Highly enantioselective synthesis of α,α-dialkylmalonates by phase-transfer catalytic desymmetrization

Synthesis of chiral malonates by α-alkylation of 2,2-diphenylethyl tert-butyl malonates via enantioselective phase-transfer catalysis

An efficient synthetic approach for chiral malonates was established via enantioselective phase transfer catalysis. The α-alkylation of 2,2-diphenylethyl tert-butyl α-methylmalonates with (S,S)-3,4,5-trifluorophenyl-NAS bromide as a phase-transfer catalyst under phase-transfer catalytic conditions successfully produced corresponding α-methyl-α-alkylmalonates; these compounds are versatile chiral building blocks containing a quaternary carbon center in high chemical yields (up to 99%) with excellent enantioselectivities (up to 98% ee). α,α-Dialkylmalonates were selectively hydrolyzed to the corresponding chiral malonic monoacids under basic (KOH/MeOH) and acidic conditions (TFA/CH₂Cl₂), showing the practicality of the method.

Enantioselective Synthesis of Quaternary Oxindoles: Desymmetrizing Staudinger-Aza-Wittig Reaction Enabled by a Bespoke HypPhos Oxide Catalyst

This paper describes a catalytic asymmetric Staudinger-aza-Wittig reaction of (o-azidoaryl)malonates, allowing access to chiral quaternary oxindoles through phosphine oxide catalysis. We designed a novel HypPhos oxide catalyst to enable the desymmetrizing Staudinger-aza-Wittig reaction through the PIII/PV═O redox cycle in the presence of a silane reductant and an IrI-based Lewis acid. The reaction occurs under mild conditions, with good functional group tolerance, a wide substrate scope, and excellent enantioselectivity. Density functional theory revealed that the enantioselectivity in the desymmetrizing reaction arose from the cooperative effects of the IrI species and the HypPhos catalyst. The utility of this methodology is demonstrated by the (formal) syntheses of seven alkaloid targets: (-)-gliocladin C, (-)-coerulescine, (-)-horsfiline, (+)-deoxyeseroline, (+)-esermethole, (+)-physostigmine, and (+)-physovenine.

Asymmetric Total Synthesis of Shizukaol J, Trichloranoid C and Trishizukaol A

The asymmetric total synthesis of three lindenane sesquiterpenoid oligomers, shizukaol J, trichloranoid C and trishizukaol A, has been accomplished concisely in 15, 16 and 18 longest linear steps, respectively. The expeditious construction of molecular architectures was facilitated by Nelson's catalytic asymmetric ketene-aldehyde cycloaddition, a sequence of allylic alkylation/reduction/acidic cyclization to forge a lactone, and a double aldol condensation cascade to construct the 5/6 bicyclic system. Diastereoselective nucleophilic substitution promoted by a phase transfer catalyst constructed the C₁₁ quaternary stereogenic center, thus prompting synthetic efficacy toward shizukaol J. The synthesis of trichloranoid C and trishizukaol A was achieved after a cascade involving furanyl diene formation and a Diels-Alder reaction, as well as a one-pot sequence involving furan oxidation and global deprotection. Furthermore, our biological evaluation revealed that two compounds exhibited unexpected toxicity against tumor cell lines.

Synthesis of Novel Crown Ether-Squaramides and Their Application as Phase-Transfer Catalysts

This work presents the synthesis of six new phase-transfer organocatalysts in which the squaramide unit is directly linked to the nitrogen atom of an aza-crown ether. Four chiral skeletons, namely hydroquinine, quinine, cinchonine (cinchonas), and α-d-glucopyranoside were responsible for the asymmetric construction of an all-carbon quaternary stereogenic center in α-alkylation and Michael addition reactions of malonic esters. We investigated the effects of these different chiral units and that of crown ethers with different sizes on catalytic activity and enantioselectivity. During extensive parameter investigations, both conventional and emerging green solvents were screened, providing valuable α,α-disubstituted malonic ester derivatives with excellent yields (up to 98%).

Catalytic Enantioselective Alkylation of Prochiral Enolates

The asymmetric alkylation of enolates is a particularly versatile method for the construction of α-stereogenic carbonyl motifs, which are ubiquitous in synthetic chemistry. Over the past several decades, the focus has shifted to the development of new catalytic methods that depart from classical stoichiometric stereoinduction strategies (e.g., chiral auxiliaries, chiral alkali metal amide bases, chiral electrophiles, etc.). In this way, the enantioselective alkylation of prochiral enolates greatly improves the step- and redox-economy of this process, in addition to enhancing the scope and selectivity of these reactions. In this review, we summarize the origin and advancement of catalytic enantioselective enolate alkylation methods, with a directed emphasis on the union of prochiral nucleophiles with carbon-centered electrophiles for the construction of α-stereogenic carbonyl derivatives. Hence, the transformative developments for each distinct class of nucleophile (e.g., ketone enolates, ester enolates, amide enolates, etc.) are presented in a modular format to highlight the state-of-the-art methods and current limitations in each area.

Enantioselective Synthesis of Acyclic Orthogonally Functionalized Compounds Bearing a Quaternary Stereocenter Using Chiral Ammonium Salt Catalysis

We herein report an asymmetric protocol to access a series of orthogonally functionalized acyclic chiral target molecules containing a quaternary stereogenic center by carrying out the enantioselective α-alkylation of novel orthogonally functionalized dioxolane-containing cyanoacetates under chiral ammonium salt catalysis. By using just 1 mol % of Maruoka's spirocyclic ammonium salt catalysts enantioselectivities up to e.r.=97.5 : 2.5 could be achieved and further functional group manipulations of the products were carried out as well.

Enantioselective Synthesis of (+)-Coerulescine by a Phase-Transfer Catalytic Allylation of Diphenylmethyl tert-Butyl α-(2-Nitrophenyl)Malonate

A 7-step enantioselective synthetic method for preparing (S)(+)-coerulescine is reported through the use of diphenylmethyl tert-butyl α-(2-nitrophenyl)malonate (16% overall yield, >99% ee). Allylation is the key step under phase-transfer catalytic conditions (86% ee). This synthetic method can be used as a practical route for the synthesis of various derivatives of (S)(+)-coerulescine for analyzing its structure–activity relationships against its biological activities.

Construction of Quaternary Carbon Center by Catalytic Asymmetric Alkylation of 3-Arylpiperidin-2-ones Under Phase-Transfer Conditions

A highly enantioselective synthesis of δ-lactams having a chiral quaternary carbon center at the α-position has been developed through an asymmetric alkylation of 3-arylpiperidin-2-ones under phase-transfer conditions. In this transformation, a 2,2-diarylvinyl group on the δ-lactam nitrogen atom plays a crucial role as a novel protecting group and an achiral auxiliary for improving both yield and enantioselectivity of the reaction.

Chemoselective Mono- and Difluorination of 1,3-Dicarbonyl Compounds

By altering the amount of Selectfluor, the highly selective mono- and difluorination of 1,3-dicarbonyl compounds has been achieved, affording a variety of 2-fluoro- and 2,2-difluoro-1,3-dicarbonyl compounds in good to excellent yields. The reaction can be readily performed in aqueous media without any catalyst and base, which features practical and convenient fluorination. Importantly, a gram-scale reaction, transformation of 2-fluoro-1,3-diphenylpropane-1,3-dione to 4-fluoro-1,3,5-triphenyl-1H-pyrazole, and chlorination and bromination of 1,3-dicarbonyl compounds are realized to further exhibit its synthetic utility.

Brønsted acid and Pd-PHOX dual-catalysed enantioselective addition of activated C-pronucleophiles to internal dienes

We describe the development of Pd-PHOX-catalysed enantioselective couplings of internal dienes with malononitrile and other activated C-pronucleophiles. Reactions are dramatically accelerated by the addition of Et3N·HBArF 4 as a Brønsted acid co-catalyst, enabling a suite of products bearing a variety of alkyl substituents at the stereogenic carbon to be prepared. A series of mechanism-oriented experiments reveal key aspects of the catalytic cycle and the importance of the non-coordinating BArF 4 counterion in not only promoting reactions of internal dienes but also additions of previously unreactive nucleophiles towards terminal dienes.

Enantioselective Intermolecular Pd-Catalyzed Hydroalkylation of Acyclic 1,3-Dienes with Activated Pronucleophiles

We report a highly enantioselective Pd-PHOX-catalyzed intermolecular hydroalkylation of acyclic 1,3-dienes. Meldrum's acid derivatives and other activated C-pronucleophiles, such as β-diketones and malononitriles, react with a variety of aryl- and alkyl-substituted dienes in ≤20 h at room temperature. The coupled products, obtained in up to 96% yield and 97.5:2.5 er, are easily transformed into useful chemical building blocks for downstream synthesis.

Enantioselective Synthesis of Chiral α-Thio-Quaternary Stereogenic Centers via Phase-Transfer-Catalyzed α-Alkylation of α-Acylthiomalonates

An efficient synthetic method for establishing chiral α-thio-α-quaternary stereogenic center was successfully developed. The enantioselective α-alkylation of α-acylthiomalonates under phase-transfer catalytic conditions [50% aq. KOH, toluene, -20 °C, and (S,S)-3,4,5-trifluorophenyl-NAS bromide] provided the corresponding α-acylthio-α-alkylmalonates in high chemical yields (up to 99%) and high optical yields (up to 98% ee).

Highly enantioselective synthesis of α-tertiary chiral amino acid derivatives through rhodium-catalyzed asymmetric arylation of cyclic N-sulfonyl α-ketimino esters

A simple sulfur-olefin ligand promoted Rh-catalyzed highly enantioselective arylation of cyclic α-ketimino esters with arylboronic acids is described.

Quaternized α,α'-Amino Acids via Curtius Rearrangement of Substituted Malonate-Imidazolidinones

An efficient synthesis protocol is presented for accessing quaternized α-amino acids in chiral, nonracemic form via diastereoselective malonate alkylation followed by C- to N-transposition. The key stereodifferentiating step involves a diastereoselective alkylation of an α-monosubstituted malonate-imidazolidinone, which is followed first by a chemoselective malonate PMB ester removal and then a Curtius rearrangement to provide the transposition. The method demonstrates a high product ee (89-99% for eight cases) for quaternizing a range of proteinogenic α-amino acids. The stereogenicity in targets 5a-i supports previous conclusions that the diastereoselective alkylation step proceeds via an α-substituted malonate-imidazolidinone enolate in its Z-configuration, with the auxiliary in an s-trans_C-N conformation.

Phase-transfer catalyzed enantioselective α-alkylation of α-acyloxymalonates: construction of chiral α-hydroxy quaternary stereogenic centers

The enantioselective synthesis of α-acyloxy-α-alkylmalonates was developed as an efficient method for producing chiral α-tertiary alcohols, which are potentially valuable intermediates in the synthesis of natural products and pharmaceuticals.

Organocatalytic 1,4-Addition Reaction of 2-Formyl(thio)esters to Vinylketones: An Efficient Access to Acyclic Chiral Building Blocks with a Quaternary Carbon Stereocenter

2-Formyl(thio)esters were utilized as pronucleophiles to obtain less-accessible acyclic chiral building blocks bearing versatile functional groups on a quaternary carbon atom for enantioselective 1,4-addition to vinylketones. To achieve high enantioselectivity in the present 1,4-addition reaction, thiourea-tertiary amines containing a bulky chiral backbone were developed as catalysts, and several derivatizations of the products were performed to demonstrate the synthetic utility of the products.

Efficient enantioselective total synthesis of (-)-horsfiline

A new efficient and concise enantioselective synthetic method for (-)-horsfiline is reported. (-)-Horsfiline could be obtained from diphenylmethyl tert-butyl malonate in 9 steps (32%,>99% ee) by using the enantioselective phase-transfer catalytic allylation (91% ee) as the key step. This approach can be applied as a practical route for the large-scale synthesis of spirooxindole natural products, which enables a systematic investigation of their biological activity to be performed.

Construction of a chiral quaternary carbon center by catalytic asymmetric alkylation of 2-arylcyclohexanones under phase-transfer conditions

In this paper, we present an asymmetric alkylation of modified 2-arylcyclohexanones that employs a novel chiral ammonium bromide as a phase-transfer catalyst and an achiral auxiliary as a controller to improve the enantioselectivity to afford optically enriched products having a chiral quaternary carbon center.

Catalytic asymmetric diaziridination

The first catalytic enantio- and diastereoselective synthesis of diaziridines is presented. Aziridination of N-tosyl aldimines applying modified hydroxylamine under asymmetric phase-transfer catalysis furnished optically active diaziridines. The diaziridines are formed as a single diastereomer in up to 96% ee, containing two orthogonal N-protecting groups, which can be deprotected selectively.

Recent developments in asymmetric phase-transfer reactions

Phase-transfer catalysis has been recognized as a powerful method for establishing practical protocols for organic synthesis, because it offers several advantages, such as operational simplicity, mild reaction conditions, suitability for large-scale synthesis, and the environmentally benign nature of the reaction system. Since the pioneering studies on highly enantioselective alkylations promoted by chiral phase-transfer catalysts, this research field has served as an attractive area for the pursuit of "green" sustainable chemistry. A wide variety of asymmetric transformations catalyzed by chiral onium salts and crown ethers have been developed for the synthesis of valuable organic compounds in the past several decades, especially in recent years.

2-Aryl-2-nitroacetates as central precursors to aryl nitromethanes, α-ketoesters, and α-amino acids

Nitroarylacetates are useful small molecular building blocks that act as precursors to α-ketoesters and aryl nitromethanes as well as α-amino acids. Methods were developed that produce each of these compound types in good yields. Two different conditions for decarboxylation are discussed for substrates with neutral and electron-poor aryl groups versus electron-rich aryl groups. For formation of the α-ketoesters, new mild conditions for the Nef disproportionation were identified.

Catalytic asymmetric alkylation reactions for the construction of protected ethylene-amino and propylene-amino motifs attached to quaternary stereocentres

An efficient catalytic and stereoselective method for the direct construction of protected ethylene-amino and propylene-amino scaffolds attached to quaternary stereocentres is reported. Preliminary investigations revealed a mild base catalysed nucleophilic ring opening of N-sulfonyl aziridines using the commercially available phosphazene base 2-tert-butylimino-2-diethylamino-1,3-dimethyl-perhydro-1,3,2-diazaphosphorine (BEMP) was possible and resulted in highly efficient alkylation reactions with a range of methine carbon acids. This reaction could be rendered highly asymmetric (up to 97% ee) by employing phase-transfer catalysis to control stereoinduction. Incorporation of alkyl substituents onto the aziridine electrophile, resulted in a highly diastereoselective (up to 30:1 d.r.) variant of this methodology. A further extension using N-protected cyclic sulfamidates as the electrophilic component was successful with a range of pro-nucleophiles (up to 96% ee and 45:1 d.r.) and allowed a range of nitrogen protecting groups (carbamate, sulfonyl, phosphonyl, benzyl) to be incorporated into the alkylation adducts. Finally, the utility of the products have been demonstrated in the synthesis of useful heterocycles and compounds bearing structural components of natural products.