Enantioselective organocatalytic strategies to access noncanonical α-amino acids

Organocatalytic asymmetric synthesis has evolved over the years and continues to attract the interest of many researchers worldwide. Enantiopure noncanonical amino acids (ncAAs) are valuable building blocks in organic synthesis, medicinal chemistry, and chemical biology. They are employed in the elaboration of peptides and proteins with enhanced activities and/or improved properties compared to their natural counterparts, as chiral catalysts, in chiral ligand design, and as chiral building blocks for asymmetric syntheses of complex molecules, including natural products. The linkage of ncAA synthesis and enantioselective organocatalysis, the subject of this perspective, tries to imitate the natural biosynthetic process. Herein, we present contemporary and earlier developments in the field of organocatalytic activation of simple feedstock materials, providing potential ncAAs with diverse side chains, unique three-dimensional structures, and a high degree of functionality. These asymmetric organocatalytic strategies, useful for forging a wide range of C-C, C-H, and C-N bonds and/or combinations thereof, vary from classical name reactions, such as Ugi, Strecker, and Mannich reactions, to the most advanced concepts such as deracemisation, transamination, and carbene N-H insertion. Concurrently, we present some interesting mechanistic studies/models, providing information on the chirality transfer process. Finally, this perspective highlights, through the diversity of the amino acids (AAs) not selected by nature for protein incorporation, the most generic modes of activation, induction, and reactivity commonly used, such as chiral enamine, hydrogen bonding, Brønsted acids/bases, and phase-transfer organocatalysis, reflecting their increasingly important role in organic and applied chemistry.

Iron-Catalyzed Synthesis of α-Azido α-Amino Esters via the Alkylazidation of Alkenes

An iron-catalyzed alkylazidation of dehydroamino acids using peroxides as alkyl radical precursors is described. Non-natural azidated amino esters bearing an α-alkyl chain could be obtained in 18-94% yields using TMSN3 as an azide source. The obtained α-alkyl-α-azide α-amino esters could be further functionalized through cycloaddition or azide reduction with amide couplings to afford aminal-type peptides, α-triazolo amino acids, and tetrahydro-triazolopyridine, showing the great versatility of this now easily accessible class of amino acids.

Design of Bifunctional Amino Tf-Amide Organocatalysts and Application in Various Asymmetric Transformations

In this personal account, we describe our recent developments on the four types of amino Tf-amide catalysts in asymmetric transformations. Firstly, axially chiral biaryl-based secondary-amino Tf-amide catalyzed various stereoselective reactions via enamine intermediates. Secondly, pyrrolidine-based secondary-amino aliphatic Tf-amide catalyzed asymmetric direct Mannich reaction. Thirdly, chiral primary-amino aliphatic Tf-amide catalyzed asymmetric direct aldol reaction and conjugate addition. Finally, modified chiral amino aromatic Tf-amide catalyzed asymmetric transformations. These four different strategies are illustrated by using various organocatalyzed asymmetric transformations.

Modular synthesis of chiral 1,2-dihydropyridines via Mannich/Wittig/cycloisomerization sequence that internally reuses waste

1,2-Dihydropyridines are valuable and reactive synthons, and particularly useful precursors to synthesize piperidines and pyridines that are among the most common structural components of pharmaceuticals. However, the catalytic enantioselective synthesis of structurally diverse 1,2-dihydropyridines is limited to enantioselective addition of nucleophiles to activated pyridines. Here, we report a modular organocatalytic Mannich/Wittig/cycloisomerization sequence as a flexible strategy to access chiral 1,2-dihydropyridines from N-Boc aldimines, aldehydes, and phosphoranes, using a chiral amine catalyst. The key step in this protocol, cycloisomerization of chiral N-Boc δ-amino α,β-unsaturated ketones recycles the waste to improve the yield. Specifically, recycling by-product water from imine formation to gradually release the true catalyst HCl via hydrolysis of SiCl₄, whilst maintaining a low concentration of HCl to suppress side reactions, and reusing waste Ph₃PO from the Wittig step to modulate the acidity of HCl. This approach allows facile access to enantioenriched 2-substituted, 2,3- or 2,6-cis-disubstituted, and 2,3,6-cis-trisubstituted piperidines.

Three-Component Synthesis of α-Aminoperoxides Using Primary and Secondary Dialkylzinc Reagents with O2 and α-Amido Sulfones

A straightforward strategy for the synthesis of unprecedented α-aminoperoxides bearing primary and secondary alkylperoxide substituents is described. Commercially available dialkylzinc reagents are oxidized with molecular oxygen and the consequent peroxide species react with stable (hetero)aromatic and aliphatic α-amido sulfones in excellent yields (>90%). The now available α-aminoperoxides are of potential interest in medicinal chemistry, specifically for the synthesis of antimalarial compounds. Moreover, modification of the reaction conditions selectively leads to N,O-acetals in good yields.

Stereodivergent Synthesis of 1-Hydroxymethylpyrrolizidine Alkaloids

A first stereodivergent strategy for the asymmetric synthesis of all stereoisomers of 1-hydroxymethylpyrrolizidine alkaloids is developed using an asymmetric self-Mannich reaction as a key step. An anti-selective self-Mannich reaction of methyl 4-oxobutanoate with the PMP-amine catalyzed by a chiral secondary amine is successfully optimized for the asymmetric synthesis of (+)-isoretronecanol and (-)-isoretronecanol. A syn-selective self-Mannich reaction catalyzed by proline is utilized for the asymmetric synthesis of the diastereomer, (+)-laburnine, and its enantiomer, (-)-trachelanthamidine.

Organocatalytic enantioselective aminoalkylation of pyrazol-3-ones with aldimines generated in situ from α-amido sulfones

Herein, an efficient asymmetric aminoalkylation of pyrazolones with α-amido sulfones catalyzed by a quinine-derived squaramide in dichloromethane/aqueous media has been established. A variety of chiral amines were obtained with high yields (up to 98%) and excellent enantioselectivities (up to 99% ee). The corresponding products are transformed into optically active acetylated pyrazoles after treatment with Ac2O/Et3N, because of the instability of some adducts. The reaction tolerates a wide range of α-amido sulfones and different pyrazolones.

H-bonding vs Protonation of Alkynes in Regioselective Hydroamination Reactions: A Glimpse into the Reactivity of Arylogous Ynolethers and Ynamines

In this paper is described the competition and transition between hydrogen bonding and protonation of alkynes connected, on one side, to various aromatic rings and to chiral amino ester appendages on the other side. While the first mode of activation induced the cyclization into pyrrolidines, the protonation of the alkyne led preferentially to tetrahydropyridines due to the higher level of the highest occupied molecular orbital (HOMO) of the considered arylogous ynolethers and ynamines. The transition between H-bonding and protonation was observed with the alkyne substituted with 2-methoxyphenyl, for which the cyclization delivered either five- or six-membered rings depending on the temperature of the experiment. From there, the cyclization of dialkoxy- and trialkoxyphenyl-substituted alkynes into six-membered rings, i.e., tetrahydropyridines, was developed. When next applied to alkynylindoles, the same pattern of cyclization provided six-membered rings as an illustration of the reactivity of arylogous ynamines. Thanks to the high reactivity of the intermediate cationic species, weak nucleophiles such as NH-oxazolidinone participated efficiently in the hydroamination reaction of alkynylindoles. Pyrrolidines, tetrahydropyridines, and piperidines decorated with various aromatics and substituents were thus prepared in enantio- and stereoselective manner. Capitalizing on the enamine moiety of the azaheterocycles, the molecular diversity was extended through stereoselective oxidation and ring contraction processes.

Synthesis and Reactivity of α-Haloglycine Esters: Hyperconjugation in Action

A general and efficient synthesis of α-haloglycine esters from commercially available feedstock chemicals, in a single step, is reported. The reactivity of these α-haloglycine esters with various nucleophiles was studied as surrogates of α-iminoesters upon activation with hydrogen-bond donor catalysts. DFT calculations on the α-haloglycine structures (X = F, Cl, Br) accompanied by an X-ray characterization of the α-bromoglycine ester support the existence of a "generalized" anomeric effect created by hyperconjugation. This peculiar hyperconjugative effect is proposed to be responsible for the enhanced halogen nucleofugality leading to a facile halogen abstraction by hydrogen-bond donor catalysts. This reactivity was exploited with thiourea catalysts on several catalytic transformations (aza-Friedel-Crafts and Mannich reactions) for the synthesis of several types of non-proteinogenic α-amino esters.

Recent synthetic applications of α-amido sulfones as precursors of N-acylimino derivatives

α-Amido sulfones can be directly used as N-acylimine or N-acyliminium ion precursors in several synthetic processes aimed at the preparation of nitrogen containing compounds. This review collects the most relevant and practical utilizations of α-amido sulfones appeared in the literature after 2005.

Enantioselective anti-Mannich reaction catalyzed by modularly designed organocatalysts

A highly stereoselective method for achieving the anti-Mannich reaction of aldehydes and ketones with ethyl (4-methoxyphenylimino)acetate was realized using the modularly designed organocatalysts (MDOs) self-assembled from cinchona alkaloid derivatives and (A)-pyrrolidien-3-carboxylic acid in the reaction media. The desired anti-Mannich products were obtained in good to excellent yields (up to 93%), excellent diastereoselectivities (up to 99:1 dr), and good to high enantioselectivities (up to 99% ee).

Chiral Primary Amine Catalysis for Asymmetric Mannich Reactions of Aldehydes with Ketimines: Stereoselectivity and Reactivity

The application potential of primary amine catalysts in the context of Mannich reactions of aldehydes with ketimines is exemplified by isatin-derived ketimines and cyclic trifluoromethyl ketimines. Primary amine catalysts exhibit either unusual stereoselectivity or reactivity, which is not observable with secondary amine catalysts. Moreover, reversal of diastereofacial selectivity between primary and secondary amine catalysts is disclosed. These new reactions provide useful methods for the syntheses of chiral 3-substituted 3-amino-2-oxindoles and dihydroquinazolinones bearing a trifluoromethylated quaternary stereocenter. The synthetic utility of the reactions is demonstrated through the formal synthesis of AG-041R and DPC 083 and the total synthesis of (-)-psychotriasine.

In Situ-Generated Glycinyl Chloroaminals for a One-Pot Synthesis of Non-proteinogenic α-Amino Esters

An acetyl chloride-mediated cascade transformation involving a primary carbamate, ethyl glyoxylate, and various types of nucleophiles is reported for the synthesis of orthogonally protected α-amino esters. These reactions proceeded rapidly to afford the pivotal α-chloroglycine intermediate in excellent yields, which can be directly functionalized in situ with various types of nucleophiles. A mild and unique AcOH(cat.)/AcCl system was found to promote an autocatalytic-like condensation and facilitate the multicomponent assembly of non-proteinogenic α-amino esters. To better understand this one-pot transformation and the orchestration of the components' condensations, the investigation of a broader scope of nucleophiles and some kinetic studies are presented. Our findings suggest that the halogenation step toward the formation of α-chloroglycine is the rate-determining step likely proceeding through the formation of N-carbamoyl iminium. Also, the initial kinetic profiling for the nucleophilic substitution supports an S_N1-like (S_N2C+) mechanism in which nucleophiles add to the iminium-chloride tight ionic pair. These results lead ultimately to the design of a new protocol in which an achiral hydrogen bond donor thiourea catalyst was utilized to enhance the reaction scope and enable silylated nucleophiles to be efficiently exploited to synthesize novel non-proteinogenic α-amino esters.

Asymmetric synthesis of vinylogous β-amino acids and their incorporation into mixed backbone oligomers

Chiral vinylogous β-amino acids (VBAA) were synthesized using enantioselective Mannich reactions of aldehydes with in situ generated N-carbamoyl imines followed by a Horner-Wadsworth-Emmons reaction. The efficiency with which these units could be incorporated into oligomers with different moieties on the C- and N-terminal sides was established, as was the feasibility of sequencing oligomers containing VBAAs by tandem mass spectrometry. The data show that VBAAs will be useful building blocks for the construction of combinatorial libraries of peptidomimetic compounds.

Phosphatase-Stable Phosphoamino Acid Mimetics That Enhance Binding Affinities with the Polo-Box Domain of Polo-like Kinase 1

(2S,3R)-2-Amino-3-methyl-4-phosphonobutanoic acid (Pmab) is a phosphatase-stable analogue of phosphothreonine (pThr), which has been used in a variety of biological contexts. Among these applications are peptidomimetic ligands that bind to the polo-box domain (PBD) of polo-like kinase 1 (Plk1) with affinities approaching that of the corresponding pThr-containing peptides. However, Pmab is not widely used, because there are no direct, high-yield preparations of suitably protected reagent. We have now achieved an efficient synthesis of protected Pmab, as well as variants with different substituents at the 3R center. When incorporated into our peptidomimetic scaffold, these new Pmab analogues exhibit Plk1 PBD-binding affinities that are several-fold higher than Pmab, yet retain good selectivity for Plk1 relative to the PBDs of Plk2 and Plk3. These findings will significantly impact the future development of PBD-binding inhibitors, as well as ligands directed against a broad spectrum of pThr-dependent processes.

Asymmetric Mannich synthesis of α-amino esters by anion-binding catalysis

We report a scalable, one-pot Mannich route to enantioenriched α-amino esters by direct reaction of α-chloroglycine ester as a practical imino ester surrogate. The reaction is promoted by a chiral aminothiourea, which is proposed to operate cooperatively by generating an iminium ion by chloride abstraction and an enolate by deprotonation, followed by highly stereoselective C-C bond formation between both reactive intermediates associated non-covalently within the catalyst framework.

A concise diastereoselective approach to enantioenriched substituted piperidines and their in vitro cytotoxicity evaluation

A library of diversely stereo-oriented, highly substituted 2,6-cis piperidine derivatives were synthesized, and evaluated for their anticancer activity in cancer cells that included A549 (lung cancer, CCL-185), MCF7 (breast cancer (HTB-22), DU145 (prostate cancer (HTB-81), and HeLa (cervical cancer, CCL-2). One stereo-variant emerged as a promising candidate for further design based structure-activity studies.

Stereocontrolled synthesis of vicinal diamines by organocatalytic asymmetric Mannich reaction of N-protected aminoacetaldehydes: formal synthesis of (-)-agelastatin A

The 1,2-diamine (vicinal diamine) motif is present in a number of natural products with interesting biological activity and in many chiral molecular catalysts. The efficient and stereocontrolled synthesis of enantioenriched vicinal diamines is still a challenge to modern chemical methodology. We report here both syn- and anti-selective asymmetric direct Mannich reactions of N-protected aminoacetaldehydes with N-Boc-protected imines catalyzed by proline and the axially chiral amino sulfonamide (S)-3. This organocatalytic process represents the first example of a Mannich reaction using Z- or Boc-protected aminoacetaldehyde as a new entry of α-nitrogen functionalized aldehyde nucleophile in enamine catalysis. The obtained optically active vicinal diamines are useful chiral synthons as exemplified by the formal synthesis of (-)-agelastatin A.