Recent applications of Cinchona alkaloids and their derivatives as catalysts in metal-free asymmetric synthesis

Bicarbonate-binding catalysis for the enantioselective desymmetrization of keto sulfonium salts

Herein, an enantioselective desymmetrization of cyclic keto sulfonium salts through enantioselective deprotonation/ring opening process by anion-binding catalysis is presented. We report a squaramide/HCO3- complex as catalytic active species which is able to stereo-differentiate two enantiomeric protons, triggering the ring opening event taking advantage of the great tendency of sulfonium salts to act as leaving groups. Thus, this desymmetrization methodology give rise to β-methylsulfenylated sulfa-Michael addition type products with excellent yields and very good enantioselectivities. The bifunctional organocatalyst has been demonstrated to be capable of activating simultaneously the base and the keto sulfonium salt by DFT calculations and experimental proofs.

Organocatalytic Asymmetric Synthesis of Tetrahydroquinolines from ortho-Aminophenyl para-Quinone Methides

The asymmetric catalytic [4 + 2] cycloannulation of ortho-aminophenyl p-QMs with different types of alkenes for the construction of tetrahydroquinolines containing three contiguous stereogenic centers was developed. This is the first example of catalytic asymmetric cycloannulation of ortho-aminophenyl p-QMs. This reaction exhibits excellent functional group tolerance. Excellent yields, exclusive diastereoselectivities, and high enantioselectivities were obtained in this efficient organocatalytic reaction.

Plausible Pnicogen Bonding of epi-Cinchonidine as a Chiral Scaffold in Catalysis

As a non-covalent interaction of a chiral scaffold in catalysis, pnicogen bonding of epi-cinchonidine (epi-CD), a cinchona alkaloid, was simulated to consider whether the interaction can have the potential controlling enantiotopic face like hydrogen bonding. Among five reactive functional groups in epi-CD, two stable complexes of the hydroxyl group (X-epi-CD1) at C₁₇ and of the quinoline ring (X-epi-CD2) at N₁₆ with pnictide family analytes [X = substituted phosphine (PX), i.e., F, Br, Cl, CF₃, CN, HO, NO₂, and CH₃, and pnictide family analytes, i.e., PBr₃, BiI₃, SbI₃, and AsI₃] were predicted with intermolecular interaction energies, charge transfer (Q_Mulliken and Q_NBO), and band gap energies of HOMO-LUMO (Eg) at the B3LYP/6-31G(d,p) level of density functional theory. It was found that the dominant site of pnicogen bonding in epi-CD is the quinoline ring (N₁₆ atom) rather than the hydroxyl group (O₃₆ atom). In addition, the UV-Vis spectra of the complex were calculated by time-dependent density functional theory (TD-DFT) at the B3LYP/6-31+G(d,p) level and compared with experimental measurements. Through these calculations, two intermolecular interactions (H-bond vs. pnicogen bond) of epi-CD were compared.

Enantioselective Protonation: Hydrophosphinylation of 1,1-Vinyl Azaheterocycle N-Oxides Catalyzed by Chiral Bis(guanidino)iminophosphorane Organosuperbase

Enantioselective protonation by hydrophosphinylation of diarylphosphine oxides with 2-vinyl azaheterocycle N-oxide derivatives was demonstrated using chiral bis(guanidino)iminophosphorane as the higher-order organosuperbase catalyst. It was confirmed by several control experiments that a chiral weak conjugate acid of the chiral bis(guanidino)iminophosphorane, instead of achiral diarylphosphine oxides, directly functioned as the proton source to afford the corresponding product in a highly enantioselective manner in most cases. Enantioselective protonation by a weak conjugate acid generated from the higher-order organosuperbase would broaden the scope of enantioselective reaction systems because of utilization of a range of less acidic pronucleophiles. This method is highlighted by the valuable synthesis of a series of chiral P,N-ligands for chiral metal complexes through the reduction of phosphine oxide and N-oxide units of the corresponding product without loss of enantiomeric purity.

Synthesis and Applications of Cinchona Squaramide-Modified Poly(Glycidyl Methacrylate) Microspheres as Recyclable Polymer-Grafted Enantioselective Organocatalysts

This work presents the immobilization of cinchona squaramide organocatalysts on poly(glycidyl methacrylate) solid supports. Preparation of the well-defined monodisperse polymer microspheres was facilitated by comprehensive parameter optimization. By exploiting the reactive epoxy groups of the polymer support, three amino-functionalized cinchona derivatives were immobilized on this carrier. To explore the effect of the amino linker, these structurally varied precatalysts were synthesized by modifying the cinchona skeleton at different positions. The catalytic activities of the immobilized organocatalysts were tested in the Michael addition of pentane-2,4-dione and trans-β-nitrostyrene with excellent yields (up to 98 %) and enantioselectivities (up to 96 % ee). Finally, the catalysts were easily recovered five times by centrifugation without loss of activity.

ADMET Polymerization of Dimeric Cinchona Squaramides for the Preparation of a Highly Enantioselective Polymeric Organocatalyst

Under the acyclic diene metathesis (ADMET) reaction condition, the C3-vinyl groups of cinchona alkaloids readily react with each other to form a C-C bond. A novel type of cinchona alkaloid polymers was synthesized from dimeric cinchona squaramides using the Hoveyda-Grubbs’ second-generation catalysts (HG2) by means of ADMET reaction. The chiral polymers, containing cinchona squaramide moieties in their main chains, were subsequently employed as catalysts for the enantioselective Michael reaction to give the corresponding chiral adducts in high yields with excellent enantioselectivity and diastereoselectivity. Both enantiomers from the asymmetric Michael reaction were distinctively prepared while using the polymeric catalysts, possessing pseudoenantiomeric structures. The catalysts were readily recovered from the reaction mixture and recycled several times due to the insolubility of the cinchona-based squaramide polymers.

Asymmetric Organocatalyzed Aza-Henry Reaction of Hydrazones: Experimental and Computational Studies

The first asymmetric catalyzed aza-Henry reaction of hydrazones is presented. In this process, quinine was used as the catalyst to synthesize different alkyl substituted β-nitrohydrazides with ee up to 77 %. This ee was improved up to 94 % by a further recrystallization and the opposite enantiomer can be obtained by using quinidine as the catalyst, opening exciting possibilities in fields in which the control of chirality is vital, such as the pharmaceutical industry. Additionally, experimental and ab initio studies were performed to understand the reaction mechanism. The experimental results revealed an unexpected secondary kinetic isotope effect (KIE) that is explained by the calculated reaction pathway, which shows that the protonation of the initial hydrazone and the C-C bond forming reaction occur during a concerted process. This concerted mechanism makes the catalysis conceptually different to traditional base-promoted Henry and aza-Henry reactions.

The Synthesis of α-Aminophosphonates via Enantioselective Organocatalytic Reaction of 1-(N-Acylamino)alkylphosphonium Salts with Dimethyl Phosphite

α-Aminophosphonic acids are phosphorus analogues of α-amino acids. Compounds of this type find numerous applications in medicine and crop protection due to their unique biological activities. A crucial factor in these activities is the configuration of the stereoisomers. Only a few methods of stereoselective transformation of α-amino acids into their phosphorus analogues are known so far and all of them are based on asymmetric induction, thus involving the use of a chiral substrate. In contrast, we have focused our efforts on the development of an effective method for this type of transformation using a racemic mixture of starting N-protected α-amino acids and a chiral catalyst. Herein, a simple and efficient stereoselective organocatalytic α-amidoalkylation of dimethyl phosphite by 1-(N-acylamino)alkyltriphenylphosphonium salts to enantiomerically enriched α-aminophosphonates is reported. Using 5 mol% of chiral quinine- or hydroquinine-derived quaternary ammonium salts provides final products in very good yields up to 98% and with up to 92% ee. The starting phosphonium salts were easily obtained from α-amino acid derivatives by decarboxylative methoxylation and subsequent substitution with triphenylphosphonium tetrafluoroborate. The appropriate self-disproportionation of enantiomers (SDE) test for selected α-aminophosphonate derivatives via achiral flash chromatography was performed to confirm the reliability of the enantioselectivity results that were obtained.

Comparison of Cinchona Catalysts Containing Ethyl or Vinyl or Ethynyl Group at Their Quinuclidine Ring

Numerous cinchona organocatalysts with different substituents at their quinuclidine unit have been described and tested, but the effect of those saturation has not been examined before. This work presents the synthesis of four widely used cinchona-based organocatalyst classes (hydroxy, amino, squaramide, and thiourea) with different saturation on the quinuclidine unit (ethyl, vinyl, ethynyl) started from quinine, the most easily available cinchona derivative. Big differences were found in basicity of the quinuclidine unit by measuring the pKa values of twelve catalysts in six solvents. The effect of differences was examined by testing the catalysts in Michael addition reaction of pentane-2,4-dione to trans-β-nitrostyrene. The 1.6-1.7 pKa deviation in basicity of the quinuclidine unit did not result in significant differences in yields and enantiomeric excesses. Quantum chemical calculations confirmed that the ethyl, ethynyl, and vinyl substituents affect the acid-base properties of the cinchona-thiourea catalysts only slightly, and the most active neutral thione forms are the most stable tautomers in all cases. Due to the fact that cinchonas with differently saturated quinuclidine substituents have similar catalytic activity in asymmetric Michael addition application of quinine-based catalysts is recommended. Its vinyl group allows further modifications, for instance, recycling the catalyst by immobilization.

A Local-Desymmetrization-Based Divergent Synthesis of Quinine and Quinidine

Herein we report a novel synthetic entry to the legendary quinuclidine natural products quinine and quinidine. The developed strategy is based on the use of a symmetrical and nonstereogenic precursor to access quinine and quinidine through a "local-desymmetrization" approach, in stark contrast conceptually to the preparation of stereodefined disubstituted piperidines (or their acyclic precursors) as featured in all past syntheses. The developed strategy also provided quinine and quinidine derivatives that could not be readily obtained through previous total syntheses or by modification of the naturally occurring substances.

Asymmetric synthesis of β-amino ketones by using cinchona alkaloid-based chiral phase transfer catalysts

A highly enantioselective nucleophilic addition of ketones to imines catalyzed by chiral phase-transfer catalysts (N-quaternised cinchona alkaloid ammonium salts) has been developed, and the process affords the Mannich reaction products with tertiary stereocenters in good to high yields (up to 95%) with excellent enantioselectivities (up to 97% ee).

Cinchona Squaramide-Based Chiral Polymers as Highly Efficient Catalysts in Asymmetric Michael Addition Reaction

We have synthesized novel chiral polymers containing a cinchona-based squaramide in the main chain. We designed a novel cinchona squaramide dimer that contains two cinchona squaramide units connected by diamines. The olefinic double bonds in the cinchona squaramide dimer were used for Mizoroki-Heck (MH) polymerization with aromatic diiodides. The MH polymerization of the cinchona squaramide dimer and aromatic diiodide proceeded well to give the corresponding chiral polymers in good yields. The catalytic activity of the chiral polymers was investigated for asymmetric Michael addition reactions. The effect of the squaramide structure of the polymeric catalyst on the catalytic performance is discussed in detail. We have surveyed the influence of the chiral polymer structure on the catalytic activity and enantioselectivity of the asymmetric reaction. The asymmetric Michael addition of β-ketoesters to nitroolefins was successfully catalyzed by polymeric cinchona squaramide organocatalysts to obtain the corresponding Michael adducts in good yields with excellent enantio- and diastereoselectivities. The polymeric catalysts were insoluble in commonly used organic solvents and easily recovered from the reaction mixture and reused several times without the loss of catalytic activity.

Quantitative determination of major alkaloids in Cinchona bark by Supercritical Fluid Chromatography

Chinoline alkaloids found in Cinchona bark still play an important role in medicine, for example as antimalarial and antiarrhythmic drugs. For the first time Supercritical Fluid Chromatography has been utilized for their separation. Six respective derivatives (dihydroquinidine, dihydroquinine, quinidine, quinine, cinchonine and cinchonidine) could be resolved in less than 7 min, and three of them quantified in crude plant extracts. The optimum stationary phase showed to be an Acquity UPC² Torus DEA 1.7 μm column, the mobile phase comprised of CO₂, acetonitrile, methanol and diethylamine. Method validation confirmed that the procedure is selective, accurate (recovery rates from 97.2% to 103.7%), precise (intra-day ≤2.2%, inter-day ≤3.0%) and linear (R² ≥ 0.999); at 275 nm the observed detection limits were always below 2.5 μg/ml. In all of the samples analyzed cinchonine dominated (1.87%-2.30%), followed by quinine and cinchonidine. Their total content ranged from 4.75% to 5.20%. These values are in good agreement with published data, so that due to unmatched speed and environmental friendly character SFC is definitely an excellent alternative for the analysis of these important natural products.

Direct Access of the Chiral Quinolinyl Core of Cinchona Alkaloids via a Brønsted Acid and Chiral Amine Co-catalyzed Chemo- and Enantioselective α-Alkylation of Quinolinylmethanols with Enals

A strategy for the facile construction of the chiral quinolinylmethanolic structure, a core featured in cinchona alkaloids, is reported. A new reactivity is harnessed by TfOH-promoted chemoselective activation of α-C-H over O-H bond in quinolinylmethanols. The new reactivity is successfully engineered with an iminium catalysis in a synergistic manner to create a powerful conjugate addition-cyclization cascade process for synthesis of chiral quinoline derived γ-butyrolactones in good yields and with good to excellent enantioselectivities. The method enables the first total synthesis of natural product broussonetine in three steps.

Cinchona derivatives as sustainable and recyclable homogeneous organocatalysts for aza-Markovnikov addition

Aza-Markovnikov additions were achieved with up to 98% yields using cinchona based organocatalysts that were recycled by organic solvent nanofiltration.

An asymmetric organocatalytic vinylogous Mannich reaction of 3-methyl-5-arylfuran-2(3H)-ones with N-(2-pyridinesulfonyl) imines: enantioselective synthesis of δ-amino γ,γ-disubstituted butenolides

A series of δ-amino γ,γ-disubstituted butenolides were obtained with satisfactory results via an asymmetric vinylogous Mannich reaction of 3-methyl-5-arylfuran-2(3H)-ones with N-(2-pyridinesulfonyl)imines.

Recent Advances in Organocatalyzed Domino C-C Bond-Forming Reactions

Reactions that form a C-C bond make up a foundational pillar of synthetic organic chemistry. In addition, organocatalysis has emerged as an easy, environmentally-friendly way to promote this type of bond formation. Since around 2000, organocatalysts have been used in a variety of C-C bond-forming reactions including Michael and aldol additions, Mannich-type reactions, and Diels-Alder reactions, to name a few. Many of these methodologies have been refined and further developed to include cascade and domino processes. This review will focus on recent advances in this area with an emphasis on methodologies having applications in the synthesis of biologically-significant compounds.

Mechanistic Study of the Gas-Phase In-Source Hofmann Elimination of Doubly Quaternized Cinchona-Alkaloid Based Phase-Transfer Catalysts by (+)-Electrospray Ionization/Tandem Mass Spectrometry

An unusual in-source fragmentation pattern observed for 14 doubly quaternized cinchona alkaloid-based phase-transfer catalysts (PTC) was studied using (+)-ESI high resolution mass spectrometry. Loss of the substituted benzyl cation (R1 or R2) was found to be the major product ion [M²⁺ - R₁⁺ or R₂⁺]⁺ in MS spectra of all PTC compounds. A Hofmann elimination product ion [M - H]⁺ was also observed. Only a small amount of the doubly charged M²⁺ ions were observed in the MS spectra, likely due to strong Columbic repulsion between the two quaternary ammonium cations in the gas phase. The positive voltage in the MS inlet but not the ESI probe was found to induce this extensive fragmentation for all PTC diboromo-salts. Compound 1 was used as an example to illustrate the proposed in-source fragmentation mechanism. The mechanism of formation of the Hofmann elimination product ion [M - H]⁺ was further investigated using HRMS/MS, H/D exchange, and DFT calculations. The proposed formation of 2b as the major Hofmann elimination product ion was supported both by HRMS/MS and DFT calculations. Formation of product ion 2b through a concerted unimolecular E_i elimination pathway is proposed rather than a bimolecular E2 elimination pathway for common solution Hofmann eliminations. Graphical Abstract ᅟ.

Enantioselective Organocatalyzed Transformations of β-Ketoesters

The β-ketoester structural motif continues to intrigue chemists with its electrophilic and nucleophilic sites. Proven to be a valuable tool within organic synthesis, natural product, and medicinal chemistry, reports on chiral β-ketoester molecular skeletons display a steady increase. With the reignition of organocatalysis in the past decade, asymmetric methods available for the synthesis of this structural unit has significantly expanded, making it one of the most exploited substrates for organocatalytic transformations. This review provides comprehensive information on the plethora of organocatalysts used in stereoselective organocatalyzed construction of β-ketoester-containing compounds.

A Series of Cinchona-Derived N-Oxide Phase-Transfer Catalysts: Application to the Photo-Organocatalytic Enantioselective α-Hydroxylation of β-Dicarbonyl Compounds

A series of cinchona-derived N-oxide asymmetric phase-transfer catalysts were synthesized and applied in the enantioselective photo-organocatalytic α-hydroxylation of β-keto esters and β-keto amides (23 examples) using molecular oxygen in excellent yields (up to 98%) and high enantioselectivities (up to 83% ee). These new catalysts could be recycled and reused six times for such a reaction with almost the original reactivity and enantioselectivity.