Catalytic asymmetric synthesis of alpha-amino acids

Homogeneous Catalysis: A Powerful Technology for the Modification of Important Biomolecules

Homogeneous catalysis plays an important and ubiquitous role in the synthesis of simple and complex molecules, including drug compounds, natural products, and agrochemicals. In recent years, the wide-reaching importance of homogeneous catalysis has made it an indispensable tool for the modification of biomolecules, such as carbohydrates (sugars), amino acids, peptides, nucleosides, nucleotides, and steroids. Such a synthetic strategy offers several advantages, which have led to the development of new molecules of biological relevance at a rapid rate relative to the number of available synthetic methods. Given the powerful nature of homogeneous catalysis in effecting these synthetic transformations, this Focus Review has been compiled to highlight these important developments.

Employing Photoredox Catalysis for DNA-Encoded Chemistry: Decarboxylative Alkylation of α-Amino Acids

A new procedure for the photoredox-mediated conjugate addition of radicals that can be conveniently generated from α-amino acids to DNA-tagged Michael acceptors and styrenes is presented. This C(sp³ )-C(sp³ ) coupling tolerates a broad array of structurally diverse radical precursors, including all of the 20 proteinogenic amino acids. Importantly, this reaction proceeds under mild conditions and in DNA-compatible aqueous media. Furthermore, the presented reaction conditions are compatible with DNA, making this reaction platform well suited for the construction of DNA-encoded libraries. The scope and limitations of the chemistry are discussed herein along with proposals for how this methodology might be used to construct DNA-encoded libraries.

Synthesis of α-(4-Oxazolyl)amino Esters via Brønsted Acid Catalyzed Tandem Reaction

A one-step, Brønsted acid catalyzed tandem reaction for the synthesis of α-(4-oxazolyl)amino esters was developed. 4-Nitrobenzenesulfonic acid was found to be an efficient catalyst for the coupling of ethyl 2-oxobut-3-ynoates with amides to provide various α-(4-oxazolyl)amino esters. The experimental and X-ray crystallographic data suggest that a series of bond-forming reactions including imine formation, intermolecular Michael addition, and intramolecular Michael addition are involved to generate both the oxazole and amino acid functionalities.

2-Hydroxybenzophenone as a Chemical Auxiliary for the Activation of Ketiminoesters for Highly Enantioselective Addition to Nitroalkenes under Bifunctional Catalysis

An organocatalytic system is presented for the Michael addition of monoactivated glycine ketimine ylides with a bifunctional catalyst. The ketimine bears an ortho hydroxy group, which increases the acidity of the methylene hydrogen atoms and enhances the reactivity, thus allowing the synthesis of a large variety of α,γ-diamino acid derivatives with excellent stereoselectivity.

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

Remote C-H Hydroxylation by an α-Ketoglutarate-Dependent Dioxygenase Enables Efficient Chemoenzymatic Synthesis of Manzacidin C and Proline Analogs

Selective C-H functionalization at distal positions remains a highly challenging problem in organic synthesis. Though Nature has evolved a myriad of enzymes capable of such feat, their synthetic utility has largely been overlooked. Here, we functionally characterize an α-ketoglutarate-dependent dioxygenase (Fe/αKG) that selectively hydroxylates the δ position of various aliphatic amino acids. Kinetic analysis and substrate profiling of the enzyme show superior catalytic efficiency and substrate promiscuity relative to other Fe/αKGs that catalyze similar reactions. We demonstrate the practical utility of this transformation in the concise syntheses of a rare alkaloid, manzacidin C, and densely substituted amino acid derivatives with remarkable step efficiency. This work provides a blueprint for future applications of Fe/αKG hydroxylation in complex molecule synthesis and the development of powerful synthetic paradigms centered on enzymatic C-H functionalization logic.

Copper-catalyzed amination of an α-C(sp3)–H bond in inactivated ethers to synthesize α-aminonitriles

A copper-catalyzed functionalization of inert cyclic ethers was developed to provide α-aminonitriles via a cascade oxidation/amination/ring-opening/cyanation reaction.

Pd/Cu dual catalysis: highly enantioselective access to α-substituted α-amino acids and α-amino amides

In this work, we have developed a synergistic Pd/Cu catalyst system for the asymmetric allylation of glycine iminoesters/amides, affording a range of α-substituted α-amino acids/amides in high yields and with excellent enantioselectivities (88 → 99% ee).

Copper-catalyzed synthesis of α-amino nitriles through methyl transfer from DMF to aromatic amines

A copper-catalyzed activation of C(sp³)–H bonds of DMF at room temperature was developed, which results in methyl transfer to aromatic amines for efficient synthesis of exceedingly valuable α-amino nitriles.

Enzymatic asymmetric synthesis of chiral amino acids

This review summarizes the progress achieved in the enzymatic asymmetric synthesis of chiral amino acids from prochiral substrates.

Enantioselective synthesis of α-perfluoroalkylated prolines, their 6,7-membered homologues and derivatives

The highly enantioselective Strecker reaction with alpha-perfluoroalkylated cyclic ketimines was reported. The corresponding asymmetric cyclic nitriles are a useful tool for obtaining amino acids and their derivatives.

Selective alkylation/oxidation of N-substituted isoindolinone derivatives: synthesis of N-phthaloylated natural and unnatural α-amino acid analogues

The interchangeability of the isoindolinone group as a nitrogen protecting group for amino acid intermediates is demonstrated by the preparation of several natural and unnatural α-amino acid derivatives using a two-carbon N-isoindolinone (phthalimidine) scaffold. Using a selective benzylic oxidation, the N-isoindolinone group is then converted to the N-phthaloyl group for convenient removal (65-98%). For preparation of the isoindolinone products which were to be the substrates for benzylic oxidation, a range of side chains were installed on the isoindolinone-protected glycine equivalent on deprotonation to demonstrate the utility of the N-protected isoindolinone synthon (51-93%). While the ensuing benzylic oxidation is employed successfully for converting the N-isoindolinone group to the N-phthaloyl group in simple substrates, substrates bearing unsaturated or electron-rich side chains respond poorly to the oxidation.

Development of the Large-Scale Synthesis of Tetrahydropyran Glycine, a Precursor to the HCV NS5A Inhibitor BMS-986097

An efficient large-scale synthesis of acid 1, a penultimate precursor to the HCV NS5A inhibitor BMS-986097, along with the final API step are described. Three routes were devised for the synthesis of 1 at the various stages of the program. The third generation route, the one that proved scalable and is the main subject of this paper, features a one-step Michael addition of t-butyl 2-((diphenylmethylene)amino)acetate (24) to (E)-benzyl 4-(1-hydroxycyclopropyl)but-2-enoate (28) followed by cyclization and chiral separation to form 27c, the core skeleton of cap piece 1. The epimerization and chiral resolution of 27c followed by further synthetic manipulations involving the carbamate formation, lactone reduction and cyclization, afforded cyclopropyl pyran 1. A detailed study of diphenylmethane deprotection via acid hydrolysis as well as a key lactone to tetrahydropyran conversion, in order to avoid a side reaction that afforded an alternative cyclization product, are discussed. This synthesis was applied to the preparation of more than 100 g of the final API BMS-986097 for toxicology studies.

Chiral phase-transfer catalysis in the asymmetric α-heterofunctionalization of prochiral nucleophiles

Chiral phase-transfer catalysis is one of the major catalytic principles in asymmetric catalysis. A broad variety of different catalysts and their use for challenging applications have been reported over the last decades. Besides asymmetric C–C bond forming reactions the use of chiral phase-transfer catalysts for enantioselective α-heterofunctionalization reactions of prochiral nucleophiles became one of the most important field of application of this catalytic principle. Based on several highly spectacular recent reports, we thus wish to discuss some of the most important achievements in this field within the context of this review.

Tandem Palladium and Isothiourea Relay Catalysis: Enantioselective Synthesis of α-Amino Acid Derivatives via Allylic Amination and [2,3]-Sigmatropic Rearrangement

A tandem relay catalytic protocol using both Pd and isothiourea catalysis has been developed for the enantioselective synthesis of α-amino acid derivatives containing two stereogenic centers from readily accessible N,N-disubstituted glycine aryl esters and allylic phosphates. The optimized process uses a bench-stable succinimide-based Pd precatalyst (FurCat) to promote Pd-catalyzed allylic ammonium salt generation from the allylic phosphate and the glycine aryl ester. Subsequent in situ enantioselective [2,3]-sigmatropic rearrangement catalyzed by the isothiourea benzotetramisole forms syn-α-amino acid derivatives with high diastereo- and enantioselectivity. This methodology is most effective using 4-nitrophenylglycine esters and tolerates a variety of substituted cinnamic and styrenyl allylic ethyl phosphates. The use of challenging unsymmetrical N-allyl-N-methylglycine esters is also tolerated under the catalytic relay conditions without compromising stereoselectivity.