Asymmetric Strecker Synthesis of α-Amino Acids via a Crystallization-Induced Asymmetric Transformation Using (R)-Phenylglycine Amide as Chiral Auxiliary

Construction of metal-organic framework-based multienzyme system for L-tert-leucine production

Chiral compounds are important drug intermediates that play a critical role in human life. Herein, we report a facile method to prepare multi-enzyme nano-devices with high catalytic activity and stability. The self-assemble molecular binders SpyCatcher and SpyTag were fused with leucine dehydrogenase and glucose dehydrogenase to produce sc-LeuDH (SpyCatcher-fused leucine dehydrogenase) and GDH-st (SpyTag-fused glucose dehydrogenase), respectively. After assembling, the cross-linked enzymes LeuDH-GDH were formed. The crosslinking enzyme has good pH stability and temperature stability. The coenzyme cycle constant of LeuDH-GDH was always higher than that of free double enzymes. The yield of L-tert-leucine synthesis by LeuDH-GDH was 0.47 times higher than that by free LeuDH and GDH. To further improve the enzyme performance, the cross-linked LeuDH-GDH was immobilized on zeolite imidazolate framework-8 (ZIF-8) via bionic mineralization, forming LeuDH-GDH @ZIF-8. The created co-immobilized enzymes showed even better pH stability and temperature stability than the cross-linked enzymes, and LeuDH-GDH@ZIF-8 retains 70% relative conversion rate in the first four reuses. In addition, the yield of LeuDH-GDH@ZIF-8 was 0.62 times higher than that of LeuDH-GDH, and 1.38 times higher than that of free double enzyme system. This work provides a novel method for developing multi-enzyme nano-device, and the ease of operation of this method is appealing for the construction of other multi-enzymes @MOF systems for the applications in the kinds of complex environment.

Enzyme- and Chemo-enzyme-Catalyzed Stereodivergent Synthesis

Multiple stereoisomers can be found when a substance contains chiral carbons in its chemical structure. To obtain the desired stereoisomers, asymmetric synthesis was proposed in the 1970s and developed rapidly at the beginning of this century. Stereodivergent synthesis, an extension of asymmetric synthesis in organic synthesis with the hope to produce all stereoisomers of chiral substances in high conversion and selectivity, enriches the variety of available products and serves as a reference suggestion for the synthesis of their derivatives and other compounds. Since biocatalysis has outstanding advantages of economy, environmental friendliness, high efficiency, and reaction at mild conditions, the biocatalytic reaction is regarded as an efficient strategy to perform stereodivergent synthesis. Thus, in this review, we summarize the stereodivergent synthesis catalyzed by enzymes or chemo-enzymes in cases where a compound contains two or three chiral carbons, i.e., at most four or eight stereoisomers are present. The types of reactions, including reduction of substituent ketones, cyclization reactions, olefin addition, and nonredox transesterification reactions, are also discussed for the understanding of the progress and application of biocatalysis in stereodivergent synthesis.

A Four-Step Enzymatic Cascade for Efficient Production of L-Phenylglycine from Biobased L-Phenylalanine

Enantiopure amino acids are of particular interest in the agrochemical and pharmaceutical industries. Here, we reported a multi-enzyme cascade for efficient production of L-phenylglycine (L-Phg) from biobased L-phenylalanine (L-Phe). We first attempted to engineer Escherichia coli for expressing L-amino acid deaminase (LAAD) from Proteus mirabilis, hydroxymandelate synthase (HmaS) from Amycolatopsis orientalis, (S)-mandelate dehydrogenase (SMDH) from Pseudomonas putida, the endogenous aminotransferase (AT) encoded by ilvE and L-glutamate dehydrogenase (GluDH) from E. coli. However, 10 mM L-Phe only afforded the synthesis of 7.21 ± 0.15 mM L-Phg. The accumulation of benzoylformic acid suggested that the transamination step might be rate-limiting. We next used leucine dehydrogenase (LeuDH) from Bacillus cereus to bypass the use of L-glutamate as amine donor, and 40 mM L-Phe gave 39.97 ± 3.84 mM (6.04 ± 0.58 g/L) L-Phg, reaching 99.9% conversion. In summary, this work demonstrated a concise four-step enzymatic cascade for the L-Phg synthesis from biobased L-Phe, with a potential for future industrial applications.

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.

Influence of Ostwald's Rule of Stages in the Deracemization of a Compound Using a Racemic Resolving Agent

The stereoisomeric system of rac-2-phenylglycinamide (PGA) and rac-N-acetyl tryptophan (NAT) is significant in the application of chiral resolution because it has been shown that this system can be used for enantioseparation of PGA and/or NAT using a novel deracemization route of the conglomerate salt formed. However, it was also found that the conglomerate salt eventually converted into different crystal forms that limited the time available for the separation. Herein, we try to understand the phase conversion occurring in this system using DSC, PXRD, and SC-XRD. The related structures of the salt (two polymorphs of the more stable homochiral (dd- and ll-) salts and one polymorph of the less stable heterochiral (dl- and ld-) monohydrate salts) are demonstrated and discussed relating to their relative stabilities. The successful deracemization was demonstrated using the heterochiral (dl- or ld-) monohydrate salts. However, following Ostwald's rule of stages, only limited time is available for the deracemization before the metastable compound converts into the more stable homochiral (dd- and ll-) pair. Moreover, the occurrence of the (dd- and ll-) phase always coincides with the formation of yet another phase of the racemic compound containing four components in a crystal. Ostwald's rule of stages here thus involves three steps and phases and is highly significant during the deracemization of the homochiral species.

Cyclic tailor-made amino acids in the design of modern pharmaceuticals

Tailor-made AAs are indispensable components of modern medicinal chemistry and are becoming increasingly prominent in new drugs. In fact, about 30% of small-molecule pharmaceuticals contain residues of tailor-made AAs or structurally related diamines and amino-alcohols. Cyclic tailor-made AAs present a particular value to rational structural design by virtue of their local conformational constraints and are widely used in lead optimization programs. The present review article highlights 34 compounds, all of which are derived from cyclic AAs, representing recently-approved, small-molecule pharmaceuticals as well as promising drug candidates currently in various phases of clinical study. For each compound, the discussion includes the discovery, therapeutic profile and optimized synthesis, with a focus on the preparation of cyclic tailor-made AA as the principal structural feature. The present review article is intended to serve as a reference source for organic, medicinal and process chemists along with other professionals working in the fields of drug design and pharmaceutical discovery.

The Strecker reaction coupled to Viedma ripening: a simple route to highly hindered enantiomerically pure amino acids

The Strecker reaction is broadly used for the preparation of α-amino acids. However, control of enantioselectivity remains challenging. We here couple the Strecker reaction to Viedma ripening for the absolute asymmetric synthesis of highly sterically hindered α-amino acids. As proof-of-principle, the enantiomerically pure α-amino acids tert-leucine and α-(1-adamantyl)glycine were obtained.

Design, synthesis, and evaluation of new α-aminonitrile-based benzimidazole biomolecules as potent antimicrobial and antitubercular agents

The study explores the one-pot synthesis of novel α-aminonitriles by reacting 4-[(1H-benzimidazol-2-yl)methoxy]benzaldehyde, substituted anilines and sodium cyanide using a catalytic amount of copper dipyridine dichloride (CuPy₂ Cl₂ ) and employing the Strecker reaction under mild conditions. All the synthesized compounds were screened for antimicrobial and antitubercular activity. The promising lead compounds 4d and 4e were identified, with MIC values ranging between 3.9 and 7.8 µg/mL against different bacterial strains. Compounds 4c-e and 4g also showed good antifungal activities against the tested fungal strain. Among those tested, compound 4e exhibited excellent antitubercular activity (MIC 0.05 μg/mL) with a low level of cytotoxicity, suggesting that compound 4e is a promising lead for subsequent investigations in search for new antitubercular agents.

Betti's base for crystallization-induced deracemization of substituted aldehydes: synthesis of enantiopure amorolfine and fenpropimorph

The acid-promoted crystallization-induced diastereoisomer transformation (CIDT) of naphthoxazines derived from racemic O-protected 2-substituted 4-hydroxybutyraldehydes and enantiopure Betti's base allows the deracemization of the starting aldehydes with ee up to 96%. As an alternative, reduction with lithium aluminum hydride of the diastereoisomerically enriched naphthoxazines leads to enantioenriched primary amines. The utility of the latter strategy was demonstrated by applying it to the synthesis of enantioenriched fenpropimorph and to the first synthesis of enantiopure amorolfine, with ee up to 99.5%.

A short synthesis of d-[1-(14) C]-serine of high enantiomeric purity

Herein, we report a short, three-step synthesis of d-[1-(14) C]-serine (4) in high enantiomeric purity. Starting from [(14) C]-KCN and 2-(benzyloxy)acetaldehyde, Strecker reaction using (R)-1-phenylethylamine as the chiral auxiliary gave two diastereomeric aminonitriles 1 and 2 in the ratio of 4:3, which were conveniently separated and purified chromatographically. Following hydrolysis and subsequent hydrogenolysis, the purified major diastereomer 1, was smoothly converted to d-[1-(14) C]-serine (4) in an enantiomeric excess of >99%, thus circumventing time intensive chiral HPLC enantiomeric resolution.

TiCl4-promoted condensation of methyl acetoacetate, isobutyraldehyde, and indole: a theoretical and experimental study

A combined DFT and experimental study of the title reaction points to a three-step mechanism: (1) titanium enolate formation, (2) Knoevenagel condensation of enolate and aldehyde, and (3) Michael addition of indole.

Yonemitsu-type condensations catalysed by proline and Eu(OTf)3

The reaction of aromatic heterocycles, aldehydes, and active methylene compounds affords polyfunctionalised heterocycles. The reaction is green and provides access to compounds that cannot be obtained by classical methodology.

Radical migration-addition of N-tert-butanesulfinyl imines with organozinc reagents

A novel migration-addition sequence was discovered for the reaction of enantioenriched N-tert-butanesulfinyl iminoacetate 1a with functionalized benzylzinc bromide reagents, producing tert-leucine derivatives in excellent diastereoselectivity (dr 98:2). The absolute configurations of two new chiral centers were unambiguously assigned by chemical transformations and X-ray crystallography. In addition, the regio- and diastereoselectivities of this novel reaction were both explained through the key N-sulfinamine intermediate M6 generated by the tert-butyl radical attack on the imine. Computational analysis of this reaction process, which was performed at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31G*-LANL2DZ level, also supported our proposed two-stage mechanism.

Recent Progress on the Stereoselective Synthesis of Cyclic Quaternary alpha-Amino Acids

The most recent papers describing the stereoselective synthesis of cyclic quaternary alpha-amino acids are collected in this review. The diverse synthetic approaches are classified according to the size of the ring and taking into account the bond that is formed to complete the quaternary skeleton.

A Novel Crystallization-Induced Diastereomeric Transformation Based on a Reversible Carbon−Sulfur Bond Formation. Application to the Synthesis of a γ-Secretase Inhibitor

This paper describes a remarkably efficient process for the preparation of gamma-secretase inhibitor 1. The target is synthesized in only five steps with an overall yield of 58%. The key operation is a highly selective and practical, crystallization-driven transformation for the conversion of a mixture of tertiary benzylic alcohols into the desired sulfide diastereomer with 94:6 dr. This unprecedented process is based upon a reversible carbon-sulfur bond formation under acidic conditions.

Asymmetric multicomponent reactions (AMCRs): the new frontier

Asymmetric multicomponent reactions involve the preparation of chiral compounds by the reaction of three or more reagents added simultaneously. This kind of addition and reaction has some advantages over classic divergent reaction strategies, such as lower costs, time, and energy, as well as environmentally friendlier aspects. All these advantages, together with the high level of stereoselectivity attained in some of these reactions, will force chemists in industry as in academia to adopt this new strategy of synthesis, or at least to consider it as a viable option. The positive aspects as well as the drawbacks of this strategy are discussed in this Review.

Copper(ii)-mediated resolution of α-halo carboxylic acids with chiral O,O′-dibenzoyltartaric acid: spontaneous racemization and crystallization-induced dynamic resolution

Herein we present a new example of coordination-mediated resolution of racemic acids by a chiral acid. The reaction of copper(II) acetate monohydrate, optically pure O,O'-dibenzoyltartaric acid (DBTA) and racemic alpha-bromo-2-chlorophenylacetic acid (HL1) in acetonitrile solution afforded a binuclear copper(II) complex with D-DBTA dianion, alpha-bromo-2-chlorophenylacetate and acetate as ligands. After decomposition of the complex with acid, the optically active acid ((R)-HL1) was obtained. Similarly, alpha-bromo-2-fluorophenylacetic acid (HL2), alpha-bromo-2-bromophenylacetic acid (HL3), alpha-chloro-2-chlorophenylacetic acid (HL4), alpha-chloro-2-fluorophenylacetic acid (HL5), alpha-bromophenylacetic acid (HL6), alpha-bromo-4-chlorophenylacetic acid (HL7), 2-bromopropionic acid (HL8) and 2-chloropropionic acid (HL9) were resolved by the same method. Satisfactory results were obtained for HL2 to HL5. For HL6 and HL7, only racemic acids were obtained. For the two alpha-halo aliphatic acids (HL8 and HL9), poor enantioselectivity was obtained. It is more interesting that three acids (HL1, HL2 and HL3) could spontaneously racemize in acetonitrile solution, which resulted in crystallization-induced dynamic resolution (CIDR) with greater than 50% yield.

Epimerization of Diastereomeric α-Amino Nitriles to Single Stereoisomers in the Solid State

[reaction: see text] A diastereomeric mixture of the alpha-amino nitrile prepared by the Strecker reaction of benzaldehyde, (1S,2R)-1-aminoindan-2-ol, and cyanotrimethylsilane thermally epimerizes in the solid state to give a single diastereomer with an (S)-configuration at the alpha position to the nitrile moiety. This shows a sharp contrast to the reaction conducted in DMSO at room temperature, which gives a 1:1 mixture of (S)- and (R)-isomers. Several other alpha-amino nitriles also epimerize in the solid-state toward single diastereomers.

New Resolution of 2-Formyl-1,4-DHP Derivatives Using CIDR Methodology. Facile Access to New Chiral Tricyclic Thiolactam

(R)- and (S)-alpha-phenylethylamine (alpha-PEA: 7) have been used separately to resolve successfully a racemate 2-formyl-1,4-DHP derivative 4. The process was based on the difference of the solubility of both Schiff bases (6) since one of them crystallized out from the solution. These imines obtained by condensation of (R)-alpha-PEA (7) or (S)-alpha-PEA (7) with aldehyde (rac-4) were separated and analyzed by X-ray diffraction, and their exposition to an hydrochloric hydrolysis conditions led to the enantiopure (4R)-4 or (4S)-4 in excellent yields. Separate condensation of other chiral (8 and 13) and racemic (18) amino thiols as auxiliary with rac-4, (4S)-4, or (4R)-4 is accompanied by an in situ crystallization-induced dynamic resolution, whereby one distereomer of thiazole template selectively precipitates and can be isolated by simple filtration in 76-82% yield with dr > 99. The thiazole species isolated from this process resulted from an amino aldehyde condensation followed by a spontaneous thiol-imine cycloaddition. Finally, the racemate (+/-)-(4R,2'R)-19 and the diastereomerically pure homologous (4S,2'R)-23 and (4R,2'S)-20 (obtained in good yields (79-82%) from 2-aminoethanethiol (18) and 2-formyl-1,4-DHP derivative rac-4, (4S)-4, or (4R)-4, respectively) were converted conveniently in a one-pot procedure into newly tricyclic thiolactams in the DHP series in racemic ((+/-)-(6R,9bR)-21, 72% yield)) and enantiopure ((6S,9bR)-24, 71% yield); (6R,9bS)-24, 70% yield) forms.

Diastereoselective addition of allylzinc bromide to imines derived from (R)-phenylglycine amide

The highly diastereoselective addition of allylzinc bromide to imines derived from (R)-phenylglycine amide is reported. Homoallylamines with high enantiomeric purity are obtained from the adducts in three steps on removal of the chiral auxiliary by means of a nonreductive protocol. Removal of the auxiliary by hydrogenation leads to the saturated amines, also in high enantiomeric purity. [reaction: see text]