Enantioselective hydrogenation of alpha-aminomethylacrylates containing a free NH group for the synthesis of beta-amino acid derivatives

α-(Aminomethyl)acrylates as acceptors in radical-polar crossover 1,4-additions of dialkylzincs: insights into enolate formation and trapping

We demonstrate that α-(aminomethyl)acrylates are suitable acceptors for 1,4-additions of dialkylzincs in aerobic conditions. The air-promoted radical-polar crossover process involves the 1,4-addition of an alkyl radical followed by homolytic substitution at the zinc atom of dialkylzinc. Coordination of the nitrogen atom to zinc enables this SH2 process which represents a rare example of alkylzinc-group transfer to a tertiary α-carbonyl radical. The zinc enolate thus formed readily undergoes β-fragmentation unless it is trapped by electrophiles in situ. Enolates of substrates having free N-H bonds undergo protodemetalation to provide ultimately the 1,4-addition adduct. In the presence of carbonyl acceptors, aldol condensation occurs providing overall a tandem 1,4-addition-aldol process. When a tert-butanesulfinyl moiety is present on the nitrogen atom, these electrophilic substitution reactions occur with good levels of chiral induction, paving the way to enantioenriched β2-amino acids and β2,2-amino acids.

CO2-Assisted asymmetric hydrogenation of prochiral allylamines

A new methodology for the asymmetric hydrogenation of allylamines takes advantage of a reversible reaction between amines and carbon dioxide (CO₂) to suppress unwanted side reactions. The effects of various parameters (pressure, time, solvent, and base additives) on the enantioselectivity and conversion of the reaction were studied. The homogeneously-catalyzed asymmetric hydrogenation of 2-arylprop-2-en-1-amine resulted in complete conversion and up to 82% enantiomeric excess (ee). Added base, if chosen carefully, improves the enantioselectivity and chemoselectivity of the overall reaction.

CO₂ can act as an in situ protecting group for the asymmetric hydrogenation of allylamines.

On the Origin of Rh-Catalyzed Selective Ring-Opening Amidation of Substituted Cyclopropanols to Access β2-Amino Ketones

β²-Amino carbonyls, an α-substituted β-amino scaffold, hold a prominent place in the development of new pharmaceuticals and peptidomimetics. Herein, we report a highly efficient Rh-catalyzed ring-opening amidation of substituted cyclopropanols, which turned out to serve as a linchpin for the selective synthesis of β²-amino ketones to outcompete the formation of β³-isomers. Instead of the generally accepted rationale to consider steric factors for the β²-selectivity, orbital interaction was elucidated to play a more critical role in the amidative ring-opening of cyclopropanols to generate the key Rh-C intermediate. Subsequent inner-sphere acylnitrene transfer was achieved in excellent efficiency (TON > 5000) by using readily accessible dioxazolones as the amino source to afford β²-amino ketones with broad applicability.

Catalytic Asymmetric Synthesis of Unprotected β2-Amino Acids

We report here a scalable, catalytic one-pot approach to enantiopure and unmodified β²-amino acids. A newly developed confined imidodiphosphorimidate (IDPi) catalyzes a broadly applicable reaction of diverse bis-silyl ketene acetals with a silylated aminomethyl ether, followed by hydrolytic workup, to give free β²-amino acids in high yields, purity, and enantioselectivity. Importantly, both aromatic and aliphatic β²-amino acids can be obtained using this method. Mechanistic studies are consistent with the aminomethylation to proceed via silylium-based asymmetric counteranion-directed catalysis (Si-ACDC) and a transition state to explain the enantioselectivity is suggested on the basis of density functional theory calculation.

Water as a Direct Proton Source for Asymmetric Hydroarylation Catalyzed by a Rh(I)-Diene: Access to Nonproteinogenic β2/γ2/δ2-Amino Acid Derivatives

A highly enantioselective rhodium-catalyzed intermolecular hydroarylation of α-aminoalkyl acrylates using water as a direct proton source has been realized by employing a chiral bicyclo[3.3.0] diene ligand, allowing efficient access to a broad range of α-aryl-methyl-substituted β²-, γ²-, and δ²-amino esters with excellent enantioselectivities (up to 98% ee) under exceptionally mild conditions. By utilizing this method, a series of structurally interesting benzo-fused heterocyclic molecules and the corresponding β²-, γ²-, and δ²-amino acids are facilely constructed.

A Bulky Three-Hindered Quadrant Bisphosphine Ligand: Synthesis and Application in Rhodium-Catalyzed Asymmetric Hydrogenation of Functionalized Alkenes

A bulky three-hindered quadrant bisphosphine ligand, di-1-adamantylphosphino(tert-butylmethylphosphino)methane, named BulkyP*, has been synthesized via a convergent short pathway with chromatography-free procedures. The ligand is a crystalline solid and can be readily handled in air. Its rhodium(I) complex exhibits very high enantioselectivities and catalytic activities in the asymmetric hydrogenation of functionalized alkenes.

Access to β2-Amino Acids via Enantioselective 1,4-Arylation of β-Nitroacrylates Catalyzed by Chiral Rhodium Catalysts

The highly enantioselective conjugate addition of a variety of arylboronic acids to β-nitroacrylates is reported to provide optically active α-aryl β-nitropropionates in up to 70% yields and >99.5% ee's, which are useful building blocks for preparing chiral β²-amino acids. The applicability of this transformation is demonstrated by converting 3aa into the β²-amino acid 5 and transforming 3ap to β-amino ester 7 via reduction and reductive N-alkylation. The latter compound is a precursor for preparing ent-ipatasertib.

Catalyst-controlled switch of regioselectivity in the asymmetric allylic alkylation of oxazolones with MBHCs

A catalyst-controlled switch of regioselectivity in asymmetric allylic alkylation of oxazolones with MBHCs was described.

Highly enantioselective synthesis of non-natural aliphatic α-amino acids via asymmetric hydrogenation

A series of non-natural aliphatic α-amino acids were synthesized with high ee and yield by employing a rhodium–Duanphos complex as the catalyst. An efficient approach for the synthesis of the intermediate of the anti-diabetic drugs Alogliptin and Linagliptin in the DPP-4 inhibitor class was also developed.

Highly efficient synthesis of heterocyclic and alicyclic β2-amino acid derivatives by catalytic asymmetric hydrogenation

A valuable class of new heterocyclic and alicyclic prochiral α-aminomethylacrylates has been conveniently synthesized through a three-step transformation involving a Baylis-Hillman reaction, O-acetylation, and a subsequent allylic amination. The corresponding novel β(2)-amino acid derivatives were prepared with excellent enantioselectivities and high yields by catalytic asymmetric hydrogenation using the catalyst rhodium(Et-Duphos) (Et-Duphos = 2',5',2'',5''-tetraethyl-1,2-bis(phospholanyl)benzene)) under mild reaction conditions (up to 99 % ee and S/C = 1000). The influence of the substrate on the enantioselectivity and reactivity is investigated, and the most suitable substrate configuration for the highly efficient enantioselective hydrogenation of β-substituted α-aminomethylacrylates under the Rh-Duphos system is reported. The current protocol provides a very practical, facile, and scalable method for the preparation of heterocyclic and alicyclic β(2)-amino acids and their derivatives.

Asymmetric allylic alkylation of isatin-derived Morita-Baylis-Hillman carbonates with nitroalkanes

A stereoselective allylic alkylation of isatin-derived Morita-Baylis-Hillman (MBH) carbonates with nitroalkanes has been developed. In the presence of 10 mol % β-isocupreidine (β-ICD), 3,3'-disubstituted oxindoles were prepared with moderate diastereoselectivities and excellent enantioselectivities.