Anti Diastereomers Research Articles

Fluorene and Fluorenone Unnatural Amino Acid Motifs via Diastereoselective Pd(II)‐Catalyzed sp3C−H Functionalization

AbstractThis paper reports the construction of racemic and enantiopure (D‐ and L‐) fluorene and fluorenone‐based novel unnatural amino acid derivatives. The Pd(II)‐catalyzed bidentate directing group‐aided arylation of prochiral β‐C(sp3)‐H bonds in racemic, enantiopure carboxamides of amino acids with iodofluorenes was reported. The construction of novel examples of fluorene and fluorenone motifs containing amino acids comprising of norvaline, leucine, phenylalanine, norleucine, 2‐aminooctanoic acid derivatives having the anti‐stereochemistry (with good enantiopurity) was accomplished. The construction of bis fluorenyl alanine scaffolds and fluorenyl motif‐containing non‐α‐amino acid (aminoalkanoic acid) derivatives was also reported. Examples of removal of the directing group (8‐aminoquinoline) and phthalimide‐protecting group present in the fluorenyl‐based unnatural amino acid derivatives have been shown. Accordingly, fluorene motif‐based amino acid esters and free amino group‐containing unnatural amino acid derivatives were synthesized. The stereochemistry of the major (anti) diastereomers obtained via the prochiral β‐C(sp3)‐H arylation of norvaline, leucine, phenylalanine, norleucine, 2‐aminooctanoic acid derivatives was ascertained from the X‐ray structure of a representative compound. Fluorene‐ and fluorenone‐based motifs are vital skeletons in materials and medicinal chemistry research. Accordingly, this work is a contribution towards enriching the library of fluorene and fluorenone unnatural amino acid scaffolds.

Construction of C5‐Indole Unnatural Amino Acid Motifs via Diastereoselective Pd(II)‐Catalyzed β‐C(sp3)‐H Functionalization

We report the construction of C5‐indole unnatural amino acid derivatives via the diastereoselective Pd(II)‐catalyzed prochiral β‐C(sp3)‐H arylation of racemic (DL), and enantiopure L‐ and D‐carboxamides of amino acids with 5‐iodoindoles. Independently, indole‐based compounds and amino acid motifs are important small molecules in organic synthesis and drug discovery research. This report aimed to contribute to enriching the library of indole‐based unnatural amino acid derivatives. The synthesis of examples of C5‐indole motif‐installed amino acid derivatives comprising norvaline, phenylalanine, leucine, norleucine, 2‐aminooctanoic acid having anti‐stereochemistry was accomplished. The synthesis of the C5‐indole motif‐containing non‐α‐amino acid (aminoalkanoic acid) derivatives was also reported. The C‐H arylation reactions of racemic and enantiopure carboxamides with 5‐iodoindoles afforded the corresponding indole unnatural amino acid derivatives with good diastereoselectivity (anti, dr >95:5 and enantiopurity (er up to 99:1). Removal of the directing group (8‐aminoquinoline), phthalimide group and preparation of indole amino acid esters and free amino group‐containing indole unnatural amino acid motifs were also shown. The utility of the methodology was shown by synthesizing indole motif‐containing pyrrolidone and peptide molecules. The stereochemistry of the major (anti) diastereomers was ascertained from the X‐ray structure of a representative compound.

Isomer-Dependent Escape Rate of Xenon from a Water-Soluble Cryptophane Cage Studied by Ab Initio Molecular Dynamics.

The escape of xenon from the anti and syn diastereomers of hexacarboxylic-cryptophane-222 in water has been studied by ab initio molecular dynamics simulations. The structures of both complexes, when the xenon atom is trapped inside their cages, have been compared and show no major differences. The free-energy profiles corresponding to the escape reaction have been calculated with the Blue Moon ensemble method using the distance between Xe and the center of mass of the cage as the reaction coordinate. The resulting free-energy barriers are very different; the escape rate is much faster in the case of the syn diastereomer, in agreement with experimental data obtained in hyperpolarized 129 Xe NMR. Our simulations reveal the mechanistic details for each diastereomer and provide an explanation for the different in-out xenon rates based on the solvation structure around the cages.

Pnictogen‐Bonding Catalysts, Compared to Tetrel‐Bonding Catalysts: More Than Just Weak Lewis Acids

AbstractA hyperresponsive tool to assess supramolecular catalysis, the cyclization of di‐epoxides into cyclic ethers is used to elucidate the difference between pnictogen‐bonding and Lewis acid catalysis systematically. For all stereoisomers, most tested catalysts follow the Baldwin rules. Brønsted acid and anion–π catalysis afford almost only Baldwin products. Lewis acids such as SbCl3, BF3 and BiCl3 give the poorest selectivity, with at least 50 % Baldwin (B) products for all stereoisomers. In clear contrast, optimized pnictogen‐bonding catalysts operating on the Sb(III) and the Sb(V) level give the fused anti‐Baldwin (A) bicycles as the main product of trans epoxides, independent of syn or anti relation of the two epoxides. In the cis series, Sb(III) pnictogen‐bonding catalysts afford BA products almost exclusively for syn diastereomers, while anti diastereomers give equal amounts of BA and AB products. Sb(V) pnictogen‐bonding catalysts show similarly special trends. These unique characteristics support that pnictogen‐bonding catalysis differs from Lewis acid catalysis and can arguably be defined as its non‐covalent counterpart, just like hydrogen‐bonding catalysis is understood and appreciated as the non‐covalent counterpart of Brønsted acid catalysis. Computational studies on the origin of anti‐Baldwin selectivity reveal Sb(V) catalysts with an introvert deep σ hole surrounded by an almost planar ring of ligands. Central pnictogen‐bond attraction against peripheral steric repulsion then forces the epoxide to break open. In contrast, transient antimony oxidation in cyclic intermediates accounts for the chemoselectivity of Sb(III) catalysts. Presumably due to insufficient accessibility of their σ holes, the activity of tetrel‐bonding catalysts is negligible. The division between powerful pnictogen‐bonding and irrelevant tetrel‐bonding catalysts does not exist with the respective orthodox Lewis acids and thus supports that σ‐hole and Lewis acid catalysis are not the same.

Visible Light-Mediated, Highly Diastereoselective Epimerization of Lactams from the Most Accessible to the More Stable Stereoisomer.

Most known methods to access δ-lactams with stereogenic centers at the α- and β-positions are highly selective for the contra-thermodynamic syn diastereomer, typically via hydrogenation of the corresponding pyridinones or quinolinones. We describe here the development of a photoredox-mediated hydrogen atom transfer (HAT) approach for the epimerization of δ-lactams to access the more stable anti diastereomers from the contra-thermodynamic syn isomers. The reaction displays broad functional group compatibility, including acid, ester, 1°, 2° and 3° amide, carbamate, and pyridyl groups, and was effective for a range of differently substituted monocyclic and bicyclic lactams. Experimentally observed diastereoselectivities are consistent with the calculated relative stabilities of lactam diastereomers. Convergence to the same diastereomer ratio from the syn- and anti- diastereomers establishes that reversible epimerization provides an equilibrium mixture of diastereomers. Additionally, deuterium labeling and luminescence quenching studies shed further light on the mechanism of the reaction.

Nickel-Catalyzed Enantioselective Synthesis of Pre-Differentiated Homoallylic syn- or anti-1,2-Diols from Aldehydes and Dienol Ethers.

Nickel catalysis allied with cyclodiphosphazane or VAPOL-derived phosphoramidite ligands provides selective access to monoprotected vicinal diols by reductive coupling of dienol ethers and aldehydes. The observed regioselectivity is unprecedented, in that the diene reacts at the least nucleophilic and most hindered C atom that is attached to the oxygen substituent rather than at the terminal position. Notably, both syn and anti diastereomers of the products can be accessed depending on the configuration of the diene partner with usually excellent diastereo- and enantioselectivity.

Stereoselective cyclopropanations of amino-acid derived enones

Stereoselective cyclopropanation of a series of amino acid-derived enones to afford cyclopropyl keto-esters is reported, using a Michael-induced ring closure. The use of quinine and quinidine ether catalysts in the cyclopropanation step afforded the cyclopropyl keto-esters with high stereoselectivity. Results follow a consistent pattern, with the pseudoenantiomeric catalysts leading to opposite stereoselectivity, allowing for synthesis of either the syn or anti diastereomers.

Thermodynamics and Mechanism of a Photocatalyzed Stereoselective [2 + 2] Cycloaddition on a CdSe Quantum Dot.

Colloidal quantum dots (QDs) have shown promise over the last few decades for a range of applications including single photon emission, in vivo imaging, and photocatalysis. Recent experiments demonstrated that QDs impart stereoselectivity to triplet excited-state [2 + 2] cycloaddition reactions of alkenes photocatalyzed by the QD through self-assembly of the reagent molecules on the QD surface, but these experiments did not reveal the precise geometries of surface-bound molecules or their interactions with surface atoms. Here, a theoretical mechanistic approach is used to study such interactions for [2 + 2] cycloadditions of 4-vinylbenzoic acid derivatives on CdSe QDs. Spin-polarized periodic density functional theory (DFT) and nonperiodic DFT calculations are deployed to determine the origin of the selectivity for the syn diastereomer of the resultant tetrasubstituted cyclobutane product via atomistic modeling of the CdSe surface and substrates, determination of the thermodynamic energies of reactions for each step, the intermolecular interactions between the substrates, and the triplet state reaction paths. The calculations indicate that reaction selectivity arises from preferred binding of pairs through intermolecular interactions of substrate molecules on the QD surface in a syn-precursor structure followed by dimerization after triplet excitation. These mechanisms are generalizable to other metal-enriched QD surfaces that have a similar surface structure as that of CdSe, such as InSe or CdTe. Design principles for anti diastereomer derivatives are also discussed.

Stereodivergent Asymmetric Synthesis of α,β-Disubstituted β-Aminoalkylboronic Acid Derivatives via Group-Selective Protodeboronation Enabling Access to the Elusive Anti Isomer.

Chiral β-aminoalkylboronates generate growing interest as versatile synthetic building blocks to access β-aminoalcohols and other useful compounds, and also as bioisosteres of β-amino acids in drug discovery. In this study, the lack of methodology to access both syn and anti diastereomers of optically enriched, acyclic α,β-disubstituted β-aminoalkylboronates is addressed with the development of a divergent, diastereoselective strategy for the monoprotodeboration of β-amino gem-bis(boronate) precursors. To this end, new reaction conditions were successfully optimized to provide the elusive anti diastereomer by inverting a sequence of desulfinylation and protodeboronation. The desired syn or anti isomers are isolated independently in good yields and excellent diastereoselectivity (up to >20:1 dr) for a wide scope of substituents. The diastereotopic group selectivity of the new conditions yielding the anti isomer is rationalized by invoking a reactive rotamer featuring two ammonium-boronate hydrogen bonds, which enables phosphate coordination to boron with a concomitant, stereoretentive protonation of the least sterically hindered C-B bond. The accessibility and utility of both diastereomers of these α,β-disubstituted β-aminoalkylboronates is exemplified with the functionalization of the amino group, stereospecific oxidation to β-amino alcohols and C-C bond transformations of the secondary alkylboronate, and the preparation of free boronic acids and hemiboronic heterocycles.

Stereoselective Reduction of α‐Fluoro‐β‐keto Esters by NADH and NADPH‐Dependent Ketoreductases

Racemic α‐fluoro‐β‐keto esters were stereoselectively transformed to α‐fluoro‐β‐hydroxy esters through dynamic reductive kinetic resolution (DYRKR) using commercially available NAD(P)H‐dependent ketoreductases. Aromatic, alkenyl, and alkyl substrates were all reduced in high optical purities and yields. For most substrates, either anti or syn diastereomers could be produced with high enantiomeric excess, depending on the enzyme employed. The enzyme reactions with ethyl α‐fluoroacetoacetate were conveniently monitored in real time by in situ 19F NMR spectroscopy. These commercially available enzymes provide convenient access to stereoisomers of α‐fluoro‐β‐hydroxy esters from easily accessible racemic substrates.

Diastereoselective Allylation of Aldehydes by Dual Photoredox and Chromium Catalysis.

Herein, we report the redox-neutral allylation of aldehydes with readily available electron-rich allyl (hetero-) arenes, β-alkyl styrenes and allyl-diarylamines. This process was enabled by the combination of photoredox and chromium catalysis, which allowed a range of homoallylic alcohols to be prepared with high levels of selectivity for the anti diastereomer. Mechanistic investigations support the formation of an allyl chromium intermediate from allylic C(sp3)-H bonds and thus significantly extends the scope of the venerable Nozaki-Hiyama-Kishi reaction.

Employing Small Polyfunctionalized Molecules for a Diastereoselective Synthesis of Highly Substituted Indolines

An efficient approach has been developed for the diastereoselective synthesis of 1,2,3‐trisubstituted indolines. The reaction sequence includes the highly diastereoselective reductive amination of 2‐oxo‐1,3‐propanediols, which are prepared by postfunctionalization of Morita–Baylis–Hillman (MBH) adducts, to give substituted 2‐amino‐1,3‐propanediols with an anti relative configuration. A subsequent intramolecular palladium‐catalyzed Buchwald coupling reaction provided the 1,2,3‐trisubstituted indolines in 47–82 % yield. The preference for the anti diastereomer was investigated by Gibbs free energy diagrams and applying theoretical calculations at the M06–2X/6‐31+G** level.

Surface-induced formation of stereogenic centers on gold nanoparticles through diastereoselective interfacial Henry reaction: an NMR investigation

This study aims at the synthesis of gold nanoparticles (2–3 nm) functionalized with different nitroalkane-terminated thiols and investigates the chemical reactivity of these confined nitroalkanes towards aldehydes to yield β-nitroalcohols. Interfacial Henry reaction between fixed nitroalkane-terminated thiols with various aromatic and heteroaromatic aldehydes resulted in stereogenic centers on the surface of mixed-monolayer-protected gold nanoparticles (MMPNs). The ratio of the resulting diastereomers was determined by 1H NMR spectroscopy. It was found that some parameters such as the chain length of nitroalkyl and the nature of aromatic aldehyde play the main role in affecting the diastereomeric ratio on the surface of MMPNs. Certain trends have been analyzed from the data, and it can be inferred that some aldehydes approximately prefer the formation of anti diastereomers as the predominant products, while others give syn β-nitroalcohols. We have attributed this stereoselectivity to the packed layers, forceful lateral interactions (van der Waals), and intramolecular hydrogen bonding on the surface of modified gold nanoparticles that are able to suppress the role of solvent and intermolecular interactions.

Crystal structure of a fluorescent C-shaped molecule containing closely stacked bithiophene-substituted quinoxaline rings.

Molecules with well-defined structures that feature closely stacked aromatic rings are important for understanding π-π interactions. A previously reported C-shaped molecule with bithiophene-substituted quinoxaline rings suspended from an aliphatic bridge that holds the aromatic rings in close proximity exists as a pair of syn and anti diastereomers. The anti isomer, namely (1α,2β,4β,5α,16α,17β,19β,20α)-1,5,16,20-tetrachloro-31,31,32,32-tetramethoxy-11,26-bis[5-(thiophen-2-yl)thiophen-2-yl]-7,14,22,29-tetraazanonacyclo[18.10.1.15,16.02,19.04,17.06,15.08,13.021,30.023,28]dotriaconta-6(15),7,9,11,13,21(30),22,24,26,28-decaene chloroform monosolvate, C48H36Cl4N4O4S4·CHCl3, whose X-ray structure is described herein, has cofacial quinoxaline rings with bithiophene rings attached on opposite sides. The molecular structure is approximately C-shaped and consists of an aliphatic spacer with a boat-shaped cyclohexane ring in the middle. The centroid-to-centroid distance between the quinoxaline rings is 3.950 (1) Å, with ring-offset distances of 0.354 (3) and 0.816 (2) Å. The pendant bithiophene rings are oriented parallel to one another, which results from the thiophene rings connected to the quinoxaline rings being oriented such that their S atoms are rotated inward toward one another, but are not overlapped. Intermolecular packing is largely governed by van der Waals forces and a few weak C-H...X (X = N or O) interactions.

Synthesis of PhTAD-substituted dihydropyrrole derivatives via stereospecific C–H amination

Stereospecific α-amination has been accomplished via addition of N-phenyltriazolinedione (PhTAD) to the allylic position of dihydropyrroles. The aim of this study is to evaluate new PhTAD derivatives of biologically active bicyclic dihydropyrroles. Ene reaction was accomplished via addition of PhTAD to the allylic position to react with syn and anti diastereomers for α-amination. The α-amination depends on the stereochemistry, proceeding faster with syn than anti diastereoisomers. Steric hindrance from sulfide substituent slows down the transformation. Although the results are in accordance with an ene reaction followed by 1,3-shift of the urazole moiety, deuteration at the allylic position by simply stirring the diastereoisomeric dihydropyrroles in MeOD revealed a [1,3-H] shift. The stereospecificity of the transformation is attributed to steric hindrance during the allylic transposition step.

Stereochemistry of N-Benzoyl-5-substituted-1-benzazepines Revisited: Synthesis of the Conformationally Biased Derivatives and Revision of the Reported Structure.

The syn (aR*,5R*) and anti (aS*,5R*) diastereomers of N-benzoyl-C5-substituted-1-benzazepines originating in the chiralities at C5 and the Ar-N(C═O) axis were first stereoselectively synthesized by biasing the conformation with a substituent at C6 and C9, respectively. Detailed examination of the stereochemistry (i.e., conformation and configuration) of these N-benzoyl-1-benzazepines by X-ray crystallographic analysis, VT NMR, and DFT calculations revealed new physicochemical aspects of these heterocycles including revision of the stereochemistry previously reported.

Computational and NMR Spectroscopic Evidence for Stereochemistry-Dependent Conformations of 2,2,6,6-Tetramethylpiperidinyl-Masked 1,2-Diols.

2,2,6,6-Tetramethylpiperidinyl-masked 1,2-diols exhibited stereochemistry-dependent hydroxyl proton chemical shifts: ca. 7 ppm for the syn diastereomer and ca. 2 ppm for the anti diastereomer. A computational search for low energy geometries revealed that the syn isomer favors a six-membered ring hydrogen bond to nitrogen and the anti isomer favors a five-membered ring hydrogen bond to oxygen. The computed low energy conformations were found to have a large difference in hydroxyl proton shielding that was reflected in the experimental chemical shift difference. This chemical shift difference was observed in a broad range of solvents, and thus may be useful as a stereochemical probe. The stereochemistry-dependent conformation and chemical shift signature appeared to be due to a syn pentane interaction between the gem-dimethyl groups on the 2,2,6,6-tetramethylpiperidinyl moiety.

Structural Peculiarities of Cyanoximes and their Anions: Co-crystallization of Two Diastereomers and Formation of Acid-salts

Cyanoximes represent a new subclass of oximes and have the general formula NCC(= NOH)-R, where R is an electron-withdrawing group such as CN, amide / thioamide, keto-, carboxylic ester, and a variety of aryls and heterocycles. A group of 40 cyanoximes (including 4 biscyanoximes) and their metal complexes has been synthesized and characterized using a variety of spectroscopic methods and X-ray crystallography. Cyanoximes demonstrate a rich stereochemistry and exist as 4 main principal geometrical isomers such as cis / trans (with respect to the oxime C=N bond orientation relative to the size/heteroatom ranking) and syn / anti (with respect to the oxime N-OH fragment orientation relative to the CNgroup). The latter isomers are also erroneously called E or Z by analogy with isomers of C=C compounds. Contrary to organic compounds, cyanoximes frequently undergo a thermally-induced interconversion of syn and anti isomers around the C=N bond at relatively low temperatures, and, in this case the latter terminology was offered to distinguish them from classic, organic C=C compounds. All four isomers represent diastereomers. In this work, we summarize literature data for simultaneously co-crystallized syn and anti diastereomers of several cyanoximes and present new structural data for some metal complexes with monovalent Ag. Cyanoximes also have shown a remarkable ability to form HL2 - anions in their metal derivatives, including alkali metal salts of Werner-type complexes. In the anion, one H-atom is confined between two cyanoxime molecules via strong intermolecular H-bonding. Thus, compounds of a Kat[HL2] type represent acid salts (Kat = Na, K, Cs, where L = mono-cyanoxime; or NiL2 2+; where L = bis-cyanoxime). In this work we present our data for five new acid salts and review available literature data on that subject. Keywords: Acid-salts, cyanoximes, diastereomers, NMR spectra, pKa values, X-ray analysis.

Decarboxylative Generation of 2-Azaallyl Anions: 2-Iminoalcohols via a Decarboxylative Erlenmeyer Reaction.

Condensation between the tetrabutylammonium salt of 2,2-diphenylglycine and aldehydes results in a decarboxylative Erlenmeyer reaction, affording 1,2-diaryl-2-iminoalcohols as a mixture of diastereomers in good yields. The diastereomeric ratio shifts over time, with the anti diastereomer and the syn oxazolidine tautomer serving as the kinetic and thermodynamic products, respectively. Addition of Lewis acids can catalyze the rates of reaction and product equilibration. The results highlight the stereochemical promiscuity of 1,2-diaryl-2-iminoalcohols in the presence of Lewis acids and Brønsted bases.

ChemInform Abstract: One‐Pot Sequential Organocatalytic Michael—Tishchenko—Lactonization Reactions. Synthesis of Enantioenriched 4,5,6‐Trisubstituted δ‐Lactones.

Enantioenriched trisubstituted lactones were obtained in good yields and moderate to very good enantioselectivities in one-pot process, which implies a sequential organocatalyzed Michael addition of ketones to enals, followed by catalytic intramolecular diastereoselective Tishchenko reaction and lactonization. The final lactones were obtained as single diastereoisomers, demonstrating that the mixture of the anti and syn diastereomers epimerized to the syn hydroxy ester during the oxido-reduction step.