External Chiral Source Research Articles

Simple and Efficient Enantioselective α-Deuteration Method of α-Amino Acids without External Chiral Sources

Simple and Efficient Enantioselective α-Deuteration Method of α-Amino Acids without External Chiral Sources

Spontaneous emergence of enantioenriched chiral aldol reaction products from Achiral precursors in solution and origin of biological homochirality of sugars: a first-principles study.

Experimental reports about observation of spontaneous mirror symmetry breaking and chiral amplification in stereoselective Mannich and aldol reactions, run under fully achiral initial conditions, have drawn a lot of attention, fuelled partly by the role these reactions could have played in chemical evolution as a cause for still puzzling observed homochirality of biomolecules, often considered a prerequisite for the origin of life. We have now revisited this still unresolved problem, using DFT computation of all combinatorially possible transition states and numerical solution of complete set of resulting coupled kinetic rate equations to model the aldol reaction rigorously "from the first principles" and without making any a priori assumptions. Spontaneous mirror symmetry breaking in this autocatalytic, reversible, closed and homogenous system is explained by a supercritical pitchfork bifurcation, occurring in concentrations of enantiomers due to time-delayed kinetic instability of racemic composition of reaction mixture, when reactants are initially provided in non-stoichiometric quantities. Same process, taking place under similar conditions in primordial "soup" of chemicals, might conceivably explain origin of biological homochirality of sugar molecules on early earth billions of years ago. Our results suggest that seemingly innocuous chemical reactions could exhibit unexpected and counter-intuitive emergent behaviour, when initial conditions are appropriately chosen. Chiral amplification in self-catalyzed aldol reaction occurs during approach of thermodynamic equilibrium in accord with principle of microscopic reversibility and second law of thermodynamics.

Self-recovery of chiral microphase separation in an achiral diblock copolymer system.

Macroscopic regulation of chiral supramolecular nanostructures in liquid-crystalline block copolymers is of great significance in photonics and nanotechnology. Although fabricating helical phase structures via chiral doping and microphase separation has been widely reported, the chiral memory and self-recovery capacity of asymmetric phase structures are the major challenge and still deeply rely on the presence of chiral additives. Herein, we demonstrate the first controllable chiral microphase separation in an achiral amphiphilic block copolymer consisting of poly(ethylene oxide) and azobenzene (Azo) groups. Chirality can be transferred to the fabricated helical nanostructures by doping with chiral additives (tartaric acid, TA). After the removal of the chiral additives and then performing cross-linking, the formed helical nanostructures will completely dispense with the chiral source. The supramolecular chirality and the micron-scale phase structure can be maintained under UV irradiation and heating-cooling treatment, enabling a reversible "on-off" chiroptical switch feature. This work is expected to avoid the tedious synthesis and expensive raw materials and shows a great application prospect in chiral separation and so on.

Enantioselective C–H Functionalization Using High-Valent Group 9 Metal Catalysts

Abstract Group 9 transition metals with a cyclopentadienyl-type ligand are highly reactive catalysts for directed C–H functionalization reactions. In this review, we summarize our recent work on the development of enantioselective C–H functionalization reactions using achiral Rh(III) or Co(III) catalysts in combination with an external chiral source such as chiral sulfonates, a Lewis base, or carboxylic acids.

Use of Non‐Aromatic Hydrophobic α‐Amino Acids (α‐AA) and Non‐Amino Acid Derived Synthons: Comparative Studies Towards Total Syntheses of Selected Bioactive Natural Alkaloids

AbstractEnantiomerically pure non‐aromatic α‐amino acids having hydrophobic side chains (Isoleucine, Valine, Leucine, Methionine and Alanine) have been acknowledged as an important chiral pool. The wide range of chemical and biological applications associated with these α‐amino acids has encouraged organic chemists to develop new synthetic methodologies for the total synthesis of bioactive molecules. In this review, the synthetic routes of 10 bioactive natural alkaloids have been covered. The present article is an attempt to discuss various synthetic approaches of therapeutically active natural alkaloids and related diversity from non‐aromatic hydrophobic α‐amino acids and non‐amino acid derived synthetic approaches to present a comparative study between the methods. The synthetic steps and the key strategies involved in the total synthesis of selected natural alkaloids have been compared based on atom economy, overall chemical yield, stereoselectivity and sustainability with cost effective manner by using inexpensive commercially available starting materials but the source of chiral pool mainly α‐amino acids to target natural alkaloids in chiral synthesis will be playing an important role. In selected cases of alkaloids synthesized from non‐amino acid route, an external chiral source of induction like metal and organometallic catalysts to generate chirality in the molecule are used.

Total synthesis of selected bioactive alkaloids, their structure–function relationships and molecular target interactions: A comparative synthetic analysis of tryptophan originated chiral pool approaches vs other synthons

The use of tryptophan as an inexpensive and versatile chiral pool for alkaloid synthesis continues to influence modern synthetic chemistry. In this review, we summarized comparative strategies of tryptophan based alkaloids for the construction of architecturally complex bioactive natural compounds using tryptophan-derived chiral synthons versus non-tryptophan routes based on the basis of atom economy, overall chemical yield, stereoselectivity and sustainability. We also briefly discussed the Structure-activity relationship of selected tryptophan-based alkaloids & their modified analogous to elevate the effects of bioactive compounds in a broad spectrum of pharmacological effects. Tryptophan has an inherent chirality which makes it a great advantage as a cheap Chiron having functionalizable groups (–CO2H, –NH2 and indole) and ensures at least one fixed chiral center which governs the installation of another stereogenic center in metal and organocatalyzed asymmetric synthesis. On the contrary, the non-tryptophan route utilizes an external chiral source to generate chirality in the molecule. In this approach, metal and organometallic catalysts are used in numerous synthetic transformations. With green chemistry approach being the need of the hour in industry and research laboratories, atom economic and sustainable synthetic strategies for the total synthesis of bioactive Tryptophan-derived alkaloids will be a guiding path forward.

Atroposelective Total Syntheses of Naphthylisoquinoline Alkaloids with (P)-Configuration.

Asymmetric total syntheses of naphthylisoquinoline alkaloids with a (P)-configuration are described. An atroposelective Suzuki-Miyaura reaction between naphthyl pinacol boronate and an aryl iodide bearing an (S)-2-(N-acetylamino)propyl group at the ortho-position using Pd(OAc)2 in the presence of SPhos and Ba(OH)2 provided the (P)-selective biaryl product as the major product without any external chiral sources. This biaryl product was converted into naphthylisoquinoline alkaloids with a (P)-configuration via stereoselective construction of the isoquinoline framework with the appropriate oxidation state and stereochemistry.

Absolute Asymmetric Synthesis Involving Chiral Symmetry Breaking in Diels–Alder Reaction

Efficient generation and amplification of chirality from prochiral substrates in the Diels–Alder reaction (DA reaction) followed by dynamic crystallization were achieved without using an external chiral source. Since the DA reaction of 2-methylfuran and various maleimides proceeds reversibly, an exo-adduct was obtained as the main product as the reaction proceeded. From single crystal X-ray structure analysis, it was found that five of ten exo-adducts gave conglomerates. When 2-methylfuran and various maleimides with a catalytic amount of TFA were reacted in a sealed tube, the exo-DA adducts were precipitated from the solution, while the reaction mixtures were continuously ground and stirred using glass beads. Deracemization occurred and chiral amplification was observed for four of the substrates. Each final enantiomeric purity was influenced by the crystal structure, and when enantiomers were included in the disorder, they reached an enantiomeric purity reflecting the ratio of the disorder. The final ee value of the 3,5-dimethylphenyl derivative after chiral amplification was 98% ee.

Asymmetric Total Syntheses of Naphthylisoquinoline Alkaloids via Atroposelective Coupling Reaction Using Central Chirality as Atroposelectivity-Controlling Group.

We describe the asymmetric total syntheses of naphthylisoquinoline alkaloids. Atroposelective biaryl coupling reaction between naphthyl pinacol boronate and an aryl iodide, bearing (S)-2-aminopropyl group at the ortho-position using the existing central chirality as an atroposelectivity-controlling group, provided the desired biaryl product with high atroposelectivity, without the use of an external chiral source. From the resulting biaryl product, several naphthylisoquinoline alkaloids were prepared via the stereoselective formation of the isoquinoline framework with the appropriate oxidation state and stereochemistry.

Absolute Asymmetric Synthesis of an Aspartic Acid Derivative from Prochiral Maleic Acid and Pyridine under Achiral Conditions.

The asymmetric synthesis of an aspartic acid derivative, N-succinopyridine, from prochiral starting materials involving dynamic enantioselective crystallization was accomplished without using any external chiral source. The aza-Michael addition reaction of prochiral maleic acid and pyridine afforded racemic conglomerate N-succinopyridine in water. Continuous stirring of the suspension of the reaction mixture with acetic acid promoted gradual deracemization to afford a crystal with an excellent optical purity of 99 % in 71 % yield.

Cp*CoIII-Catalyzed C–H Functionalization and Asymmetric Reactions Using External Chiral Sources

This account describes Cp*CoIII-catalyzed C–H functionalization reactions developed in our group between 2013 and 2018. Cp*CoIII catalysts not only serve as inexpensive alternatives to Cp*RhIII catalysts but also exhibit unique reactivity and selectivity in several transformations. In the latter part of this review, we introduce catalytic asymmetric C–H functionalization reactions using achiral RhIII or CoIII catalysts with chiral disulfonates or carboxylic acids as external chiral sources.1 Introduction and Overview2 Cp*CoIII-Catalyzed C–H Functionalization Reactions2.1 C–H Addition Reactions to Polar Double Bonds2.2 Cp*Co(CO)I2 and [Cp*CoI2]2 Precursors for the C2-selective C–H Amidation of Indoles2.3 C–H Functionalization of Carbamoyl-Protected Indoles Using Alkynes2.4 C–H Allylation Using Allyl Alcohols2.5 Cyclization Reactions of O-Acyloximes and Alkynes2.6 Other Miscellaneous Reactions3 Enantioselective C–H Functionalization Reactions by Hybrid Catalysis3.1 Cp*RhIII/Chiral Disulfonate Catalysts for the Enantioselective C–H Addition to Enones3.2 Enantioselective C–H Cleavage Using Chiral Carboxylic Acids4 Summary and Perspective

Chiral diversification through the assembly of achiral phenylacetylene macrocycles with a two-fold bridge.

We demonstrate so-called "chiral diversification", which is a design strategy to create multiple chiral molecules through the assembly and double-bridging of achiral components. We used phenylacetylene macrocycles (PAMs) as an achiral element. In a molecule, two achiral rings of [6]PAM are stacked one above the other, or bound to each other mechanically. As an alternative, a single enlarged ring of [12]PAM was also assumed to be a doubled form of [6]PAM. In any case, one or two ring(s) are doubly-bridged by covalent bonds to exert chirality. Through intramolecular two-bond formation, these multiple chiral molecules were obtained as a set of products in one reaction. The dynamic chirality generated in molecules with either two helically-stacked rings of [6]PAM or a single helically-folded ring of [12]PAM was characterized by induced Cotton effects with the aid of an external chiral source. Thus, a chiral structure based on [12]PAM could be demonstrated as the first success. Alternatively, enantiomeric separation was achieved for molecules with two interlocked rings of [6]PAM to show remarkable chiroptical properties.

Asymmetric Synthesis Involving Reversible Photodimerization of a Prochiral Flavonoid Followed by Crystallization

Asymmetric synthesis involving photochemical dimerization of a prochiral flavonoid derivative in solution without any chiral source was achieved. Irradiation of ethyl 6‐bromochromonecarboxylate in solution efficiently gave a C2‐chiral anti‐head‐to‐head dimer in excellent chemical yield with good quantum efficiency (Φ365 = 0.15). X‐ray crystallographic analysis revealed that the dimer crystallized as a conglomerate of C2 space group. The crystalline dimer precipitated upon irradiation of the monomer in solution, and indirect racemization of the dimer through a reversible photoreaction in solution and selective crystallization simultaneously occurred to give the C2‐chiral dimer in optically active form with up to 80 % ee. Optically active photoproducts could be obtained by simply irradiating achiral materials in solution without an external chiral source.

Asymmetric Total Synthesis of Lepadiformine C Using Memory of Chirality in an Intramolecular Ester Enolate Michael Addition.

The asymmetric synthesis of lepadiformine C was achieved using d-proline as the only chiral source. The synthetic strategy features the use of the principle of "memory of chirality" in an intramolecular Michael addition to construct the bicyclic intermediate without the aid of external chiral sources. A brief mechanistic rationale is presented to account for the stereochemical outcome.

Asymmetric Synthesis of an Amino Acid Derivative from Achiral Aroyl Acrylamide by Reversible Michael Addition and Preferential Crystallization.

Single-handed α-amino acid derivatives were generated from achiral precursors without an external chiral source. Conjugate addition of phenethylamine to an achiral aroyl acrylamide under homogeneous conditions gave the α-amino amides in quantitative yields, which crystallized as a conglomerate of a P21 crystal system. Dynamic preferential crystallization or attrition-enhanced deracemization resulted in the formation of enantiomorphic crystals of 99 % ee.

Asymmetric transformation by dynamic crystallization of achiral succinimides

Optically active materials could be generated by simple solidification of achiral materials without an external chiral source. When achiral cis-3,4-diphenylsuccinimides were solidified in the presence of a catalytic amount of DBU by evaporating the solvent with stirring, optically active trans-3,4-diphenylsuccinimides were obtained quantitatively with high enantiomeric excesses.

A chirality rewriting cycle mediated by a dynamic cyclen–calcium complex

A Ca(2+) complex with octadentate cyclen having three pyridines and one triazole on its sidearms mediated two-point chirality transfer from an external chiral source to the coordinating pyridine and coumarin moieties. The induced chirality information was repeatedly written, deleted, and rewritten by coupling with esterification of the coordinating external chiral anion.

A Synthetic Route to Highly Substituted 1,2,3,4‐Tetrahydroisoquinolines via Yb(OTf)3‐Catalyzed Diastereoselective Ring Opening of Bridged Oxazolidines: Asymmetric Synthesis of 2‐Azapodophyllotoxin

We herein report a robust and efficient synthetic route to highly functionalized enantiopure 1,2,3,4-tetrahydroisoquinolines (THIQs) from Garner aldehyde. We utilized the inherent chirality of Garner aldehyde through 1,2- and 1,3-/1,4-asymmetric inductions iteratively to obtain 1,2,3,4-tetrasubstitued THIQs using rigid and isolable bridged oxazolidines without any external chiral sources. All possible stereoisomers of bridged oxazoliodines were efficiently synthesized from L- and D-Garner aldehydes and transformed into fully functionalized THIQs via diastereoselective ring opening with various nucleophiles in the presence of Yb(OTf)(3). This methodology furnished four out of eight possible diastereomers of 1,2,3,4-tetrasubstituted THIQs despite the electronic nature of substituents on the aryl rings. Finally, the enantioselective synthesis of 2-azapodophyllotoxin was achieved with an overall yield of 35.4% (eight steps) from D-Garner aldehyde using this synthetic route.

Homochiral Nickel Coordination Polymers Based on Salen(Ni) Metalloligands: Synthesis, Structure, and Catalytic Alkene Epoxidation

One-dimensional (1D) homochiral nickel coordination polymers [Ni(3)(bpdc)(RR-L)(2)·(DMF)](n) (2R, RR-L = (R,R)-(-)-1,2-cyclohexanediamino-N,N'-bis(3-tert-butyl-5-(4-pyridyl)salicylidene), bpdc = 4,4'-biphenyldicarboxylic acid) and [Ni(3)(bpdc)(SS-L)(2)·(DMF)](n) (2S, SS-L = (S,S)-(-)-1,2-cyclohexanediamino-N,N'-bis(3-tert-butyl-5-(4-pyridyl)salicylidene) based on enantiopure pyridyl-functionalized salen(Ni) metalloligand units NiL ((1,2-cyclohexanediamino-N,N'-bis(3-tert-butyl-5-(4-pyridyl)salicylidene))Ni(II)) have been synthesized and characterized by microanalysis, IR spectroscopy, solid-state UV-vis spectroscopy, thermogravimetric analysis (TGA), circular dichroism (CD) spectroscopy, cyclic voltammetric measurement, and powder and single crystal X-ray diffraction. Each NiL as unbridging pendant metalloligand uses one terminal pyridyl group to coordinate achiral unit (nickel and bpdc(2-)) building a helical chain, while the other pyridyl group remains uncoordinated. Both 2R and 2S contain left- and right-handed helical chains made of the achiral building blocks, while the NiL as remote external chiral source is perpendicular to the backbone of the helices. The nickel coordination polymers 2R and 2S containing unsaturated active nickel center in metalloligand NiL can be used as self-supported heterogeneous catalysts. They show catalytic activity comparable with their homogeneous counterpart in alkene epoxidation and exhibit great potential as recyclable catalysts.

Asymmetric Intramolecular Conjugate Addition of Chiral Enolates <i>via</i> Nonequilibrium

Optically active alpha,alpha-disubstituted alpha-amino acids belong to an important class of unnatural amino acids. Since the synthesis of such amino acids involves the creation of a quaternary stereocenter, methods for their synthesis have been extensively studied. We have reported that N-t-butoxycarbonyl(Boc)-N-methoxymethyl(MOM)-amino acid derivatives undergo asymmetric alpha-alkylation in up to 93% ee. Original chiral information on an amino acid is preserved in axially chiral enolate intermediates, and thus asymmetric induction is achieved without the aid of external chiral sources (i.e., memory of chirality). Recently, we have reported a new protocol for the asymmetric cyclization of amino acid derivatives, which enables straightforward synthesis of cyclic amino acids with a tetrasubstituted carbon center from the usual alpha-amino acids in up to 98% ee. Here we report the asymmetric construction of highly substituted chiral nitrogen heterocycles via intramolecular conjugate addition of chiral enolates generated from N-Boc-N-alkylylamino acid derivatives. This method is applicable to the asymmetric construction of pyrrolidine, piperidine, tetrahydroisoquinoline, and indoline derivatives with contiguous quaternary and tertiary stereocenters.