Amino Acid Ligand Chirality for Enantioselective Syntheses

N-Aminoacyl-3-amino-nido-carboranes as a Group of Boron-Containing Derivatives of Natural Amino Acids

A new group of nido-carboranyl derivatives of natural (S)-amino acids containing from 9 to 18 boron atoms was obtained in good yields as a result of acylation of 3-amino-1,2-dicarba-closo-dodecaborane followed by deboronation. The proposed approach is convenient and based on the use of readily available reagents and is suitable for the synthesis of enantiopure nido-carboranyl derivatives of amino acids with various side chains, including water-soluble boron-containing amino acids (17 examples).

An expeditious route to sterically encumbered nonproteinogenic α-amino acid precursors using allylboronic acids

A diastereoselective allylation of N-tert-butane sulfinyl α-iminoesters using allylboronic acids is developed to obtain optically active non-proteinogenic α-amino acid precursors in good yields and diastereoselectivities. Gram-scale synthesis, broad tolerance of functional groups, excellent stereodivergence, post-synthetic modifications, and easy removal of the chiral auxiliary are some of the key highlights. The protocol is applicable to various amino acids and short peptides, resulting in the incorporation of these precursors at the N-terminal position.

A diastereoselective allylation of N-tert-butane sulfinyl α-iminoesters using allylboronic acids is developed to obtain optically active non-proteinogenic α-amino acid precursors in good yield and diastereoselectivities.

Chemoselective and Enantioselective Fluorescent Identification of Specific Amino Acid Enantiomers

The enantiomers of chiral amino acids play versatile roles in biological systems including humans. They are also very useful in the asymmetric synthesis of diverse chiral organic compounds. Therefore, identification...

Amino Acid-Functionalized Metal-Organic Frameworks for Asymmetric Base-Metal Catalysis

We report a strategy to develop heterogeneous single-site enantioselective catalysts based on naturally occurring amino acids and earth-abundant metals for eco-friendly asymmetric catalysis. The grafting of amino acids within the pores of a metal-organic framework (MOF), followed by post-synthetic metalation with iron precursor, affords highly active and enantioselective (>99 % ee for 10 examples) catalysts for hydrosilylation and hydroboration of carbonyl compounds. Impressively, the MOF-Fe catalyst displayed high turnover numbers of up to 10 000 and was recycled and reused more than 15 times without diminishing the enantioselectivity. MOF-Fe displayed much higher activity and enantioselectivity than its homogeneous control catalyst, likely due to the formation of robust single-site catalyst in the MOF through site-isolation.

Determining the concentration and enantiomeric composition of histidine using one fluorescent probe

A chemoselective as well as enantioselective fluorescent probe has been developed to determine both the concentration and enantiomeric composition of the biologically important amino acid histidine by measuring the fluorescence responses when excited at two different wavelengths.

Immobilization of (l)-valine and (l)-valinol on SBA-15 nanoporous silica and their application as chiral heterogeneous ligands in the Cu-catalyzed asymmetric allylic oxidation of alkenes

Chiral heterogeneous ligands AL*-i-Pr-SBA-15 and AA*-i-Pr-SBA-15 were synthesized and then used in Cu-catalyzed asymmetric allylic oxidation of alkenes. Allylic esters with moderate enantiomeric excess and good yields were obtained.

Helical Nanostructures with Circularly Polarized Luminescence from the Multicomponent Assembly of π-Conjugated N-terminal Amino Acids

Self-assembled structures with circularly polarized luminescence (CPL) have attracted great attention in recent years. π-conjugated N-terminal amino acids with chiral amino acid residues and luminophores are capable of forming self-assembled structures at hierarchical levels, whereby chirality can be transferred to the macroscopic scale with easily modulated CPL properties. Due to the presence of multiple noncovalent binding sites, including hydrogen bonding and aromatic interactions, π-conjugated N-terminal amino acids are emerging core candidates for incorporation into multicomponent self-assembled architectures, accomplishing rational control over supramolecular chirality as well as showing rich chiroptical properties. In this Minireview, we provide a brief summary of multiple-component coassembled systems comprising π-conjugated N-terminal amino acids, small organic species and metal ions. The synthesis of helical structures and manipulation of supramolecular chirality by controlling the self-assembled species is introduced, and the CPL properties of multiple-component π-conjugated N-terminal amino acids are also briefly summarized.

Opposite Enantioselectivity of Mg(II) Versus Zn(II) in the Fluorescent Recognition of Amino Acids

The addition of Mg²⁺ is found to turn on the fluorescence response of a molecular probe, 3,3'-diformyl-1,1'-bi-2-naphthol, toward chiral amino acids with high enantioselectivity. It is further found that the enantioselective fluorescence responses of the molecular probe in the presence of Mg²⁺ toward certain amino acids are the opposite of those in the presence of Zn²⁺, that is, using Mg²⁺ with an l-amino acid generates much greater fluorescence enhancement than with the corresponding d-amino acid, but using Zn²⁺ with the d-amino acid gives much greater fluorescence than with the l-enantiomer. Thus, simply changing the metal cation additive allows the chirality sense of the fluorescence-based molecular recognition to be easily regulated.

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.

Theoretical Studies on Palladium-Mediated Enantioselective C-H Iodination

DFT calculations have been carried out to investigate the reaction mechanism for Pd(II)-mediated enantioselective C-H iodination. Iodination of the aryl ortho C-H bond of benzylamines catalyzed by Pd(II) diacetate complexes in the presence of the L-MPAA ligand experiences three main steps: first, C-H bond activation; second, oxidative addition of iodine on Pd(II) and reductive elimination of iodobenzene; third, catalyst regeneration through ligand exchange. The C-H bond activation is found to be the rate-determining step of the overall iodination due to higher activation energy. The reaction barrier for the formation of iodinated (R)-benzylamine is lower than that of (S)-benzylamine, which confirms the R enantioselectivity in iodination at room temperature. The retainment of the coordination of one acetic acid on Pd(II) and the chelating MPAA ligand during the catalyzed reaction are suggested to give space economy to facilitate the C-H bond activation. The NHTf functional group on the substrate is found to be very important for ortho C-H iodination at ambient condition. Our calculated results are consistent with the experimental observations.

Half-sandwich complexes of rhodium containing cysteine-derived ligands

Five distinct coordination modes have been disclosed for rhodium complexes containing modified cysteines. From the spectroscopic, theoretical and crystallographic data, the absolute configuration of the compounds has been established.

MoCl5 as an effective chlorinating agent towards α-amino acids: synthesis of α-ammonium-acylchloride salts and α-amino-acylchloride complexes

The reactivity of MoCl5 with α-amino acids was investigated for the first time by choosing CH2Cl2 as the reaction medium. The interaction of MoCl5 with l-proline proceeded with Cl/O interchange and led to the formation of [NH2(CH2)3CHC(O)Cl][MoOCl4], 1, in 63% yield. The reactions of MoCl5 with l-phenylalanine, sarcosine, N,N-dimethylglycine and N,N-dimethyl-l-phenylalanine afforded the α-amino-acylchloride complexes MoOCl3[O[double bond, length as m-dash]C(Cl)CH(R)N(R')(R'')] (R = CH2Ph, R' = R'' = H, 2a; R = R' = H, R'' = Me, 2b; R = H, R' = R'' = Me, 2c R = CH2Ph, R' = R'' = Me, 2d), in ca. 70% yields. According to DFT studies, 2a,d are mononuclear Mo(v) octahedral complexes bearing a N,O-coordinated α-amino-acylchloride ligand. The presumed species formed during the first stages of the MoCl5/l-phenylalanine interaction were DFT-elucidated, thus the calculated Gibbs free energy profile for the multi-step reaction of MoCl5 with l-phenylalanine was traced. [NH3CH(CH2Ph)C(O)Cl][MoOCl4], 3, acting as an intermediate in the course of the formation of 2a, was isolated by combination of [NH3CH(CH2Ph)C(O)Cl][Cl] with MoOCl3.

Synthesis of α-amino acidato derivatives of niobium and tantalum pentahalides and their conversion into iminium salts

Dinuclear complexes of formula Nb2Cl9[O2CCH(R)NR'R″2-κO,κO] (R = CH2CHMe2, R' = R″ = H, 1a; R = CH2Ph, R' = R″ = H, 1b; R = CH2CH2SCH3, R' = R″ = H, 1c; R = R' = H, R″ = Me, 1d; R = CH2Ph, R' = R″ = Me, 1e; Nb2Cl9[O2C⌈CH(CH2)3NH⌉], 1f) were prepared by allowing NbCl5 to react in dichloromethane with the appropriate α-amino acid in 1:2 amino acid/Nb molar ratio. The 1:1 reactions between MX5 (M = Nb, Ta; X = Cl, Br) and a series of α-amino acids resulted in the formation of the iminium salts [(R)CH═NR'R″2][MX6] (R = CH2Ph, R' = R″ = Me: M = Nb, X = Cl, 2a; M = Nb, X = Br, 2b; M = Ta, X = Cl, 2c; M = Ta, X = Br, 2d; R = CH2Ph, R' = R″ = H, M = Nb: X = Cl, 3a; X = Br, 3b; R = CH2CHMe2, R' = R″ = H, M = Nb, X = Cl, 4; R = R' = H, R″ = Me, M = Nb, X = Cl, 5). The formate/amino acidate derivative NbCl3(O2CH)[O2CCH(CH2Ph)NMe2], 6, was isolated and identified as coproduct of the 1:1 reaction between NbCl5 and N,N-dimethyl-l-phenylalanine, leading to 2a. All of the compounds were characterized by analytical and spectroscopic methods and by X-ray diffractometry in the cases of 2a, 2b, and 2d. Moreover, density functional theory studies were carried out to shed light on mechanistic and structural aspects.

Stochastic and empirical models of the absolute asymmetric synthesis by the Soai-autocatalysis

Absolute asymmetric synthesis (AAS) is the preparation of pure (or excess of one) enantiomer of a chiral compound from achiral precursor(s) by a chemical reaction, without enantiopure chiral additive and/or without applied asymmetric physical field. Only one well-characterized example of AAS is known today: the Soai-autocatalysis. In an attempt at clarification of the mechanism of this particular reaction we have undertaken empirical and stochastic analysis of several parallel AAS experiments. Our results show that the initial steps of the reaction might be controlled by simple normal distribution ("coin tossing") formalism. Advanced stages of the reaction, however, appear to be of a more complicated nature. Symmetric beta distribution formalism could not be brought into correspondence with the experimental observations. A bimodal beta distribution algorithm provided suitable agreement with the experimental data. The parameters of this bimodal beta function were determined by a Pólya-urn experiment (simulated by computer). Interestingly, parameters of the resulting bimodal beta function give a golden section ratio. These results show, that in this highly interesting autocatalysis two or even perhaps three catalytic cycles are cooperating. An attempt at constructing a "designed" Soai-type reaction system has also been made.

Structural characterization of α-amino acid complexes of molybdates: a spectroscopic and DFT study

A joint spectroscopic and computational study has allowed us to determine the dinuclear structural core of the products of the reactions between molybdates and α-amino acids in aqueous medium.

The chlorinating behaviour of WCl6 towards α-aminoacids

WCl₆ behaves as a selective chlorinating agent towards the carboxylic function of primary and secondary α-aminoacids. The initial formation of α-ammonium acylchloride salts may be followed by HCl elimination and, in the case of l-proline derived species, a clean cyclization reaction.