Enantioselective Organocatalytic Synthesis of Quaternary α-Amino Acids Bearing a CF3 Moiety

Photoredox Microfluidic Synthesis of Trifluoromethylated Amino Acids

Fluorinated amino acids are a class of highly valuable building blocks that are widely employed in biological science and pharmaceutical industry for improved stability, activity, and folding property of proteins. However, the synthetic approach has conventionally been constrained by harsh conditions and limited substrate range. We demonstrate a general synthetic protocol for photoinduced α-CF₃ amino acids using continuous flow technology that benefits from enhanced fusion and precise control of reaction time, making it potentially useful in large-scale peptide synthesis.

Recent advances in the synthesis of fluorinated amino acids and peptides

The site-selective modification of amino acids, peptides, and proteins has always been an intensive topic in organic synthesis, medicinal chemistry, and chemical biology due to the vital role of amino acids in life. Among the developed methods, the site-selective introduction of fluorine functionalities into amino acids and peptides has emerged as a useful approach to change their physicochemical and biological properties. With the increasing demand for life science, the direct fluorination/fluoroalkylation of proteins has also received increasing attention because of the unique properties of fluorine atom(s) that can change the protein structure, increase their lipophilicity, and enable fluorine functionality as a biological tracer or probe for chemical biology studies. In this feature article, we summarized the recent advances in the synthesis of fluorinated amino acids and peptides, wherein two strategies have been discussed. One is based on the fluorinated building blocks to prepare fluorinated amino acids and peptides with diversified structures, including the transformations of fluorinated imines and nickel-catalyzed dicarbofunctionalization of alkenes with bromodifluoroacetate and its derivatives; the other is direct fluorination/fluoroakylation of amino acids, peptides, and proteins, in which the selective transformations of the functional groups on serine, threonine, tyrosine, tryptophan, and cysteine lead to a wide range of fluorinated α-amino acids, peptides, and proteins, featuring synthetic convenience and late-stage modification of biomacromolecules. These two strategies complement each other, wherein transition-metal catalysis and new fluoroalkylating reagents provide powerful tools to selectively access fluorinated amino acids, peptides, and proteins, showing the prospect of medicinal chemistry and chemical biology.

Recent Advances in the Catalytic Asymmetric Friedel-Crafts Reactions of Indoles

Functionalized chiral indole derivatives are privileged and versatile organic frameworks encountered in numerous pharmaceutically active agents and biologically active natural products. The catalytic asymmetric Friedel-Crafts reaction of indoles, catalyzed by chiral metal complexes or chiral organocatalysts, is one of the most powerful and atom-economical approaches to access optically active indole derivatives. Consequently, a wide range of electrophilic partners including α,β-unsaturated ketones, esters, amides, imines, β,γ-unsaturated α-keto- and α-ketiminoesters, ketimines, nitroalkenes, and many others have been successfully employed to achieve a plethora of functionalized chiral indole moieties. In particular, strategies for C-H functionalization in the phenyl of indoles require incorporation of a directing or blocking group in the phenyl or azole ring of indole. The discovery of chiral catalysts which can control enantiodiscrimination has gained a great deal of attention in recent years. This review will provide an updated account on the application of the asymmetric Friedel-Crafts reaction of indoles in the synthesis of diverse chiral indole derivatives, covering the timeframe from 2011 to today.

Enantioselective synthesis of indole-based unnatural β-Alkynyl α-amino acid derivatives via chiral phosphoric acid catalysis

The synthesis of unnatural α-amino acid derivatives has attracted considerable interest in recent years, as they are ubiquitous in protein synthesis and peptide-based drug discovery. Herein, we reported the chiral phosphoric acid catalyzed asymmetric reaction of indoles with β,γ-alkynyl-α-imino esters for the enantioselective synthesis of unnatural indole-based α-amino acid derivatives. This asymmetric organocatalysis protocol enables efficient synthesis of unnatural α-amino acid derivatives with a tetrasubstituted stereogenic center and an alkyne moiety with up to 99% yield and 98% ee, resulting in operationally simple conditions, short reaction time, and broad substrate scope.

An overview of the applications of chiral phosphoric acid organocatalysts in enantioselective additions to CO and CN bonds

Since 2004, chiral phosphoric acids (CPAs) have emerged as highyl efficient organocatalysts, providing excellent results in a wide reaction scope. In this review, the applications of CPA for enantioselective additions to CO and CN bonds are covered.

Organocatalytic asymmetric synthesis of quaternary α-isoxazole–α-alkynyl amino acid derivatives

Chiral phosphoric acid catalyzed enantioselective addition of 5-amino-isoxazoles with β,γ-alkynyl-α-ketimino esters provided good yields and excellent enantioselectivities.

Enantioselective Aza-Friedel-Crafts Reaction of Indoles and Pyrroles Catalyzed by Chiral C1-Symmetric Bis(phosphoric Acid)

A hydrogen bonding network in chiral Brønsted acid catalysts is important for the construction of a chiral cavity and the enhancement of catalytic activity. In this regard, we developed a highly enantioselective aza-Friedel-Crafts reaction of indoles and pyrroles with acyclic α-ketimino esters in the presence of a chiral C₁-symmetric BINOL-derived bis(phosphoric acid) catalyst. The desired alkylation products with chiral quaternary carbon centers were obtained in high yields with high enantioselectivities on up to a 1.2-g scale with 0.2 mol % catalyst loading. Interestingly, the absolute configurations of the products from indoles and pyrroles were opposite even with the use of the same chiral catalyst. Moreover, preliminary mechanistic considerations disclosed that a unique hydrogen bonding network with or without π-π interactions among the catalyst and substrates might partially play a pivotal role.

Strategies for the Catalytic Enantioselective Synthesis of α-Trifluoromethyl Amines

The exploitation of the α-trifluoromethylamino group as an amide surrogate in peptidomimetics and drug candidates has been on the rise. In a large number of these cases, this moiety bears stereochemistry with the stereochemical identity having important consequences on numerous molecular properties, such as the potency of the compound. Yet, the majority of stereoselective syntheses of α-CF₃ amines rely on diastereoselective couplings with chiral reagents. Concurrent with the rapid expansion of fluorine into pharmaceuticals has been the development of catalytic enantioselective means of preparing α-trifluoromethyl amines. In this work, we outline the strategies that have been employed for accessing these enantioenriched amines, including normal polarity approaches and several recent developments in imine umpolung transformations.

Catalytic asymmetric synthesis of quaternary trifluoromethyl α- to ε-amino acid derivatives via umpolung allylation/2-aza-Cope rearrangement

In this study, we developed an efficient Ir-catalyzed cascade umpolung allylation/2-aza-Cope rearrangement of tertiary α-trifluoromethyl α-amino acid derivatives for the preparation of a variety of quaternary α-trifluoromethyl α-amino acids in high yields with excellent enantioselectivities. The umpolung reactivity empowered by the activation of the key isatin-ketoimine moiety obviates the intractable enantioselectivity control in Pd-catalyzed asymmetric linear α-allylation. In combination with quasi parallel kinetic resolution or kinetic resolution, the generality of this method is further demonstrated by the first preparation of enantioenriched quaternary trifluoromethyl β-, γ-, δ- and ε-amino acid derivatives.

A Broad Substrate Scope of Aza-Friedel-Crafts Alkylation for the Synthesis of Quaternary α-Amino Esters

A versatile synthetic protocol of aza-Friedel-Crafts alkylation has been developed for the synthesis of quaternary α-amino esters. This operationally simple alkylation proceeds under ambient conditions with high efficiency, regioselectivity, and an exceptionally broad scope of arene nucleophiles. A key feature of this alkylation is the role associated with the silver(I) salt counteranions liberated during the reaction. Taking advantage of a phase-transfer counteranion/Brønsted acid pair mechanism, we also report a catalytic enantioselective example of the reaction.

Regio- and Enantioselective Synthesis of Trifluoromethyl-Substituted Homoallylic α-Tertiary NH2 -Amines by Reactions Facilitated by a Threonine-Based Boron-Containing Catalyst

A method for catalytic regio- and enantioselective synthesis of trifluoromethyl-substituted and aryl-, heteroaryl-, alkenyl-, and alkynyl-substituted homoallylic α-tertiary NH₂ -amines is introduced. Easy-to-synthesize and robust N-silyl ketimines are converted to NH-ketimines in situ, which then react with a Z-allyl boronate. Transformations are promoted by a readily accessible l-threonine-derived aminophenol-based boryl catalyst, affording the desired products in up to 91 % yield, >98:2 α:γ selectivity, >98:2 Z:E selectivity, and >99:1 enantiomeric ratio. A commercially available aminophenol may be used, and allyl boronates, which may contain an alkyl-, a chloro-, or a bromo-substituted Z-alkene, can either be purchased or prepared by catalytic stereoretentive cross-metathesis. What is more, Z-trisubstituted allyl boronates may be used. Various chemo-, regio-, and diastereoselective transformations of the α-tertiary homoallylic NH₂ -amine products highlight the utility of the approach; this includes diastereo- and regioselective epoxide formation/trichloroacetic acid cleavage to generate differentiated diol derivatives.

Asymmetric synthesis of quaternary α-trifluoromethyl α-amino acids by Ir-catalyzed allylation followed by kinetic resolution

Facile access to quaternary α-trifluoromethyl α-amino acids has been developed. This sequential reaction involves an Ir-catalyzed asymmetric allylation of α-trifluoromethyl aldimine esters followed by an unprecedented kinetic resolution.

Pd-Catalyzed Allylation of Imines to Access α-CF3-Substituted α-Amino Acid Derivatives

We herein report a high yielding protocol for the direct α-allylation of easily accessible trifluoropyruvate-derived imines using Pd-catalysis. The reaction gives access to a variety of different α-allylated-α-CF3-amino acids in a straightforward manner, starting from commercially available trifluoropyruvate. We also provide a proof-of-concept for an enantioselective protocol (up to er = 75:25) by using chiral phosphane ligands.

Organocatalytic asymmetric synthesis of benzazepinoindole derivatives with trifluoromethylated quaternary stereocenters by chiral phosphoric acid catalysts

An enantioselective aza-Friedel-Crafts reaction of trifluoromethyl dihydrobenzoazepinoindoles with pyrroles catalyzed by a chiral spirocyclic phosphoric acid was developed. This methodology provides a facile route to CF₃- and pyrrole-containing benzazepinoindoles bearing quaternary stereocenters in good yields and with moderate to excellent enantioselectivities (up to 93% ee). Indoles were also investigated as electron-rich aromatic substrates to afford the corresponding chiral heterocycles with good yields and considerable enantioselectivities. The introduction of CF₃ shows a remarkable fluorine effect and increases the activation and stereoinduction.

Organocatalytic enantioselective functionalization of indoles in the carbocyclic ring with cyclic imines

An organocatalytic enantioselective functionalization in the carbocyclic ring of indoles with benzoxathiazine 2,2-dioxides is described using a quinine-derived bifunctional organocatalyst.

3-Mono-Substituted BINOL Phosphoric Acids as Effective Organocatalysts in Direct Enantioselective Friedel-Crafts-Type Alkylation of N-Unprotected α-Ketiminoester

Although BINOL-derived phosphoric acids are among the most widely used chiral Brønsted acid organocatalysts, their structures are mostly limited to 3,3'-disubstituted ones and simple 3-mono-substituted ones without any polar functionalities on the 3-substituent have not been used in highly enantioselective reactions. This work reports such 3-mono-substituted analogues as effective organocatalysts in direct highly enantioselective Friedel-Crafts-type alkylation of N-unprotected α-ketiminoester. The origin of the observed high enantioselectivity with the 3-mono-substituted catalyst is also discussed.

A Novel Sc(OTf)3 -Catalyzed (2+2+1)-Cycloannulation/Aza-Friedel-Crafts Alkylation Sequence toward Multicyclic 2-Pyrrolines

The rapid assembly of molecular complexity continues to be at the forefront of novel reaction development. In the pursuit of that goal, we herein report a novel Sc(OTf)₃ -catalyzed, one-pot multicomponent reaction that furnishes complex multicyclic 2-pyrrolines with excellent overall yields and perfect diastereocontrol. This process is based on our previously established (2+2+1)-cycloannulation of in situ generated 1-azaallyl cations, 1,3-dicarbonyls and primary amines. The newly formed and highly reactive aminal moiety is readily substituted with indoles and pyrroles both as external and internal π-nucleophiles to provide densely functionalized N-heterocycles with four new σ-bonds and two vicinal quaternary stereogenic centers. In addition, DFT calculations have been conducted to further characterize the intermediate 1-azaallyl cations.

Phosphine-Catalyzed Asymmetric Umpolung Addition of Trifluoromethyl Ketimines to Morita-Baylis-Hillman Carbonates

A novel phosphine-catalyzed, highly enantioselective umpolung addition of trifluoromethyl ketimines to Morita-Baylis-Hillman carbonates was developed and it provides facile access to optically active trifluoromethyl amines with a chiral tertiary stereocenter under mild reaction conditions. The salient features of this reaction include general substrate scope, mild reaction conditions, good yields, high enantioselectivity, ease of scale-up to gram scale, and further transformations of the products.

Oxidative Asymmetric Aza-Friedel-Crafts Alkylation of Indoles with 3-Indolinone-2-carboxylates Catalyzed by a BINOL Phosphoric Acid and Promoted by DDQ

An asymmetric aza-Friedel-Crafts alkylation reaction between indoles and indolenines that were derived in situ from 3-indolinone-2-carboxylates has been developed by using 3,3'-bis(triphenylsilyl)-1,1'-binaphthyl-2,2'-diyl hydrogen phosphate as a catalyst. The reaction proceeded under mild conditions and provided chiral indol-3-yl-3-indolinone-2-carboxylate derivatives in good yields with excellent ee values (up to 98.6 %). Similarly, the Mannich-type addition of indoline-3-ones to indolenines provided heterodimers with vicinal chiral quaternary centers. This method was successfully applied to the construction of the core structure of trigonoliimine C.

Enantioselective synthesis of cyclic quaternary α-amino acid derivatives by chiral phosphoric acid catalysis

A highly enantioselective aza-Friedel-Crafts reaction of N-sulfonyl cyclic ketimines with indoles catalyzed by chiral phosphoric acids has been developed. This methodology provides an efficient and facile route to indole-containing chiral cyclic α-amino acid derivatives bearing a quaternary stereocenter in high yields and up to 98% enantioselectivity.

Enantioselective aza-Friedel–Crafts reaction of cyclic ketimines with indoles using chiral imidazoline–phosphoric acid catalysts

A highly enantioselective aza-Friedel Crafts reaction of cyclic 4-aryl-3-oxo-1,2,5-thiadiazol-1,1-oxides as cyclic ketimines with indoles was developed.

Selecting Chiral BINOL-Derived Phosphoric Acid Catalysts: General Model To Identify Steric Features Essential for Enantioselectivity

Choosing the optimal catalyst for a new transformation is challenging because the ideal molecular requirements of the catalyst for one reaction do not always simply translate to another. Large groups at the 3,3' positions of the binaphthol rings are important for efficient stereoinduction but if they are too large this can lead to unusual or poor results. By applying a quantitative steric assessment of the substituents at the 3,3' positions of the binaphthol ring, we have systematically studied the effect of modulating this group on enantioselectivity for a wide range of reactions involving imines, and verified this analysis using ONIOM calculations. We have shown that in most reactions, the stereochemical outcome depends on both proximal and remote sterics. Summarising detailed calculations into a simple qualitative model identifies and explains the steric features required for high selectivity. This model is consistent with seventy seven papers reporting reactions (over 1000 transformations in total), and provides a straightforward decision tree for selecting the best catalyst.

Catalytic Asymmetric Conjugate Addition of Indolizines to α,β-Unsaturated Ketones

A catalytic enantioselective conjugate addition of indolizines to enones is described. The chiral phosphoric acid (S)-TRIP activates α,β-unsaturated ketones, thereby promoting an enantioface-differentiating attack by indolizines. Using this reaction, several alkylated indolizines were synthesized in good yields and with enantiomeric ratios of up to 98:2.

Asymmetric synthesis of CF3- and indole-containing tetrahydro-β-carbolines via chiral spirocyclic phosphoric acid-catalyzed aza-Friedel–Crafts reaction

An enantioselective aza-Friedel–Crafts reaction of indoles with 1-trifluoromethyl-3,4-dihydro-β-carbolines has been developed to afford tetrahydro-β-carbolines with a CF₃- and indole-containing quaternary stereocenter by using chiral phosphoric acid catalysis.