An enantioselective synthesis of β2-amino acid derivatives

Metal-Free Stereoselective Synthesis of (E)- and (Z)-N-Monosubstituted β-Aminoacrylates via Condensation Reactions of Carbamates

N-monosubstituted β-aminoacrylates are building blocks, which have been used in the preparation of amino acids and pharmaceuticals. Two efficient, stereoselective methods of preparation, via acid- or base-promoted condensation reactions of carbamates, are described. The base-promoted reaction is E-selective, while acid catalysis can, through the choice of solvent, selectively form E or Z. The acid-catalyzed E-selective process proceeds through a crystallization obviating the need for chromatographic purification.

Enantioselective Iridium-Catalyzed Allylation of Nitroalkanes: Entry to β-Stereogenic α-Quaternary Primary Amines

The first systematic study of simple nitronate nucleophiles in iridium-catalyzed allylic alkylation is described. Using a tol-BINAP-modified π-allyliridium C,O-benzoate catalyst, α,α-disubstituted nitronates substitute racemic branched alkyl-substituted allylic acetates, thus providing entry to β-stereogenic α-quaternary primary amines. DFT calculations reveal early transition states that render the reaction less sensitive to steric effects and distinct trans-effects of diastereomeric chiral-at-iridium π-allyl complexes that facilitate formation of congested tertiary-quaternary C-C bonds.

Rhodium-catalyzed asymmetric hydrogenation of β-branched enamides for the synthesis of β-stereogenic amines

β-Branched simple enamides were hydrogenated to give β-stereogenic amines in quantitative yields and with excellent enantioselectivities.

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.

rac-Ethyl (2Z)-3-{2-[(Z)-4-eth-oxy-4-oxobut-2-en-2-yl-amino]-cyclo-hexyl-amino}-but-2-enoate

The asymmetric unit of the title compound, C(18)H(30)N(2)O(4), contains two independent mol-ecules. In each mol-ecule, the cyclo-hexane ring adopts a chair conformation with equatorial orientation of the substituents, and the conformation is stabilized by two intra-molecular N-H⋯O hydrogen bonds, forming rings of S(6) graph-set motif. One eth-oxy group and one ethyl group are disordered over two sets of sites with refined occupancy ratios of 0.704 (2):0.296 (2) and 0.505 (3):0.495 (3), respectively. In the crystal, a weak inter-molecular C-H⋯O hydrogen inter-action is observed, involving the O atom of the major component of the disordered eth-oxy group.

Chiral N-phosphonyl imine chemistry: asymmetric additions of ester enolates for the synthesis of beta-amino acids

Chiral phosphonyl imines attached by 1-naphthyl protection group were found to react with lithium ester enolates smoothly and give chiral beta-amino esters in good yields (70-88%) and up to excellent diastereoselectivity (>99:1 dr). Triisopropoxytitanium (IV) chloride was found to enhance diastereoseletivity when used as the Lewis acid promoter. The chiral auxiliary can be readily removed by treating with HBr to give free amino esters. The absolute structure has been unambiguously determined by converting one of the products into an authentic sample. This reaction provides an easy access to beta-amino acid derivatives.

Direct C-Arylation of β-Enamino Esters and Ketones with Arynes

An efficient, mild, and general method for the C-arylation of beta-enamino esters and ketones with arynes has been developed. This methodology provides a facile and direct access to a variety of substituted aromatic beta-enamino compounds in moderate to excellent yield. [reaction: see text]

Enantioselective synthesis of β2-amino acids using rhodium-catalyzed hydrogenation

A series of protected beta2-dehydroamino acids has been prepared in three steps from commercially available starting materials in good yields. These were used as substrates in rhodium-catalyzed asymmetric hydrogenation applying a mixed ligand system of monodentate phosphoramidites and phosphines. Optimization of the catalyst structure was achieved by high throughput experimentation. High enantioselectivities were obtained (up to 91%) with full conversion for a number of beta-amino acids.

Asymmetric synthesis of β2-amino acids: 2-substituted-3-aminopropanoic acids from N-acryloyl SuperQuat derivatives

Conjugate addition of lithium dibenzylamide to (S)-N(3)-acryloyl-4-isopropyl-5,5-dimethyloxazolidin-2-one (derived from l-valine) and alkylation of the resultant lithium beta-amino enolate provides, after deprotection, a range of (S)-2-alkyl-3-aminopropanoic acids in good yield and high ee. Alternatively, via a complementary pathway, conjugate addition of a range of secondary lithium amides to (S)-N(3)-(2'-alkylacryloyl)-4-isopropyl-5,5-dimethyloxazolidin-2-ones, diastereoselective protonation with 2-pyridone, and subsequent deprotection furnishes a range of (R)-2-alkyl- and (R)-2-aryl-3-aminopropanoic acids in good yield and high ee. Additionally, the boron-mediated aldol reaction of beta-amino N-acyl oxazolidinones is a highly diastereoselective method for the synthesis of a range of beta-amino-beta'-hydroxy N-acyl oxazolidinones.

Asymmetric Bioreductions of β-Nitro Acrylates as a Route to Chiral β2-Amino Acids

[Structure: see text] Reductions of beta-nitroacrylates by Saccharomyces carlsbergensis old yellow enzyme is the key step in a concise route to optically active beta2-amino acids. The enzymatic reductions occur with 87-96% ee, with larger substrates providing greater stereoselectivities. This work extends enantioselective enzymatic alkene reductions to include acyclic systems with weakly coordinating substituents.

Rhodium-Catalyzed Enantioselective Hydrogenation of β-Phthalimide Acrylates to Synthesis of β2-Amino Acids

[Structure: see text] The enantioselective hydrogenation of beta-phthalimide acrylates provides the corresponding chiral beta2-amino acids in excellent enantiomeric excess catalyzed by Rh-monophosphorus.