Enantioselective Synthesis of β-Trifluoromethyl α-Amino Acids

Enantio- and Diastereoselective Total Synthesis of Belzutifan Enabled by Rh-Catalyzed Hydrogenation

Herein, we report a nine-step synthesis of belzutifan enabled by a novel Rh-catalyzed asymmetric hydrogenation to install the contiguous fluorinated stereocenters with high enantioselectivity. Moreover, the final ketone reduction in the synthesis proceeds with high diastereoselectivity, leading to the expedient assembly of the stereotriad. In contrast to the original 16-step synthesis, this route avoids a lengthy bromination-oxidation sequence and introduces the sulfone functionality via nucleophilic aromatic substitution, obviating the need for transition metal catalysis.

Electron-Deficient Fluoroarene-Mediated Synthesis of Trifluoromethyl Ketones from Carboxylic Acids

Fluoroarene-mediated trifluoromethylation of carboxylic acids for the synthesis of trifluoromethyl ketones is disclosed. The fluoroarene activates the acid group and generates the fluoride source in situ for the trifluoromethylation reaction. The present protocol is safe and metal-free, operates under mild reaction conditions, and does not require any external additives to generate trifluoromethyl anion. The current transformation provides good functional group tolerance and also delivers 92% and 88% yields of trifluoromethyl ketones in batch and continuous flow, respectively.

Gram-Scale Asymmetric Synthesis of Fluorinated Amino Acids Using a Chiral Nickel(II) Complex

Fluorinated amino acids play an important role in the field of peptide and protein engineering. Although numerous syntheses have been published in recent decades, strategies that allow routine access to fluorinated amino acids on a gram-scale have been poorly described. Furthermore, the described pathways that gain fluorinated amino acids are based on different synthetic strategies, making a uniform approach that uses similar starting materials highly beneficial. Chiral Ni(II) complexes were introduced as powerful tools in the synthesis of noncanonical amino acids. In this work, we present a strategy for the synthesis of a diverse range of fluorinated amino acids based on the corresponding Ni(II) complex from which the products can be obtained in enantiopure form (99% ee) on a gram-scale. In addition, we describe an optimized procedure for the synthesis of alkyl iodide building blocks that are required for the alkylation reactions with the corresponding Ni(II) complex. Finally, we characterized the synthesized fluorinated amino acids with regard to their hydrophobicity and α-helix propensity.

Highly Enantioselective Rhodium-Catalyzed Transfer Hydrogenation of Tetrasubstituted Olefins: Application toward the Synthesis of GPR40 Agonist MK-2305

A highly efficient enantioselective synthesis for the potent G-protein-coupled receptor 40 agonist MK-2305 was developed. The key tetrasubstituted olefin was prepared via a stereoselective Mukaiyama aldol reaction/elimination sequence. The highly enantioselective rhodium-catalyzed transfer hydrogenation of the tetrasubstituted olefin afforded the target compound MK-2305 in excellent optical and chemical purity. The key asymmetric transfer hydrogenation proceeds in excellent yields and enantioselectivities for a variety of substrates. The superior reactivity of the tethered catalysts was revealed by NMR studies.

Recent Advances in the Enantioselective Synthesis of Chiral Amines via Transition Metal-Catalyzed Asymmetric Hydrogenation

Chiral amines are key structural motifs present in a wide variety of natural products, drugs, and other biologically active compounds. During the past decade, significant advances have been made with respect to the enantioselective synthesis of chiral amines, many of them based on catalytic asymmetric hydrogenation (AH). The present review covers the use of AH in the synthesis of chiral amines bearing a stereogenic center either in the α, β, or γ position with respect to the nitrogen atom, reported from 2010 to 2020. Therefore, we provide an overview of the recent advances in the AH of imines, enamides, enamines, allyl amines, and N-heteroaromatic compounds.

Catalytic Asymmetric Hydrogenation of Tetrasubstituted Unsaturated Lactams: An Efficient Approach to Enantioenriched 3,4-Disubstituted Piperidines

Asymmetric hydrogenation of tetrasubstituted alkenes remains a formidable challenge in asymmetric catalysis. We report herein an unprecedented Rh-catalyzed enantioselective and diastereoselective hydrogenation of easily accessed α,β-disubstituted unsaturated lactams to afford synthetically valuable chiral lactams with 1,2-consecutive stereocenters. The reaction could be performed on the gram scale, and the products could be concisely transformed to enantiomerically pure trans-3,4-disubstituted piperidines, which are prevalent structural units in medicinal agents.

Catalytic enantioselective synthesis of fluoromethylated stereocenters by asymmetric hydrogenation

Fluoromethyl groups possess specific steric and electronic properties and serve as a bioisostere of alcohol, thiol, nitro, and other functional groups, which are important in an assortment of molecular recognition processes. Herein we report a catalytic method for the asymmetric synthesis of a variety of enantioenriched products bearing fluoromethylated stereocenters with excellent yields and enantioselectivities. Various N,P-ligands were designed and applied in the hydrogenation of fluoromethylated olefins and vinyl fluorides.

Herein, a catalytic asymmetric hydrogenation to synthesize various products bearing fluoromethylated stereocenters has been developed.

Enantioselective hydrogenation of cyclic tetrasubstituted-olefinic dehydroamino acid derivatives

An efficient asymmetric hydrogenation of cyclic tetrasubstituted-olefinic dehydroamino acid derivatives has been achieved with a Rh-ArcPhos catalyst, affording a series of α-acylamino-β-alkyl tetrahydropyranones with two contiguous chiral centers in up to 96% ee and 1000 TON.

Stereodivergent entry to β-branched β-trifluoromethyl α-amino acid derivatives by sequential catalytic asymmetric reactions

Currently, conventional reductive catalytic methodologies do not guarantee general access to enantioenriched β-branched β-trifluoromethyl α-amino acid derivatives. Herein, a one-pot approach to these important α-amino acids, grounded on the reduction - ring opening of Erlenmeyer-Plöchl azlactones, is presented. The configurations of the two chirality centers of the products are established during each of the two catalytic steps, enabling a stereodivergent process.

Hydrogenation of fluorinated molecules: an overview

The review aims at providing an overview on the developments made in hydrogenation reactions of molecules having various fluorinated groups (F, CF3, CF2H, CF2Rf). Indeed, the hydrogenation of fluorine-containing molecules is a straightforward and atom-economical way to access challenging (chiral) fluorinated scaffolds. This promising field is still in its infancy and milestones are expected in the coming years. To illustrate that, the review will highlight the major contributions made in that field and will be organized by fluorinated groups.

Trifluoromethylation of Benzoic Acids: An Access to Aryl Trifluoromethyl Ketones

The trifluoromethylation of benzoic acids with TMSCF₃ was achieved through nucleophilic substitution with the use of anhydrides as an in situ activating reagent. Under the reaction conditions, a wide range of carboxylic acids including the bioactive ones worked well, thus providing a facile and efficient method for preparing aryl trifluoromethyl ketones from the readily available starting materials.

An Aliphatic Bischler-Napieralski Reaction: Dihydropyridones by Cyclocarbonylation of 3-Allylimidazolidin-4-ones

The N-chloroformylimidazolidinone derivative of enantiopure l-alanine was deprotonated to form an enolate and functionalized with a series of allylic halides. Treatment of the resulting carbamoyl chlorides with potassium iodide led to cyclization of the allylic substituent onto the carbonyl group in an intramolecular aliphatic Friedel-Crafts-type acylation that corresponds to an aliphatic Bischler-Napieralski reaction. The product 3,4-dihydropyridinones were amenable to further functionalization, and finally hydrolysis, to deliver a series of enantio-enriched pipecolic acid derivatives.

Emerging Bottom-Up Strategies for the Synthesis of Graphene Nanoribbons and Related Structures

The solution-phase synthesis is one of the most promising strategies for the preparation of well-defined graphene nanoribbons (GNRs) in large scale. To prepare high quality, defect-free GNRs, cycloaromatization reactions need to be very efficient, proceed without side reaction and mild enough to accommodate the presence of various functional groups. In this Minireview, we present the latest synthetic approaches for the synthesis of GNRs and related structures, including alkyne benzannulation, photochemical cyclodehydrohalogenation, Mallory and Pd- and Ni-catalyzed reactions.

A Bulky Three-Hindered Quadrant Bisphosphine Ligand: Synthesis and Application in Rhodium-Catalyzed Asymmetric Hydrogenation of Functionalized Alkenes

A bulky three-hindered quadrant bisphosphine ligand, di-1-adamantylphosphino(tert-butylmethylphosphino)methane, named BulkyP*, has been synthesized via a convergent short pathway with chromatography-free procedures. The ligand is a crystalline solid and can be readily handled in air. Its rhodium(I) complex exhibits very high enantioselectivities and catalytic activities in the asymmetric hydrogenation of functionalized alkenes.

Approaches to Obtaining Fluorinated α-Amino Acids

Fluorine does not belong to the pool of chemical elements that nature uses to build organic matter. However, chemists have exploited the unique properties of fluorine and produced countless fluoro-organic compounds without which our everyday lives would be unimaginable. The incorporation of fluorine into amino acids established a completely new class of amino acids and their properties, and those of the biopolymers constructed from them are extremely interesting. Increasing interest in this class of amino acids caused the demand for robust and stereoselective synthetic protocols that enable straightforward access to these building blocks. Herein, we present a comprehensive account of the literature in this field going back to 1995. We place special emphasis on a particular fluorination strategy. The four main sections describe fluorinated versions of alkyl, cyclic, aromatic amino acids, and also nickel-complexes to access them. We progress by one carbon unit increments. Special cases of amino acids for which there is no natural counterpart are described at the end of each section. Synthetic access to each of the amino acids is summarized in form of a table at the end of this article with the aim to make the information easily accessible to the reader.

Stereoelectronic Effects in Ligand Design: Enantioselective Rhodium-Catalyzed Hydrogenation of Aliphatic Cyclic Tetrasubstituted Enamides and Concise Synthesis of (R)-Tofacitinib

We herein report the development of a conformationally defined, electron-rich, C₂ -symmetric, P-chiral bisphosphorus ligand, ArcPhos, by taking advantage of stereoelectronic effects in ligand design. With the Rh-ArcPhos catalyst, excellent enantioselectivities and unprecedentedly high turnovers (TON up to 10 000) were achieved in the asymmetric hydrogenation of aliphatic carbocyclic and heterocyclic tetrasubstituted enamides, to generate a series of chiral cis-2-alkyl-substituted carbocyclic and heterocyclic amine derivatives in excellent enantiomeric ratios. This method also enabled an efficient and practical synthesis of the Janus kinase inhibitor (R)-tofacitinib.

Synthesis of new fluorinated proline analogues from polyfluoroalkyl β-ketoacetals and ethyl isocyanoacetate

New straightforward synthetic approach to hitherto unknown cis-/trans-CF₃-prolines and other 3-polyfluoroalkyl proline analogues.

E,Z-Stereodivergent synthesis of N-tosyl α,β-dehydroamino esters via a Mukaiyama–Michael addition

E or Z dehydroamino esters are stereoselectively obtained from a same set of reactants depending on the reaction conditions.

Cu-Catalyzed Alkylative Carboxylation of Ynamides with Dialkylzinc Reagents and Carbon Dioxide

Alkylative carboxylation of ynamides with CO2 and dialkylzinc reagents using a N-heterocyclic carbene (NHC)-copper catalyst has been developed. A variety of ynamides, both cyclic and acyclic, undergo this transformation under mild conditions to afford the corresponding α,β-unsaturated carboxylic acids, which contain the α,β-dehydroamino acid skeleton. The present alkylative carboxylation formally consists of Cu-catalyzed carbozincation of ynamides with dialkylzinc reagents with the subsequent nucleophilic carboxylation of the resulting alkenylzinc species with CO2 . Dialkylzinc reagents bearing a β-hydrogen atom such as Et2 Zn and Bu2 Zn still afford the alkylated products despite the potential for β-hydride elimination. This protocol would be a desirable method for the synthesis of highly substituted α,β- dehydroamino acid derivatives due to its high regio- and stereoselectivity, simple one-pot procedure, and its use of CO2 as a starting material.

Catalytic, asymmetric, and stereodivergent synthesis of non-symmetric β,β-diaryl-α-amino acids

We report a concise, enantio- and diastereoselective route to novel nonsymmetrically substituted N-protected β,β-diaryl-α-amino acids and esters, through the asymmetric hydrogenation of tetrasubstituted olefins, some of the most challenging examples in the field. Stereoselective generation of an E- or Z-enol tosylate, when combined with stereoretentive Suzuki-Miyaura cross-coupling and enantioselective hydrogenation catalyzed by (NBD)2RhBF4 and a Josiphos ligand, allows for full control over the two vicinal stereogenic centers. High yields and excellent enantioselectivities (up to 99% ee) were obtained for a variety of N-acetyl, N-methoxycarbonyl, and N-Boc β,β-diaryldehydroamino acids, containing a diverse and previously unreported series of heterocyclic and aryl substituted groups (24 examples) and allowing access to all four stereoisomers of these valuable building blocks.

Reactions of β-alkoxyvinyl polyfluoroalkyl ketones with ethyl isocyanoacetate and its use for the synthesis of new polyfluoroalkyl pyrroles and pyrrolidines

The hitherto unreported reactions of β-alkoxyvinyl polyfluoroalkyl ketones with ethyl isocyanoacetate and equimolar amounts of potassium-tert-butoxide proceeded mainly in the β-position of the α,β-unsaturated ketones in cases of α-nonsubstituted 1a–e and α-methyl substituted ketones 1g–j. Other α- or β-substituted ketones 1f,k–o gave mainly products 4 of initial attack at the carbonyl carbon. Depending on the solvent, the major products of β-attack do exist in different tautomeric forms. Generally the openchain enol tautomers 5 predominate in the polar DMSO-d(6), while the cyclic γ-hemiaminals 8 are the major tautomers in the less polar CDCl(3). Acid treatment of the latter compounds 8 led to the hitherto unknown ethyl 5-polyfluoroalkyl-pyrrole-2-carboxylates 11 by elimination of formic acid. Catalytic hydrogenation of pyrrole 11a was used for the synthesis of earlier unknown 5-trifluoromethyl proline 16.