Catalytic, enantioselective alkylation of alpha-imino esters: the synthesis of nonnatural alpha-amino acid derivatives

Interrupted Polonovski Strategy for the Synthesis of Functionalized Amino Acids and Peptides

The α-functionalization of carbamate-protected hydroxylamine glycine derivatives, acting as imine surrogates via an interrupted Polonovski reaction, is described to access functionalized amino acid derivatives. The addition of C, N, O, and S nucleophiles was achieved in a one-pot procedure in 37% to 92% yield. This method could be extended to dipeptide derivatives for the functionalization of both the C-terminus and N-terminus.

Organocatalytic enantioselective Mannich and retro-Mannich reactions and combinations of these reactions to afford tetrasubstituted α-amino acid derivatives

Organocatalytic asymmetric Mannich reactions and kinetic resolutions of the products via retro-Mannich reactions that afford enantiomerically enriched tetrasubstituted α-amino acid derivatives (α,α-disubstituted-α-amino acid derivatives) were developed. Furthermore, the combination of the Mannich reaction and the retro-Mannich reaction allowed access to products with almost perfect enantiopurities.

Introducing N-Sulfinylamines into Visible-Light-Induced Carbene Chemistry for the Synthesis of Diverse Amides and α-Iminoesters

A rare example of visible-light-mediated diverse reactivity of N-sulfinylamines with different types of carbene precursors has been disclosed. Acylsilanes and aryldiazoacetates have been utilized as nucleophilic and electrophilic carbene precursors into the N═S═O linchpin, to achieve valuable amides and α-iminoesters, respectively. Interestingly, diazocarbonyls can also participate in the amidation reaction with N-sulfinylamines via in situ generated ketenes. This operationally simple modular method offers a mild, transition-metal-free, and coupling-reagent-free protocol to fabricate structurally diverse amides and a promptly accessible technique to achieve α-iminoesters, where visible light remains as a key promoter.

Application of reductive amination by heterologously expressed Thermomicrobium roseumL-alanine dehydrogenase to synthesize L-alanine derivatives

Unnatural amino acids are unique building blocks in modern medicinal chemistry as they contain an amino and a carboxylic acid functional group, and a variable side chain. Synthesis of pure unnatural amino acids can be made through chemical modification of natural amino acids or by employing enzymes that can lead to novel molecules used in the manufacture of various pharmaceuticals. The NAD+ -dependent alanine dehydrogenase (AlaDH) enzyme catalyzes the conversion of pyruvate to L-alanine by transferring ammonium in a reversible reductive amination activity. Although AlaDH enzymes have been widely studied in terms of oxidative deamination activity, reductive amination activity studies have been limited to the use of pyruvate as a substrate. The reductive amination potential of heterologously expressed, highly pure Thermomicrobium roseum alanine dehydrogenase (TrAlaDH) was examined with regard to pyruvate, α-ketobutyrate, α-ketovalerate and α-ketocaproate. The biochemical properties were studied, which included the effects of 11 metal ions on enzymatic activity for both reactions. The enzyme accepted both derivatives of L-alanine (in oxidative deamination) and pyruvate (in reductive amination) as substrates. While the kinetic K_M values associated with the pyruvate derivatives were similar to pyruvate values, the kinetic k_cat values were significantly affected by the side chain increase. In contrast, K_M values associated with the derivatives of L-alanine (L-α-aminobutyrate, L-norvaline, and L-norleucine) were approximately two orders of magnitude greater, which would indicate that they bind very poorly in a reactive way to the active site. The modeled enzyme structure revealed differences in the molecular orientation between L-alanine/pyruvate and L-norleucine/α-ketocaproate. The reductive activity observed would indicate that TrAlaDH has potential for the synthesis of pharmaceutically relevant amino acids.

B(C6 F5 )3 /Chiral Phosphoric Acid Catalyzed Ketimine-Ene Reaction of 2-Aryl-3H-indol-3-ones and α-Methylstyrenes

The enantioselective ketimine-ene reaction is one of the most challenging stereocontrolled reaction types in organic synthesis. In this work, catalytic enantioselective ketimine-ene reactions of 2-aryl-3H-indol-3-ones with α-methylstyrenes were achieved by utilizing a B(C₆ F₅ )₃ /chiral phosphoric acid (CPA) catalyst. These ketimine-ene reactions proceed well with low catalyst loading (B(C₆ F₅ )₃ /CPA=2 mol %/2 mol %) under mild conditions, providing rapid and facile access to a series of functionalized 2-allyl-indolin-3-ones with very good reactivity (up to 99 % yield) and excellent enantioselectivity (up to 99 % ee). Theoretical calculations reveal that enhancement of the acidity of the chiral phosphoric acid by B(C₆ F₅ )₃ significantly reduces the activation free energy barrier. Furthermore, collective favorable hydrogen-bonding interactions, especially the enhanced N-H⋅⋅⋅O hydrogen-bonding interaction, differentiates the free energy of the transition states of CPA and B(C₆ F₅ )₃ /CPA, thereby inducing the improvement of stereoselectivity.

Enantioselective Synthesis of α-Allyl Amino Esters via Hydrogen-Bond-Donor Catalysis

We report a chiral-squaramide-catalyzed enantio- and diastereoselective synthesis of α-allyl amino esters. The optimized protocol provides access to N-carbamoyl-protected amino esters via nucleophilic allylation of readily accessible α-chloro glycinates. A variety of useful α-allyl amino esters were prepared, including crotylated products bearing vicinal stereocenters that are inaccessible through enolate alkylation, with high enantioselectivity (up to 97% ee) and diastereoselectivity (>10:1). The reactions display first-order kinetic dependence on both the α-chloro glycinate and the nucleophile, consistent with rate-limiting C-C bond formation. Computational analysis of the uncatalyzed reaction predicts an energetically inaccessible iminium intermediate, and a lower energy concerted SN2 mechanism.

Selective Aerobic Oxidation of Benzylic Alcohols Catalyzed by a Dicyclopropenylidene-Ag(I) Complex

The unprecedented synthesis, single-crystal X-ray structure, and first catalytic application of a dicarbene-Ag(I) complex [Ag(BAC)₂][CO₂CF₃] (BAC = bis(diisopropyl)aminocyclopropenylidene) is reported. This novel complex provides a versatile catalytic platform for selective aerobic oxidation of benzylic alcohols to aldehyde or ketone products in high yields. Ease of experimental execution coupled with the use of abundant atmospheric molecular oxygen as an oxidant and low catalyst loading are inherit strengths of these oxidations.

F10BINOL-derived chiral phosphoric acid-catalyzed enantioselective carbonyl-ene reaction: theoretical elucidation of stereochemical outcomes

An enantioselective carbonyl-ene reaction of 1,1-disubstituted olefins with ethyl glyoxylate was accomplished using an F₁₀BINOL-derived chiral phosphoric acid of which the perfluoro-binaphthyl skeleton is beneficial not only for adopting high catalytic activity but also for creating an effective chiral environment.

An F₁₀BINOL-derived chiral phosphoric acid was shown to be an effective catalyst for an enantioselective carbonyl-ene reaction of 1,1-disubstituted olefins with ethyl glyoxylate as the common enophile. The perfluoro-binaphthyl skeleton is beneficial not only for adopting high catalytic activity but also for creating an effective chiral environment for enantioselective transformations. Indeed, the reaction afforded enantio-enriched homoallylic alcohols in high yields with high enantioselectivities. Theoretical studies identified that the multi-point C–H···O hydrogen bonds and the π interactions between the substrates and the 6-methoxy-2-naphthyl substituents at the 3,3′-positions of the F₁₀BINOL skeleton play a crucial role in determining the stereochemical outcomes. The significance of the perfluoro-binaphthyl skeleton in achieving the high enantioselectivity was also evaluated through a structural analysis of the catalysts.

Enantioselective Cycloaddition of Styrenes with Aldimines Catalyzed by a Chiral Magnesium Potassium Binaphthyldisulfonate Cluster as a Chiral Brønsted Acid Catalyst

A chiral magnesium potassium binaphthyldisulfonate cluster, as a chiral Brønsted acid catalyst, was shown to catalyze an enantioselective cycloaddition of styrenes with aldimines for the first time. The strong Brønsted acidity of the catalyst precursors, which might dissolve drying agents and take up the leached Mg²⁺ and K⁺, serendipitously led to good enantioselectivity. Mechanistic aspects were supported by X-ray and ESI-MS analysis of the catalyst and a kinetics study of the reaction. Useful transformations to optically active 1,3-amino alcohols on a gram scale were also demonstrated.

Enantioselective copper catalysed, direct functionalisation of allenes via allyl copper intermediates

We explore the breadth of the copper catalysed functionalisation of allenes, which enables efficient access to enantioenriched, densely functionalised molecules.

The direct functionalisation of allenes under copper catalysis enables efficient access to enantioenriched, densely functionalised molecules. In this review we explore the breadth and depth of a versatile reaction manifold, which involves the element-cupration of allenes to generate allyl copper intermediates that are subsequently coupled with diverse arrays of electrophiles.

Enantioselective Generation of Adjacent Stereocenters in a Copper-Catalyzed Three-Component Coupling of Imines, Allenes, and Diboranes

A highly enantio- and diastereoselective copper-catalyzed three-component coupling affords the first general synthesis of homoallylic amines bearing adjacent stereocenters from achiral starting materials. The method utilizes a commercially available NHC ligand and copper source, operates at ambient temperature, couples readily available simple imines, allenes, and diboranes, and yields high-value homoallylic amines that exhibit versatile amino, alkenyl, and boryl units.

Lewis Acid Catalyzed Borotropic Shifts in the Design of Diastereo- and Enantioselective γ-Additions of Allylboron Moieties to Aldimines

Catalytic allylboron additions to aldimines are presented for which small amounts of Zn(OMe)2 serve as the co-catalyst to accelerate allyl exchange and 1,3-borotropic shift processes. Low-yielding and moderately α- and diastereoselective reactions are thus turned into highly efficient γ-, diastereo-, and enantioselective transformations that exhibit considerable scope.

A Au(I)-Precatalyst with a Cyclopropenium Counterion: An Unusual Ion Pair

The synthesis and X-ray crystal structure of a novel Au(I)-precatalyst applied to intermolecular alkyne hydroamination is reported. Density functional theory (DFT) calculations revealed the cyclopropenium counterion of this Au(I)-precatalyst imparts stability through H-bonding and other noncovalent interactions.

A highly enantioselective Hg(ii)-catalyzed Sakurai–Hosomi reaction of isatins with allyltrimethylsilanes

A chiral complex derived from (S)-difluorophos and Hg(OTf)₂ is identified as a powerful catalyst for the Sakurai–Hosomi reaction of isatins with allyltrimethylsilane, allowing the facile synthesis of valuable building blocks 3-allyl-3-hydroxyoxindoles in up to 97% ee, with only 0.5–1.0 mol% of catalyst loading.

Facile Alder-Ene Reactions of Silylallenes Involving an Allenic C(sp(2) )-H Bond

Facile and selective Alder-ene reactions of silylallenes involving the activation of an allenic C(sp(2) )H over an allylic C(sp(3) )H bond is described. In this ene reaction, the presence of a silyl substituent was found to be critical for the observed reactivity and selectivity since the corresponding alkyl-substituted allenes show different reaction profiles. Computational studies show that the origin of this unusual reactivity is the lower bond dissociation energy of the α-C(sp(2) )H bond in silylallenes compared to the corresponding nonsilylated allenes.

Stereoselective ZrCl4-Catalyzed Mannich-type Reaction of β-Keto Esters with Chiral Trifluoromethyl Aldimines

A method for the synthesis of fluorinated β'-amino β-dicarbonyl compounds using a Zr-catalyzed Mannich-type reaction has been developed, starting from N-protected trifluoromethyl aldimines and cyclic or acyclic β-keto esters bearing different ester residues. The in situ generated metallic complex reacted with optically pure trifluoromethyl aldimine derived from (R)-α-methylbenzylamine, giving a highly diastereoselective asymmetric Mannich-type addition with formation of a chiral quaternary center. The absolute configuration at the new chiral centers was assigned through two-dimensional nuclear Overhauser effect spectroscopic analysis coupled with computational studies.

Asymmetric allylation of sulfonyl imines catalyzed by in situ generated Cu(ii) complexes of chiral amino alcohol based Schiff bases

We have developed a catalytic route for enantioselective synthesis of homoallyl amines through Cu(ii)-Schiff base catalyzed allylation of aryl N-sulfonylimines.

Simple organic molecules as catalysts for enantioselective synthesis of amines and alcohols

The discovery of catalysts that can be used to synthesize complex organic compounds by enantioselective transformations is central to advances in the life sciences; for this reason, many chemists aim to discover catalysts that allow for preparation of chiral molecules as predominantly one mirror-image isomer. The ideal catalyst should not contain precious elements and should bring reactions to completion in a few hours through operationally simple procedures. Here we introduce a set of small organic molecules that can catalyse reactions of unsaturated organoboron reagents with imines and carbonyls; the products of the reactions are enantiomerically pure amines and alcohols, which might serve as intermediates in the preparation of biologically active molecules. A distinguishing feature of this catalyst class is the presence of a 'key' proton embedded within their structure. Catalysts are derived from the abundant amino acid valine and are prepared in large quantities in four steps with inexpensive reagents. Reactions are scalable, do not demand stringent conditions, and can be performed with as little as 0.25 mole per cent catalyst in less than six hours at room temperature to generate products in more than 85 per cent yield and ≥97:3 enantiomeric ratio. The efficiency, selectivity and operational simplicity of the transformations and the range of boron-based reagents are expected to render this advance important for future progress in syntheses of amines and alcohols, which are useful in chemistry, biology and medicine.