Pseudoephedrine-Directed Asymmetric α-Arylation of α-Amino Acid Derivatives

Regioselective Synthesis of α-Quaternary Amino Acid Derivatives Enabled by Photoinduced Energy Transfer

α-Quaternary amino acids have found application in many biologically relevant compounds and pharmaceuticals. Although there are many methods for the synthesis of α-quaternary amino acids, most of them are mainly realized with the aid of transition metals and complex ligands. We present herein a 2,7-Br-4CzIPN catalyzed regioselective alkylation of azlactones with redox-active esters via radical-radical couplings. Strikingly, this approach is devoid of any metal or additive and shows broad scope and superior sensitive functional group compatibility.

Challenges and recent advancements in the synthesis of α,α-disubstituted α-amino acids

α,α-Disubstituted α-amino acids (α-AAs) have improved properties compared to other types of amino acids. They serve as modifiers of peptide conformation and as precursors of bioactive compounds. Therefore, it has been a long-standing goal to construct this highly valuable scaffold efficiently in organic synthesis and drug discovery. However, access to α,α-disubstituted α-AAs is highly challenging and largely unexplored due to their steric constraints. To overcome these, remarkable advances have been made in the last decades. Emerging strategies such as synergistic enantioselective catalysis, visible-light-mediated photocatalysis, metal-free methodologies and CO2 fixation offer new avenues to access the challenging synthesis of α,α-disubstituted α-AAs and continuously bring additional contributions to this field. This review article aims to provide an overview of the recent advancements since 2015 and discuss existing challenges for the synthesis of α,α-disubstituted α-AAs and their derivatives.

Enantioselective Synthesis of Arylglycines via Pd-Catalyzed Coupling of Schöllkopf Bis-Lactim Ethers with Aryl Chlorides

Arylglycines are important pharmacophores present in several top-selling drugs. This compound class has now been made accessible from abundant aryl chlorides by a Pd-catalyzed Schöllkopf-type amino acid synthesis. In the presence of the catalyst methylnaphthyl(XPhos)-palladium bromide, the base lithium 2,2,6,6-tetramethylpyrrolidide and the additive ZnCl2 , tert-leucine-derived bis-lactim ethers were efficiently arylated at room temperature, reaching yields of 95 % and diastereoselectivities of 98 : 2. Hydrolysis gave the corresponding arylglycines in high enantiomeric excess.

Cu-Catalyzed Three-Component Carboamination of Electron Deficient Olefins

The copper-catalyzed three-component carboamination of atropates for the synthesis of α-aryl amino acid derivatives is presented. The scope of the reaction is explored with respect to all three coupling partners: the alkyl halide, the atropate, and the aryl amine. A total of 41 examples are included, with yields of ≤92%. Both primary and secondary aryl amines participate in the carboamination along with α-haloesters, nitriles, and perfluoroiodoalkanes. Mechanistic investigations support a radical mechanism involving Cu-mediated C-N bond formation with the radical adduct.

Enantioselective Intramolecular α-Arylation of Benzylamine Derivatives: Synthesis of a Precursor to Levocetirizine

A practical, transition metal-free method allows the enantioselective synthesis of α,α-diarylmethylamines by asymmetric α-arylation of benzylamines. Enantioselective lithiation of N'-aryl-N-benzyl-N-isopropyl ureas using a chiral lithium amide base generates a benzyllithium that undergoes an unactivated stereospecific intramolecular nucleophilic aromatic substitution to generate an α,α-diarylmethylamine in the form of its urea derivative, in up to >99 % ee. Treatment with acid induces an "azatropic shift" with retention of configuration, the product of which may be hydrolysed to the target amine.

Rigidified Bis(sulfonyl)ethylenes as Effective Michael Acceptors for Asymmetric Catalysis: Application to the Enantioselective Synthesis of Quaternary Hydantoins

The catalytic, enantio- and diastereoselective addition of hydantoin surrogates II to "rigidified" vinylidene bis(sulfone) reagents is developed, thus overcoming the inability of commonly employed β-substituted vinylic sulfones to react. Adducts are transformed in enantioenriched 5,5-disubstituted hydantoins through hydrolysis and reductive desulfonylation processes providing new structures for eventual bioassays. Density functional theory studies that rationalize the observed reactivity and stereoselectivity trends are also provided.

C(sp3)-Arylation by Conformationally Accelerated Intramolecular Nucleophilic Aromatic Substitution (SNAr)

The asymmetric synthesis of heavily substituted benzylic stereogenic centers, prevalent in natural products, therapeutics, agrochemicals, and catalysts, is an ongoing challenge. In this Account, we outline our contribution to this endeavor, describing our discovery of a series of new reactions that not only have synthetic applicability but also present significant mechanistic intrigue. The story originated from our longstanding interest in the stereochemistry and reactivity of functionalized organolithiums. While investigating the lithiation chemistry of ureas (a “Cinderella” sister of the more established amides and carbamates), we noted an unexpected Truce–Smiles (T-S) rearrangement involving the 1,4-N → C transposition of a urea N′-aryl group to the α-carbanion of an adjacent N-benzyl group. Despite this reaction formally constituting an S_NAr substitution, we found it to be remarkably tolerant of the electronic properties of the migrating aryl substituent and the degree of substitution at the carbanion. Moreover, in contrast to classical S_NAr reactions, the rearrangement was sufficiently rapid that it took place under conditions compatible with configurational stability in an organolithium intermediate, enabling enantiospecific arylation at benzylic stereogenic centers. Experimental and computational studies confirmed a low kinetic barrier to the aryl migration arising from the strong preference for a trans arrangement of the urea N′-aryl and carbonyl groups, populating a reactive conformer in which spatial proximity was enforced between the carbanion and N′-aryl group, hugely accelerating ipso-substitution.

This discovery led us to uncover a whole series of conformationally accelerated intramolecular N → C aryl transfers using different anilide-based functional groups, including a diverse range of urea, carbamate, and thiocarbamate-substituted anions. Products included enantioenriched α-tertiary amines (including α-arylated N-heterocycles) and alcohols, as well as rare α-tertiary thiols. Synthetically challenging diarylated centers with differentiated aryl groups featured heavily in all product sets. The absolute enantiospecificity (retention versus inversion) of the reaction was dependent on the heteroatom α to the lithiation site: the origin of this stereodivergence was probed both experimentally and computationally. Asymmetric variants of the rearrangement were realized by enantioselective deprotonation, and connective strategies were developed in which an intermolecular C–C bond-forming event preceded the anionic rearrangement. Substrates where the N′-nucleofuge (at the aryl ipso position) was tethered to the migrating arene allowed us to use the rearrangement as a ring expansion method to generate 8- to 12-membered medium-ring N-heterocycles from very simple precursors. Stabilized carbon nucleophiles such as alkali metal enolates also readily promoted intramolecular N → C aryl transfer in N′-arylureas, opening up access to biologically relevant hydantoins, and enabling a “chiral memory” approach for the (hetero)arylation of chiral α-amino acids with programmable retention or inversion of configuration. Collectively, our studies of electronically versatile T-S rearrangements in anilide-based systems have culminated in a practical and general strategy for transition metal-free C(sp³)-arylation. More broadly, our results highlight the power of conformational activation to achieve unprecedented reactivity in the construction of challenging C–C bonds.

Recent Advances in the Smiles Rearrangement: New Opportunities for Arylation

The Smiles rearrangement has undergone a renaissance in recent years providing new avenues for non-canonical arylation techniques in both the radical and polar regimes. This short review will discuss recent applications of the reaction (from 2017 to late 2021), including its relevance to areas such as heterocycle synthesis and the functionalization of alkenes and alkynes as well as glimpses at new directions for the field.

1 Introduction

2 Polar Smiles Rearrangements

3 Radical Smiles: Alkene and Alkyne Functionalization

4 Radical Smiles: Rearrangements via C–X Bond Cleavage

5 Radical Smiles: Miscellaneous Rearrangements

6 Conclusions

Pd-Catalysed oxidative carbonylation of α-amino amides to hydantoins under mild conditions

The first example of palladium-catalysed oxidative carbonylation of unprotected α-amino amides to hydantoins is described here. The selective synthesis of the target compounds was achieved under mild conditions (1 atm of CO), without ligands and bases. The catalytic system overrode the common reaction pathway that usually leads instead to the formation of symmetrical ureas.

Triarylmethanes and their Medium-Ring Analogues by Unactivated Truce-Smiles Rearrangement of Benzanilides

Intramolecular nucleophilic aromatic substitution (Truce–Smiles rearrangement) of the anions of 2‐benzyl benzanilides leads to triarylmethanes in an operationally simple manner. The reaction succeeds even without electronic activation of the ring that plays the role of electrophile in the S_NAr reaction, being accelerated instead by the preferred conformation imposed by the tertiary amide tether. The amide substituent of the product may be removed or transformed into alternative functional groups. A ring‐expanding variant (n to n+4) of the reaction provided a route to doubly benzo‐fused medium ring lactams of 10 or 11 members. Hammett analysis returned a ρ value consistent with the operation of a partially concerted reaction mechanism.

Hydantoin-bridged medium ring scaffolds by migratory insertion of urea-tethered nitrile anions into aromatic C-N bonds

Bicyclic or tricyclic nitrogen-containing heterocyclic scaffolds were constructed rapidly by intramolecular nucleophilic aromatic substitution of metallated nitriles tethered by a urea linkage to a series of electronically unactivated heterocyclic precursors. The substitution reaction constitutes a ring expansion, enabled by the conformationally constrained tether between the nitrile and the heterocycle. Attack of the metallated urea leaving group on the nitrile generates a hydantoin that bridges the polycyclic products. X-ray crystallography reveals ring-dependant strain within the hydantoin.

A Cascade Reaction of Michael Addition and Truce-Smiles Rearrangement to Synthesize Trisubstituted 4-Quinolone Derivatives

A novel transition-metal-free cascade reaction to synthesize 4-quinolone derivatives has been demonstrated. Michael addition and Truce-Smiles rearrangement are included in this protocol, providing a broad scope of 4-quinolones in moderate-to-excellent yields. This work serves as an example of the use of sulfonamides through Truce-Smiles rearrangement to build heterocyclic compounds under mild conditions.

Axially chiral hemiaminals from nonracemic α-amino acid derivatives (thiohydantoins): Synthesis and isomer identification

Stable, nonracemic axially chiral hemiaminals (O,N-hemiacetals) have been synthesized stereoselectively from lithium aluminum hydride (LiAlH₄ ) reductions of nonracemic 5-methyl- and 5-isopropyl-3-(o-aryl)-2-thioydantoins in tetrahydrofuran (THF) at room temperature in 10 min. Predominantly S-configured hemiaminals at C-4 of the heterocyclic ring were produced from the S-configured thiohydantoins at C-5 (by 80% when the C5 substituent is methyl and by 97% when it is isopropyl). The configuration at C-5 was retained during the reduction reaction. The stereochemical outcome of the axially chiral hemiaminals resulted from their conformational preferences.

Synthesis of chiral α-substituted α-amino acid and amine derivatives through Ni-catalyzed asymmetric hydrogenation

Highly efficient Ni-catalyzed asymmetric hydrogenation of cyclic N-sulfonyl ketimino esters was, for the first time, successfully developed, providing various chiral α-monosubstituted α-amino acid derivatives with excellent results (97-99% yields, 90 to >99% ee). Cyclic N-sulfonyl ketimines were also hydrogenated well to afford chiral amine derivatives with 98-99% yields and 97 to >99% ee. The gram-scale asymmetric hydrogenation was performed well with 85% yield and 99% ee using only 0.2 mol% catalyst.

HFIP-Promoted de Novo Synthesis of Biologically Relevant Nonnatural α-Arylated Amino Esters and Dipeptide Mimetics

Amino acids are fundamental building blocks, which have been extensively used in drug design and organic synthesis. However, nonnatural amino acids are relatively less studied. In this work, the authors report the first HFIP-promoted de novo synthesis of nonnatural α-arylated amino esters and dipeptide mimetics (27 examples, up to 99 % yield) from readily available amines, ethyl glyoxylate and electron-rich arenes under mild conditions, in which one C-C bond, one C-N bond and one chiral center were established simultaneously. The reaction was also performed on a gram scale, giving compound 4 a in 96 % yield. In addition, this protocol was successfully applied to the late-stage elaboration of drug molecules, such as tranylcypromine (TCP or PCPA) and troxipide. Interestingly, compound 4 h inactivated histone lysine specific demethylase 1 (LSD1) potently with an IC₅₀ value of 0.296 μm. To the best of our knowledge, compound 4 h is the first LSD1 inhibitor derived from nonnatural α-arylated amino esters, and therefore could be used as a hit compound for the development of new LSD1 inhibitors. The synthesized nonnatural α-arylated amino esters and dipeptide mimetics as unique building blocks may have potential synthetic utilities.

Connective synthesis of 5,5-disubstituted hydantoins by tandem α-amination and α-arylation of silyl ketene acetals

Amination of a silylated ester generates an intermediate urea that transfers an aryl ring to the aminated centre and cyclises to a hydantoin.

5,5-Disubstituted hydantoins, formally the cyclisation products of quaternary amino acids, were formed connectively from simple ester-derived starting materials by a one-pot tandem method. Amination of the silyl ketene acetal derivative of a methyl ester takes place by silver-catalysed addition to the N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N bond of an azocarboxamide, generating a N-amino-N′-aryl urea derivative of a substituted aminoester. Treatment with a base forms an ester enolate which undergoes arylation by intramolecular migration of an aryl ring to the α-position of the ester. The product undergoes ring closure to a hydantoin, which may itself be deprotected and functionalised. Aryl migration is successful with rings of various electronic character and with esters bearing functionalised and unfunctionalised chains, and the products have features in common with several bioactive compounds.

α-Methyl phenylglycines by asymmetric α-arylation of alanine and their effect on the conformational preference of helical Aib foldamers

α-Arylated alanine derivatives were made enantioselectively by migratory rearrangement of a urea derivative using (R,R)-pseudoephedrine as a chiral auxiliary. Incorporation of a single residue of the product α-methyl phenylglycine into an otherwise achiral oligomer of aminoisobutyric acid oligomer induced a preferred screw sense, detectable by a NMR reporter located at the remote terminus of the oligomer. The magnitude of the screw sense induction was greater when the chiral residue was located at the N-terminus of the foldamer, and in some cases the sense of induction was opposite to that of related α-methylated amino acids with α-substituents other than aryl.

Asymmetric α-arylation of amino acids

Quaternary amino acids, in which the α-carbon that bears the amino and carboxyl groups also carries two carbon substituents, have an important role as modifiers of peptide conformation and bioactivity and as precursors of medicinally important compounds^1,2. In contrast to enantioselective alkylation at this α-carbon, for which there are several methods^3-8, general enantioselective introduction of an aryl substituent at the α-carbon is synthetically challenging⁹. Nonetheless, the resultant α-aryl amino acids and their derivatives are valuable precursors to bioactive molecules^10,11. Here we describe the synthesis of quaternary α-aryl amino acids from enantiopure amino acid precursors by α-arylation without loss of stereochemical integrity. Our approach relies on the temporary formation of a second stereogenic centre in an N'-arylurea adduct¹² of an imidazolidinone derivative⁶ of the precursor amino acid, and uses readily available enantiopure amino acids both as a precursor and as a source of asymmetry. It avoids the use of valuable transition metals, and enables arylation with electron-rich, electron-poor and heterocyclic substituents. Either enantiomer of the product can be formed from a single amino acid precursor. The method is practical and scalable, and provides the opportunity to produce α-arylated quaternary amino acids in multi-gram quantities.

Enantioselectively functionalised phenytoin derivatives by auxiliary-directed N to C aryl migration in lithiated α-amino nitriles

Lithiation of N'-arylureas derived from amino nitriles incorporating a (1R,2R)-2-aminocyclohexanol chiral auxiliary leads to diastereoselective migration of the aryl ring to the position α to the nitrile. The resulting N'-lithiated ureas undergo spontaneous cyclisation to iminohydantoins, which may be hydrolysed to give chiral 5,5-diarylhydantoins related to phenytoin, in enantioenriched form.

Enantioselective Synthesis of 5,5-Disubstituted Hydantoins by Brønsted Base/H-Bond Catalyst Assisted Michael Reactions of a Design Template

A new method for the enantioselective synthesis of 5,5-disubstituted (quaternary) hydantoins was developed on the basis of an organocatalytic Michael reaction approach involving the use of 2-benzylthio-3,5-dihydroimidazol-4-ones as key hydantoin surrogates. The method is general with respect to the substitution pattern at the hydantoin N¹ (alkyl, aryl, acyl), N³ (aryl), and C⁵ (linear/branched alkyl, aryl) positions and affords essentially single diastereomeric products with enantioselectivities higher than 95 % ee in most cases. Among the bifunctional Brønsted base/H-bond catalysts examined, a known squaramide-tertiary amine catalyst and a newly prepared squaramide-tertiary amine catalyst provide the highest selectivity so far with either nitroolefins or vinyl ketones as the acceptor components. Kinetic measurements support a first-order rate dependence on both reaction partners, the donor template and the Michael acceptor, whereas competitive ¹ H NMR spectroscopy experiments reveal the high ability of the template for catalyst binding.

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.

Recent Advances in the Synthesis of Hydantoins: The State of the Art of a Valuable Scaffold

The review highlights the hydantoin syntheses presented from the point of view of the preparation methods. Novel synthetic routes to various hydantoin structures, the advances brought to the classical methods in the aim of producing more sustainable and environmentally friendly procedures for the preparation of these biomolecules, and a critical comparison of the different synthetic approaches developed in the last twelve years are also described. The review is composed of 95 schemes, 8 figures and 528 references for the last 12 years and includes the description of the hydantoin-based marketed drugs and clinical candidates.

Stereoselective Arylation of Amino Aldehydes: Overriding Natural Substrate Control through Chelation

The chelation-controlled arylation reaction of chiral, enantiopure acyclic α-amino aldehydes enabled by a B/Zn exchange reaction between arylboronic acids and Et₂Zn is reported. The presence of dibenzyl substituents at the nitrogen plays a key role in the stereochemical outcome of the reaction, and chelation is favored over the natural tendency of this type of substrate to undergo Felkin-Anh controlled additions with organomagnesium and organolithium reagents.