Enantioselective desymmetrisation of achiral epoxides

Epoxides: Small Rings to Play with under Asymmetric Organocatalysis

Optically pure epoxides are recognized as highly valuable products and key intermediates, useful in different areas from pharmaceutical and agrochemical industries to natural product synthesis and materials science. The predictable fate of the ring-opening process, in terms of stereoselectivity and often of regioselectivity, enables useful functional groups to be installed at vicinal carbon atoms in a desired manner. In this way, products of widespread utility either for synthetic applications or as final products can be obtained. The advent of asymmetric organocatalysis provided a new convenient tool, not only for their preparation but also for the elaboration of this class of heterocycles. In this review, we focus on recent developments of stereoselective organocatalytic ring-opening reactions of meso-epoxides, kinetic resolution of racemic epoxides, and Meinwald-type rearrangement. Examples of asymmetric organocatalytic processes toward specific synthetic targets, which include ring opening of an epoxide intermediate, are also illustrated.

Ketoreductase Catalyzed (Dynamic) Kinetic Resolution for Biomanufacturing of Chiral Chemicals

Biocatalyzed asymmetric reduction of ketones is an environmentally friendly approach and one of the most cost-effective routes for producing chiral alcohols. In comparison with the well-studied reduction of prochiral ketones to generate chiral alcohols with one chiral center, resolution of racemates by ketoreductases (KREDs) to produce chiral compounds with at least two chiral centers is also an important strategy in asymmetric synthesis. The development of protein engineering and the combination with chemo-catalysts further enhanced the application of KREDs in the efficient production of chiral alcohols with high stereoselectivity. This review discusses the advances in the research area of KRED catalyzed asymmetric synthesis for biomanufacturing of chiral chemicals with at least two chiral centers through the kinetic resolution (KR) approach and the dynamic kinetic resolution (DKR) approach.

Brønsted Acid-Catalysed Epoxide Ring-Opening Using Amine Nucleophiles: A facile access to β-amino alcohols

A mild, efficient, and metal-free synthetic protocol for the synthesis of β-amino alcohols is reported. The reaction proceeds at room temperature with only 0.5 mol% catalyst loading and affords β-amino alcohol derivatives in excellent yield. This protocol is well-tolerated by a wide range of styrene oxide and aniline derivatives. A notably efficacious gram-scale synthesis is also reported with a high TON = 842. Further, the Hammett correlation study was also performed to identify the rate-determining step.

Proton-accelerated Lewis acid catalysis for stereo- and regioselective isomerization of epoxides to allylic alcohols

The isomerization of epoxides to allylic alcohols was developed via proton-accelerated Lewis acid catalysis. The addition of tBuOH as a proton source is the key to the efficient catalytic cycle. Trisubstituted epoxides, including enantioenriched derivatives, were selectively converted to secondary-allylic alcohols without loss of enantiopurity.

Recent Advances in Enantioselective Desymmetrizations of Prochiral Oxetanes

Strain relief of oxetanes offers a plethora of opportunities for the synthesis of chiral alcohols and ethers. In this context, enantioselective desymmetrization has been identified as a powerful tool to construct molecular complexity and this has led to the development of elegant strategies on the basis of transition metal, Lewis acid, and Brønsted acid catalysis. This review highlights recent examples that harness the inherent reactivity of prochiral oxetanes and offers an outlook on the immense possibilities for synthetic application.

Metal-Free β-Amino Alcohol Synthesis: A Two-step Smiles Rearrangement

A novel method for the synthesis of β-amino alcohols has been demonstrated under mild reaction conditions with a broad scope via a two-step Smiles rearrangement. What is more, theoretical calculations have been performed to confirm the rationality of the mechanism. The method has been proved to be notably effective for N-arylated amino alcohols, which are difficult to synthesize by traditional methods.

Architectural and catalytic aspects of designer materials built using metalloligands of pyridine-2,6-dicarboxamide based ligands

This perspective presents the design, synthesis and crystal structures of a large number of architectures constructed using assorted metalloligands of pyridine-2,6-dicarboxamide based ligands. The metalloligands offered various appended functional groups, whereas design strategies included varying both their position and number. A combination of these parameters resulted in the development of assorted architectures including discrete trinuclear and tetranuclear complexes as well as 1D/2D/3D coordination polymers. The metalloligand strategy not only assisted in the construction of ordered crystalline materials with varied dimensionalities but also judiciously allowed the incorporation of Lewis acidic and redox-active secondary metals in the resultant architectures. As a result, such designer architectures illustrated their noteworthy role both as homogenous and heterogeneous catalysts in different organic transformation reactions.

Isomerization of Epoxides into Allylic Alcohols Catalyzed by Supported Au Nanoparticles on TiO2: A Mechanistic Insight

Based on primary and β-secondary isotope effects, it is established that the isomerization of epoxides into allylic alcohols catalyzed by supported Au nanoparticles proceeds via an unsymmetrical concerted transition state in which there is partial positive charge developing on the C-O carbon atom progressing C-H abstraction. A carbocationic intermediate can be formed only in cases of sufficient stabilization of the positive charge by appropriate substituents.

Ruthenium-Catalyzed Deaminative Hydrogenation of Amino Nitriles: Direct Access to 1,2-Amino Alcohols

A new approach for the efficient and highly selective synthesis of 1,2-amino alcohols by direct reductive hydrolysis of N-formyl-protected α-amino nitriles is reported. The commercially available RuHCl(CO)(PPh₃ )₃ complex was found to be a suitable catalyst for this operationally simple protocol, in which no stoichiometric amounts of undesired metal waste are generated. The deaminative hydrogenation is performed at 55 bar of H₂ , using a 6:1 mixture of 1,4-dioxane/water as solvent. In addition, hydroxymethyl alcohols were prepared from cyanoketones under very similar conditions.

β-Hydroxy sulfides and their syntheses

Sulfur-containing natural products are ubiquitous in nature, their most abundant source being marine organisms since sulfur, in the form of the sulfate ion, is the second most abundant anion in sea water after chloride. As part of natural products, sulfur can appear in a multitude of combinations and oxidation states: thiol, sulfide (acyclic or heterocyclic), disulfide, sulfoxide, sulfonate, thioaminal, hemithioacetal, various thioesters, thiocarbamate and isothiocyanate. This review article focuses on β-hydroxy sulfides and analogs; their presence in natural products, general protocols for their synthesis, and examples of their application in target oriented synthesis.

Synthesis and Desymmetrization of meso Tricyclic Systems Derived from Benzene Oxide

Ozonolysis of the Diels-Alder adducts derived from benzene oxides and N-alkylmaleimides resulted in fully substituted, meso bicyclic systems bearing six contiguous stereocenters, isolated as diols upon reductive workup with NaBH₄. Variation in the workup allowed for isolation of two different diastereoisomers, through double epimerization of the imide stereocenters. Desymmetrization of the resulting meso diols via asymmetric nucleophilic epoxide opening and acylation reactions provided access to highly substituted, enantioenriched fused rings.

Cooperative Lewis acid-onium salt catalysis as tool for the desymmetrization of meso-epoxides

Epoxide desymmetrizations by bromide are very rare despite the large synthetic potential of chiral bromohydrins. Herein we present a new concept for epoxide desymmetrizations in which a bifunctional Lewis acid/ammonium salt catalyst allows for efficient enantioselective epoxide ring openings by Br^-. With acetylbromide as a Br^- source bromohydrin esters are formed.

Biocatalytic approaches towards the stereoselective synthesis of vicinal amino alcohols

The global need for clean manufacturing technologies and the management of hazardous chemicals and waste present new research challenges to both chemistry and biotechnology.

Copper based coordination polymers based on metalloligands: utilization as heterogeneous oxidation catalysts

This work presents two copper-based coordination polymers and their utilization as stable, reusable and heterogeneous catalysts for the epoxidation of olefins using O₂ and for peroxide-mediated oxidation of benzyl alcohols under solvent-free conditions.

An efficient method for regioselective ring opening of epoxides by amines under microwave irradiation using Bi(NO3)3·5H2O as a catalyst

Bi(NO₃)₃·5H₂O a highly efficient environmentally benign catalyst, is used for the nucleophilic ring opening of epoxides with aromatic, aliphatic and heteroaromatic amines under solvent free microwave conditions to afford the corresponding β-amino alcohols in good to excellent yields with high regioselectivity.

The mechanism of an asymmetric ring-opening reaction of epoxide with amine catalyzed by a metal–organic framework: insights from combined quantum mechanics and molecular mechanics calculations

QM/MM computations suggest that the asymmetric ring-opening reaction of epoxide with amine is controlled by CH–π interactions between aniline and a naphthol moiety.

Enantioselective Synthesis of Silacyclopentanes

A variety of functionalized silacyclopentanes were synthesized by highly enantioselective β-eliminations of silacyclopentene oxides followed by stereospecific transformations. The reaction mechanism of the β-elimination was elucidated by DFT calculations. An in vitro biological assay with an oxy-functionalized silacyclopentane showed substantial binding to a serotonin receptor protein.

Efficient epoxide isomerization within a self-assembled hexameric organic capsule

The supramolecular organic nano-capsule formed by the self-assembly of six resorcin[4]arene units efficiently promotes the isomerization of epoxides (Meinwald isomerization) to the corresponding carbonyl compounds.

Lanthanide-based coordination polymers as promising heterogeneous catalysts for ring-opening reactions

Eu and Tb based coordination polymers function as reusable heterogeneous catalysts for ring-opening reactions with amines, alcohols, thiols, and azides as nucleophiles.

Enantioselective syntheses of β-amino alcohols catalyzed by recyclable chiral Fe(iii) metal complex

An efficient asymmetric desymmetrization of meso-epoxides with anilines catalyzed by a series of simple and environmentally benign in situ generated Fe(iii) complexes based on chiral tridentate ligands L1–L7 was carried out at rt.

Gold(I)-catalyzed asymmetric desymmetrization of meso-alkynyl diols and kinetic resolution of the corresponding DL-diols: effects of celite filtration and silver salts

The asymmetric desymmetrization of meso-2-alkynylbenzenediols through the use of a combination of axially chiral diphosphine(AuCl)2 precatalysts and silver salt co-catalysts gave optically active isochromene compounds with high enantioselectivities in good yields. The corresponding DL-diol isomers underwent efficient kinetic resolution to give the cyclized isochromenes and recovered diols with high enantioselectivities under similar conditions. The high reactivity and selectivity in the desymmetrization of the meso-diols is independent of the combination of axially chiral diphosphine(AuCl)2 precatalyst and silver salt co-catalyst, whereas the corresponding tricarbonylchromium complexes of alkynylbenzenediols were affected by the combination of the diphosphine(AuCl)2 and silver salt. The reactivity was largely dependent on the nature of the gold(I) species.

mCPBA-mediated metal-free intramolecular aminohydroxylation and dioxygenation of unfunctionalized olefins

We report a mild and metal-free process to prepare a variety of 2-hydroxymethylpyrrolidine and oxolanimine compounds.

A broadly applicable and practical oligomeric (salen) Co catalyst for enantioselective epoxide ring-opening reactions

The (salen) Co catalyst (4a) can be prepared as a mixture of cyclic oligomers in a short, chromatography-free synthesis from inexpensive, commercially available precursors. This catalyst displays remarkable enhancements in reactivity and enantioselectivity relative to monomeric and other multimeric (salen) Co catalysts in a wide variety of enantioselective epoxide ring-opening reactions. The application of catalyst 4a is illustrated in the kinetic resolution of terminal epoxides by nucleophilic ring-opening with water, phenols, and primary alcohols; the desymmetrization of meso epoxides by addition of water and carbamates; and the desymmetrization of oxetanes by intramolecular ring opening with alcohols and phenols. The favorable solubility properties of complex 4a under the catalytic conditions facilitated mechanistic studies, allowing elucidation of the basis for the beneficial effect of oligomerization. Finally, a catalyst selection guide is provided to delineate the specific advantages of oligomeric catalyst 4a relative to (salen) Co monomer 1 for each reaction class.

Synthesis of 1,2-amino alcohols via catalytic C–H amidation of sp3 methyl C–H bonds

A new route to 1,2-amino alcohols is presented by C–H amidation of sp³ methyl C–H bonds as a key step. Various alcohols were employed after converting them to removable ketoxime chelating groups. Iridium-catalyzed C–H amidation and following LAH reduction furnished β-amino alcohol products.

Asymmetric organocatalytic desymmetrization of 4,4-disubstituted cyclohexadienones at high pressure: a new powerful strategy for the synthesis of highly congested chiral cyclohexenones

A highly diastereoselective and enantioselective method for the asymmetric desymmetrization of 4,4-disubstituted cyclohexadienones at high pressure was developed.

An air-stable cationic iridium hydride as a highly active and general catalyst for the isomerization of terminal epoxides

We describe the use of an air-stable iridium hydride catalyst for the isomerization of terminal epoxides into aldehydes with perfect regioselectivity.

Nickel-catalyzed regiodivergent opening of epoxides with aryl halides: co-catalysis controls regioselectivity

Epoxides are versatile intermediates in organic synthesis, but have rarely been employed in cross-coupling reactions. We report that bipyridine-ligated nickel can mediate the addition of functionalized aryl halides, a vinyl halide, and a vinyl triflate to epoxides under reducing conditions. For terminal epoxides, the regioselectivity of the reaction depends upon the cocatalyst employed. Iodide cocatalysis results in opening at the less hindered position via an iodohydrin intermediate. Titanocene cocatalysis results in opening at the more hindered position, presumably via Ti(III)-mediated radical generation. 1,2-Disubstituted epoxides are opened under both conditions to form predominantly the trans product.