Asymmetric Malonic and Acetoacetic Acid Syntheses – A Century of Enantioselective Decarboxylative Protonations

Organocatalytic enantioselective decarboxylative protonation of α-alkyl-α-aryl malonate monoesters

In contrast to the well-established enzymatic enantioselective decarboxylative protonation (EDP), the corresponding chemocatalytic reactions of acyclic malonic acid derivatives remain challenging. Herein, we developed a biomimetic EDP of α-alkyl-α-aryl malonate monoesters using a chiral 1,2-trans-diaminocyclohexane-based N-sulfonamide as an organocatalyst. The method demonstrates excellent chemical yields, good enantioselectivity, mild reaction conditions, and the generation of only CO2 as waste.

Modular and diverse synthesis of amino acids via asymmetric decarboxylative protonation of aminomalonic acids

Stereoselective protonation is a challenge in asymmetric catalysis. The small size and high rate of transfer of protons mean that face-selective delivery to planar intermediates is hard to control, but it can unlock previously obscure asymmetric transformations. Particularly, when coupled with a preceding decarboxylation, enantioselective protonation can convert the abundant acid feedstocks into structurally diverse chiral molecules. Here an anchoring group strategy is demonstrated as a potential alternative and supplement to the conventional structural modification of catalysts by creating additional catalyst-substrate interactions. We show that a tailored benzamide group in aminomalonic acids can help build a coordinated network of non-covalent interactions, including hydrogen bonds, π-π interactions and dispersion forces, with a chiral acid catalyst. This allows enantioselective decarboxylative protonation to give α-amino acids. The malonate-based synthesis introduces side chains via a facile substitution of aminomalonic esters and thus can access structurally and functionally diverse amino acids.

Asymmetric Retro-Claisen Reaction Catalyzed by Chiral Aza-Bisoxazoline-Zn(II) Complex: Enantioselective Synthesis of α-Arylated Ketones

An asymmetric retro-Claisen reaction of α-monosubstituted β-diketones and quinones (or quinone imine) has been developed under the catalysis of a chiral aza-bisoxazoline-Zn(II) complex. The reaction proceeds via a sequence of conjugate addition, arylation, hemiketal anion-initiated C-C bond cleavage, and enantioselective protonation of enolate to provide various functionalized α-arylated ketones bearing a tertiary stereogenic center with high enantioselectivities. Notably, biologically important benzofuran and γ-butyrolactone derivatives could be synthesized by application of the developed protocol.

Biomimetic Synthesis of Chejuenolides A-C by a Cryptic Lactone-Based Macrocyclization: Stereochemical Implications in Biosynthesis

A hypothetical Mannich macrocyclization in the biosynthesis of chejuenolides A-C served as the basis for the synthetic design herein. Using a lactone-based linear precursor constructed via a tactic sequence of aldol-Julia-aldol reactions on a gram scale, the biomimetic total synthesis and structural validation of chejuenolides A-C were successfully achieved for the first time. The β-oxo-δ-lactone unit in the macrocyclized adducts was fragile and readily converted to a series of C2/C18-diastereoisomers via a decarboxylation and protonation pathway. Stereochemical identification of the biosynthetic precursor (O3P2) confirmed structural adherence to the given macrocycles and previously clarified lankacidins. Moreover, the stereovariants of the linear precursor designed for the macrocyclization event highlighted the unparalleled impact of using this biomimetic approach to determine the stereoselectivity in the proposed enzymatic reaction by reviving the lost or unstable intermediate.

Growth and Characterization of Second and Third Order Acentric Studies of l-Phenylalanine l-Phenylalaninium Malonate Single Crystal

A single crystal of l-phenylalanine l-phenylalanininum malonate (LPPMA) was synthesized by slow evaporation and was subjected to nonlinear optical examination and physio-chemical characterization. Studies on single X-ray diffraction confirm the arrangement of monoclinic space group P21 which is a vital criterion for the NLO phenomenon. The assessment of functional groups and diverse vibration modes responsible for the characteristics of the material was performed with an FTIR analysis. The UV-visible spectral examination found the wavelength of UV-cutoff at 233 nm and various optical parameters were evaluated. The mechanical strength and different criteria associated with it were assessed. The electric field response of the material was examined in terms of the dielectric constant, dielectric loss, ac conductivity and activation energy. The spectra of emission were detailed. The efficacy of second harmonic generation was studied. The parameters of nonlinearity were investigated to analyse the third-order acentric optical response in the LPPMA by Z-scan procedure.

Recent Advances in the Synthesis of Ibuprofen and Naproxen

Herein, we review the recent progress in the synthesis of representative nonsteroidal anti-inflammatory drugs (NSAIDs), ibuprofen and naproxen. Although these drugs were discovered over 50 years ago, novel practical and asymmetric approaches are still being developed for their synthesis. In addition, this endeavor has enabled access to more potent and selective derivatives from the key frameworks of ibuprofen and naproxen. The development of a synthetic route to ibuprofen and naproxen over the last 10 years is summarized, including developing methodologies, finding novel synthetic routes, and applying continuous-flow chemistry.

Enantioselective Protonation of Cyclic Carbonyl Ylides by Chiral Lewis Acid Assisted Alcohols

Chiral Lewis acid-catalyzed asymmetric alcohol addition reactions to cyclic carbonyl ylides generated from N-(α-diazocarbonyl)-2-oxazolidinones featuring a dual catalytic system are reported. Construction of a chiral quaternary heteroatom-substituted carbon center was accomplished in which the unique heterobicycles were obtained in good yields with high stereoselection. The alcohol adducts were successfully converted to optically active oxazolidine-2,4-diones by hydrolysis. Mechanistic studies by DFT calculations revealed that alcohols could be activated by Lewis acids, enabling enantioselective protonation of the carbonyl ylides.

Enantioselective decarboxylative protonation and deuteration of β-ketocarboxylic acids

Enantioselective decarboxylative protonation of tetralone-derived β-ketocarboxylic acids was achieved with up to 89% enantiomeric excess (ee)-in the presence of a chiral primary amine catalyst. Furthermore, this method was applied to enantioselective deuteration to afford the corresponding α-deuterioketones with up to 88% ee.

Catalytic enantioselective decarboxylative nucleophilic addition reactions using chiral organocatalysts

Catalytic decarboxylative reactions are attractive as biomimetic and environmentally friendly reaction processes. This review summarizes the recent results of organocatalytic enantioselective decarboxylative reactions of malonic acid half oxy- or thioesters, β-ketoacids, and related compounds from October 2013 to December 2019.

Catalytic asymmetric enamine protonation reaction

Enantioselective protonation, delivery of a proton to a carbanion intermediate, is the most straightforward and fundamental method for the preparation of a chiral tertiary carbon stereocenter. Recent efforts for this objective have been realized through enamine catalysis, which has now become a prominent catalytic strategy enabling a range of fascinating chiral transformations. This review will summarize recent advances in the field of enantioselective enamine protonation for the synthesis of optically active carbonyl compounds. Dynamic kinetic resolutions of α-substituted carbonyl compounds through enamine intermediates will be discussed as well.

Synthesis of Thymoquinone-Artemisinin Hybrids: New Potent Antileukemia, Antiviral, and Antimalarial Agents

A series of hybrid compounds based on the natural products artemisinin and thymoquinone was synthesized and investigated for their biological activity against the malaria parasite Plasmodium falciparum 3D7 strain, human cytomegalovirus (HCMV), and two leukemia cell lines (drug-sensitive CCRF-CEM and multidrug-resistant subline CEM/ADR5000). An unprecedented one-pot method of selective formation of C-10α-acetate 14 starting from a 1:1 mixture of C-10α- to C-10β-dihydroartemisinin was developed. The key step of this facile method is a mild decarboxylative activation of malonic acid mediated by DCC/DMAP. Ether-linked thymoquinone-artemisinin hybrids 6a/b stood out as the most active compounds in all categories, while showing no toxic side effects toward healthy human foreskin fibroblasts and thus being selective. They exhibited EC₅₀ values of 0.2 μM against the doxorubicin-sensitive as well as the multidrug-resistant leukemia cells and therefore can be regarded as superior to doxorubicin. Moreover, they showed to be five times more active than the standard drug ganciclovir and nearly eight times more active than artesunic acid against HCMV. In addition, hybrids 6a/b possessed excellent antimalarial activity (EC₅₀ = 5.9/3.7 nM), which was better than that of artesunic acid (EC₅₀ = 8.2 nM) and chloroquine (EC₅₀ = 9.8 nM). Overall, most of the presented thymoquinone-artemisinin-based hybrids exhibit an excellent and broad variety of biological activities (anticancer, antimalarial, and antiviral) combined with a low toxicity/high selectivity profile.

Reactivity of Lithium β-Ketocarboxylates: The Role of Lithium Salts

Lithium β-ketocarboxylates 1(COOLi), prepared by the reaction of lithium enolates 2(Li⁺) with carbon dioxide, readily undergo decarboxylative disproportionation in THF solution unless in the presence of lithium salts, in which case they are indefinitely stable at room temperature in inert atmosphere. The availability of stable THF solutions of lithium β-ketocarboxylates 1(COOLi) in the absence of carbon dioxide allowed reactions to take place with nitrogen bases and alkyl halides 3 to give α-alkyl ketones 1(R) after acidic hydrolysis. The sequence thus represents the use of carbon dioxide as a removable directing group for the selective monoalkylation of lithium enolates 2(Li⁺). The roles of lithium salts in preventing the disproportionation of lithium β-ketocarboxylates 1(COOLi) and in determining the course of the reaction with bases and alkyl halides 3 are discussed.

Transition metal-free decarboxylative alkylation reactions

This review summarizes recent advances in the transition metal-free decarboxylative alkylation of carboxylic acids and their derivatives.

Enzymatic enantioselective decarboxylative protonation of heteroaryl malonates

The enzyme aryl/alkenyl malonate decarboxylase (AMDase) catalyses the enantioselective decarboxylative protonation (EDP) of a range of disubstituted malonic acids to give homochiral carboxylic acids that are valuable synthetic intermediates. AMDase exhibits a number of advantages over the non-enzymatic EDP methods developed to date including higher enantioselectivity and more environmentally benign reaction conditions. In this report, AMDase and engineered variants have been used to produce a range of enantioenriched heteroaromatic α-hydroxycarboxylic acids, including pharmaceutical precursors, from readily accessible α-hydroxymalonates. The enzymatic method described here represents an improvement upon existing synthetic chemistry methods that have been used to produce similar compounds. The relationship between the structural features of these new substrates and the kinetics associated with their enzymatic decarboxylation is explored, which offers further insight into the mechanism of AMDase.

Catalytic enantioselective protoboration of disubstituted allenes. Access to alkenylboron compounds in high enantiomeric purity

Proto-boryl additions to 1,1-disubstituted allenes in the presence of 1.0-5.0 mol % of chiral NHC-Cu complexes, B2(pin)2, and t-BuOH proceed to afford alkenyl-B(pin) products in up to 98% yield, >98:2 site selectivity, and 98:2 er. The enantiomerically enriched alkenylboron products can be converted to otherwise difficult-to-access alkenyl bromides, methyl ketones or carboxylic acids. What's more, the corresponding boronic acids may be used in highly stereoselective NHC-Cu-catalyzed allylic substitution reactions.

Accurate predictions of C–SO2R bond dissociation enthalpies using density functional theory methods

M06-2X/6-31G(d) was found to be accurate in calculating C–S BDEs, and preliminary mechanistic studies were performed using it.

Catalytic enantioselective decarboxylative reactions using organocatalysts

Catalytic decarboxylative reactions are attractive as biomimetic reactions and environmentally friendly reaction processes. In this review, the origin and recent development of organocatalytic enantioselective decarboxylative reactions of malonic acid half oxy- or thioesters, or β-ketoacids are summarized.

Direct synthesis of β-hydroxy-α-amino acids via diastereoselective decarboxylative aldol reaction

A straightforward metal-free synthesis of anti-β-hydroxy-α-amino acids is described. The organic base-mediated decarboxylative aldol reaction of cheap, readily available α-amidohemimalonates with various aldehydes afforded under very mild conditions anti-β-hydroxy-α-amido esters in high yields and complete diastereoselectivity. Simple one-pot subsequent transformations enabled the corresponding anti-β-hydroxy-α-amino acids or in a few examples their syn diastereomers to be obtained directly using epimerization conditions.

Chiral primary-amine-catalyzed conjugate addition to α-substituted vinyl ketones/aldehydes: divergent stereocontrol modes on enamine protonation

Enantioselective protonation with a catalytic enamine intermediate represents a challenging, yet fundamentally important process for the synthesis of α-chiral carbonyls. We describe herein chiral primary-amine-catalyzed conjugate additions of indoles to both α-substituted acroleins and vinyl ketones. These reactions feature enamine protonation as the stereogenic step. A simple primary-tertiary vicinal diamine 1 with trifluoromethanesulfonic acid (TfOH) was found to enable both of the reactions of acroleins and vinyl ketones with good activity and high enantioselectivity. Detailed mechanistic studies reveal that these reactions are rate-limiting in iminium formation and they all involve a uniform H2 O/acid-bridged proton transfer in the stereogenic steps but divergent stereocontrol modes for the protonation stereoselectivity. For the reactions of α-branched acroleins, facial selections on H2 O-bridged protonation determine the enantioselectivity, which is enhanced by an OH⋅⋅⋅π interaction with indole as uncovered by DFT calculations. On the other hand, the stereoselectivity of the reactions with vinyl ketones is controlled according to the Curtin-Hammett principle in the CC bond-formation step, which precedes a highly stereospecific enamine protonation.

Palladium and organocatalysis: an excellent recipe for asymmetric synthesis

The dual activation of simple substrates by the combination of organocatalysis and palladium catalysis has been successfully applied in a variety of different asymmetric transformations. Thus, the asymmetric α-allylation of carbonyl compounds, α-fluorination of acyl derivatives, decarboxylative protonation of β-dicarbonyl compounds, cyclization reactions of alkynyl carbonyl compounds and β-functionalization of aldehydes have been efficiently achieved employing this double-catalytic methodology.