applied in the kinetic resolution of peroxides to access chiral peroxides and epoxides. The reaction provided a pathway for the preparation of two useful synthetic and biologically important structural motifs through a single-step reaction. A range of substrates has been resolved with a selectivity factor up to 63. The obtained enantioenriched peroxides and epoxides allowed a series of transformations with retained optical purities.

Enantioselective Control of Both Helical and Axial Stereogenic Elements though an Organocatalytic Approach

A highly diastereo- and enantio-selective method for the asymmetric synthesis of molecules containing helicenes and stereogenic axes was developed based on organocatalysis. Various compounds bearing both helical and axial stereogenic elements were obtained in excellent enantioselectivities. The mechanism study revealed that the reaction proceeded through two stages: 1) The first cyclization produces a reaction intermediate containing a stereogenic axis. 2) The dynamic kinetic resolution of helix reaction intermediate following with cyclization generates a helix and another stereogenic axis.

Ni(II)-Catalyzed Enantioselective Synthesis of β-Hydroxy Esters with Carboxylate Assistance

A Ni-oxazoline complex-catalyzed asymmetric decarboxylative aldol reaction between malonic acid half-oxyesters and various carbonyls with carboxylate assistance was developed, affording structurally diverse β-hydroxy esters with good yields and enantioselectivities under mild conditions. Importantly, the broad substrate scope of this methodology enabled rapid accesses to several natural products and their analogues as exemplified by phenylpropanoid, phaitanthrin B, and phthalide.

Catalyst-Free Synthesis of α-Functionalized 2H-Chromenes in Water: A Tandem Self-Promoted pseudo-Substitution and Decarboxylation Process

A catalyst-free decarboxylative reaction between β-keto acids and 2H-chromene acetals in water was developed. This reaction featured a broad substrate scope and easily obtainable starting materials to afford α-functionalized 2H-chromenes in high yields. The synthetic value of this protocol was also demonstrated by the scale-up synthesis and versatile conversions of the title product into other useful compounds. In addition, control experiments indicated that water was essential for the reactivity. Mechanistic studies further revealed that the reaction proceeded through a self-promoted tandem pseudo-substitution and decarboxylation process.

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.

Multicomponent synthesis of pyroglutamic acid derivatives via Knoevenagel-Michael-hydrolysis-lactamization-decarboxylation (KMHL-D) sequence

A novel and practical method for the synthesis of 3-substituted pyroglutamic acid derivatives is described. One pot multicomponent reaction of Meldrum's acid, aldehyde and Schiff's base followed an unprecedented chemoselective Knoevenagel-Michael-hydrolysis-lactamization domino sequence to afford 4-carboxy 3-substituted pyroglutamic acid derivatives under mild conditions. A carboxy intermediate formed appears to accelerate its own formation. The generality of the synthesis is exemplified by the use of a wide variety of aldehydes including enolizable aliphatic aldehydes, while substrates are stable under reaction conditions.

Organocatalytic atropo- and E/Z-selective Michael addition reaction of ynones with α-amido sulfones as sulfone-type nucleophile

We describe herein the first enantioselective organocatalytic Michael addition reaction of α-amido sulfones to ynones to access axially chiral styrenes.

Unraveling and Manipulating the Stereospecific Retro-Aldol Reaction in the Organocatalytic Asymmetric Aldol Reaction of Isatin and Cyclohexanone

An l-pyroglutamic acid-derived bifunctional organocatalyst was designed and applied in an organocatalytic asymmetric direct aldol reaction between isatins and cyclohexanone, in which an erosion of enantiomeric excess of aldol adduct was unexpectedly observed. Through closely monitoring the reaction and performing extensive control experiments, it was determined that the erosion of ee was attributed to a rare stereospecific retro-aldol process. Moreover, effective manipulation of the retro-aldol process by tuning the use of starting materials was ultimately accomplished, leading to evidently upgraded enantioselectivity and functional group tolerance. This study demonstrates the impact of the hidden reaction pathway on the enantioselectivity in asymmetric transformation.

Nickel/Copper Dual Catalysis for Sequential Nazarov Cyclization/Decarboxylative Aldol Reaction

A nickel/copper cocatalyzed sequential Nazarov cyclization/decarboxylative aldol reaction is reported. Various β-hydroxycyclopentenones containing three contiguous stereocenters were prepared in good yields with high diastereoselectivities. A gram-scale reaction and further derivatizations were easily achieved to highlight the synthetic utility of this transformation. Preliminary attempts for sequential asymmetric transformation were also conducted.

Access to Anti or Syn 2-Amino-1,3-diol Scaffolds from a Common Decarboxylative Aldol Adduct

A straightforward synthetic pathway allowing the access to anti or syn 2-amino-1,3-diol scaffolds is presented. The strategy relies on a diastereoselective organocatalyzed decarboxylative aldol reaction of a N-Boc-hemimalonate that is easily formed from commercial N-Boc-diethyl malonate. Although this method has been optimized previously with the N-Bz-hemimalonate analogue, this key step was reinvestigated with the N-Boc derivative to improve the required reaction time, the yield, and the diastereoselectivity. The new conditions enhance this transformation, and quantitative yields and anti/syn ratios up to 96:4 can be obtained. The anti aldol product was easily isolated in pure form and then taken forward as the key precursor in the preparation of both a set of ten N-/O-alkylated anti 2-amino-1,3-diol derivatives and the syn congeners.

Kinetic Resolution of β-Hydroxy Carbonyl Compounds via Enantioselective Dehydration Using a Cation-Binding Catalyst: Facile Access to Enantiopure Chiral Aldols

A practical and highly enantioselective nonenzymatic kinetic resolution of racemic β-hydroxy carbonyl (aldol) compounds through enantioselective dehydration process was developed using a cation-binding Song's oligoethylene glycol (oligoEG) catalyst with potassium fluoride (KF) as base. A wide range of racemic aldols was resolved with extremely high selectivity factors ( s = up to 2393) under mild reaction conditions. This protocol is easily scalable. It provides an alternative approach for the syntheses of diverse biologically and pharmaceutically relevant chiral aldols in enantiomerically pure form. For example, racemic gingerols could participate in this kinetic resolution with superb efficiency ( s > 240), affording both enantiomerically pure gingerols and corresponding shogaols simultaneously in a single step. The dramatic effectiveness of such kinetic resolution process can be ascribed to systematic cooperative hydrogen-bonding catalysis in a densely confined supramolecular chiral cage in situ generated from the chiral catalyst, substrate, and KF.

Iterative Polyketide Synthesis via a Consecutive Carbonyl-Protecting Strategy

To address the difficulty in protecting a β-polycarbonyl compound, a method for the sequential protection of elongating carbonyl groups was demonstrated. The iterative chain elongation of a carboxylic acid with malonic acid half thioester followed by the protection of the resulting β-ketothioester was performed via the stepwise formation of an isoxazole ring using an O-protected oxime functionality. Yangonin and isosakuranetin were synthesized according to this procedure.

Biomimetic iterative method for polyketide synthesis

An iterative method for synthesizing polyketides was demonstrated, in which the chain elongation of a carboxylic acid was performed by decarboxylative dehydration condensation with a malonic acid half thioester. After transforming the resulting β-ketothioester into an appropriate form, the carboxylic acid functionality was regenerated for the next elongation step.

Chiral bisoxazoline catalyzed decarboxylative aldol reactions between β-carbonyl acids and trifluoroacetaldehyde hemiacetals as well as trifluoroacetaldehyde: the mechanism, the origin of enantioselectivity and the role of a catalyst

The role of a catalyst in the decarboxylative aldol reactions between β-carbonyl acids and trifluoroacetaldehyde hemiacetals as well as trifluoroacetaldehyde catalyzed by chiral bisoxazoline were unveiled theoretically.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Regioselective decarboxylative addition of malonic acid and its mono(thio)esters to 4-trifluoromethylpyrimidin-2(1H)-ones

Background: Due to the high reactivity towards various C-nucleophiles, trifluoromethylketimines are known to be useful reagents for the synthesis of α-trifluoromethylated amine derivatives. However, decarboxylative reactions with malonic acid and its mono(thio)esters have been poorly investigated so far despite the potential to become a convenient route to β-trifluoromethyl-β-amino acid derivatives and to their partially saturated heterocyclic analogues. Results: In this paper we show that 4-trifluoromethylpyrimidin-2(1H)-ones, unique heterocyclic ketimines, react with malonic acid under organic base catalysis to regioselectively provide either Michael- or Mannich-type decarboxylative addition products depending on solvent polarity. Malonic mono(thio)esters give exclusively Michael-type products. The two regioisomeric products can be converted into saturated (2-oxohexahydropyrimidin-4-yl)acetic acid derivatives by mild hydrogenation of the endocyclic C=C double bond in the presence of Pd/C as catalyst. The cis-stereoisomers selectively formed upon reduction of the Michael-type products were structurally determined by X-ray diffraction. As a result of this study, a number of novel acetic acid derivatives containing trifluoromethylated, partially or fully saturated 2-oxopyrimidine cores were prepared and characterized as promising building blocks. Conclusions: Regio- and stereoselective protocols have been developed for the synthesis of novel isomeric 4(6)-trifluoromethylated 1,2,3,4-tetrahydro- and perhydro-(2-oxopyrimidin-4-yl)acetic acid derivatives.

Decarbethoxylative Arylation Employing Arynes: A Metal-Free Pathway to Arylfluoroamides

An efficient, metal-free decarbethoxylative arylation protocol for the synthesis of α-aryl-α-fluoroamides from fluoromalonamates, under ambient reaction conditions using aryne as an electrophilic arylating agent, is reported. This decarbethoxylative arylation proceeds under mild conditions and provides a practical and effective entry to a wide range of α-aryl-α-fluoroacetamides. Interestingly, the use of the tert-butyl ester of fluoromalonamate prevented the otherwise rapid decarboxylation step, affording the arylated fluoromalonamate in moderate yield.