Tungsten-catalyzed regio- and enantioselective aminolysis of trans-2,3-epoxy alcohols: an entry to virtually enantiopure amino alcohols

Bis-Boric Acid-Mediated Regioselective Reductive Aminolysis of 3,4-Epoxy Alcohols

Herein we report a bis-boric acid-mediated regioselective reductive aminolysis of 3,4-epoxy alcohols, providing new access to prepare amino diols in high diastereofidelity directly starting from nitroarenes. Notably, this step-economical process is enabled by the essential dual function of bis-boric acid, which is engaged initially in the 4,4'-bipyridine-catalyzed reduction of nitro compounds as the reductant and subsequently promotes the ring opening reaction of 3,4-epoxy alcohols with the in situ-generated anilines.

Diarylborinic Acid-Catalyzed Regioselective Ring Openings of Epoxy Alcohols with Pyrazoles, Imidazoles, Triazoles, and Other Nitrogen Heterocycles

A method for regioselective ring openings of 3,4- and 2,3-epoxy alcohols with ambident nitrogen heterocycles is described. Using a diarylborinic acid catalyst, a single regioisomer is favored in couplings of nucleophile and electrophile partners that display low regioselectivity under conventional conditions. The method provides access to aromatic heterocycles bearing stereochemically defined, functionalized alkyl substituents, a product class similar in structure to medicinally relevant compounds such as the acyclic nucleoside analogues.

Highly Regioselective 5-endo-tet Cyclization of 3,4-Epoxy Amines into 3-Hydroxypyrrolidines Catalyzed by La(OTf)3

Highly regioselective intramolecular aminolysis of 3,4-epoxy amines has been achieved. Key features of this reaction are (1) chemoselective activation of epoxides in the presence of unprotected aliphatic amines in the same molecules by a La(OTf)₃ catalyst and (2) excellent regioselectivity for anti-Baldwin 5-endo-tet cyclization. This reaction affords 3-hydroxy-2-alkylpyrrolidines stereospecifically in high yields. DFT calculations revealed that the regioselectivity might be attributed to distortion energies of epoxy amine substrates. The use of this reaction was demonstrated by the first enantioselective synthesis of an antispasmodic agent prifinium bromide.

Catalytic Asymmetric 1,3-Dipolar Cycloaddition of α,β-Unsaturated Amide and Azomethine Imine

α,β-Unsaturated amides were incorporated as viable dipolarophiles in a catalytic asymmetric 1,3-dipolar cycloaddition of azomethine imines. The use of a 7-azaindoline auxiliary was essential to acquire sufficient reactivity with excellent diastereoselectivity, likely due to the chelating activation of the amide by the In(III)/bishydroxamic acid complex. Although the enantioselectivity remains unsatisfactory, this work is an important step toward the development of an asymmetric catalysis utilizing stable and low-reactive substrates.

Nucleophilic Transformations of Lewis Acid-Activated Disubstituted Epoxides with Catalyst-Controlled Regioselectivity

Due to their inherent ring strain and electrophilicity, epoxides are highly attractive building blocks for fundamental organic reactions. However, controlling the regioselectivity of disubstituted epoxide transformations is often particularly challenging. Most Lewis acid-mediated processes take advantage of intrinsic steric or electronic substrate bias to influence the site of nucleophilic attack. Therefore, the scope of many of these systems is frequently quite limited. Recent efforts to generate catalysts that can overcome substrate bias have expanded the synthetic utility of these well-known reactions. In this Perspective, we highlight various regioselective transformations of disubstituted epoxides, emphasizing those that have inspired the production of challenging, catalyst-controlled processes.

2-Oxiranyl-pyridines: Synthesis and Regioselective Epoxide Ring Openings with Chiral Amines as a Route to Chiral Ligands

New epoxides, derivatives of pyridine, 2,2′-bipyridine, and 1,10-phenanthroline, were synthesized from the respective α-methylazaarenes. The obtained racemic 2-oxiranyl-azaarenes along with styrene oxide and trans-stilbene oxide were submitted to the ring opening with chiral primary amines as a chiral auxiliary. The most effective reaction was run in the presence of Sc(OTf)3/diisopropylethylamine for 7 days at 80°C, affording a good yield of the amino alcohols. Except for styrene oxide which gave both α- and β-amino alcohols, the reactions led regioselectively to the corresponding diastereomeric β-amino alcohols. The resulting diastereomers were separated, and the configurations of their stereogenic centers were established. The obtained enantiomerically pure 2-pyridinyl- and 6-(2,2′-bipyridinyl)-β-amino alcohols were tentatively tested as chiral ligands in the zinc-catalyzed aldol reaction.

Boronic acid-catalysed C-3 selective ring opening of 3,4-epoxy alcohols with thiophenols and thiols

In this protocol we described a boronic acid-catalysed C-3 selective ring opening of 3,4-epoxy alcohols with thiophenols and thiols as nucleophiles. This diastereo- and enantiospecific reaction provides an efficient entry to prepare a variety of hydroxyl sulfides. Through the directing effect of the hydroxyl group, nucleophilic attack on the C-3 position of the epoxide moiety is favoured. It can be rationalized in a proposed transition state, in which the boronic acid catalyst tethers both epoxides and S-nucleophiles.

Substrate-Directed Catalytic Selective Chemical Reactions

The development of highly efficient reactions at only the desired position is one of the most important subjects in organic chemistry. Most of the reactions in current organic chemistry are reagent- or catalyst-controlled reactions, and the regio- and stereoselectivity of the reactions are determined by the inherent nature of the reagent or catalyst. In sharp contrast, substrate-directed reaction determines the selectivity of the reactions by the functional group on the substrate and can strictly distinguish sterically and electronically similar multiple reaction sites in the substrate. In this Perspective, three topics of substrate-directed reaction are mainly reviewed: (1) directing group-assisted epoxidation of alkenes, (2) ring-opening reactions of epoxides by various nucleophiles, and (3) catalytic peptide synthesis. Our newly developed synthetic methods with new ligands including hydroxamic acid derived ligands realized not only highly efficient reactions but also pinpointed reactions at the expected position, demonstrating the substrate-directed reaction as a powerful method to achieve the desired regio- and stereoselective functionalization of molecules from different viewpoints of reagent- or catalyst-controlled reactions.

Umpolung Synthesis of 1,3-Amino Alcohols: Stereoselective Addition of 2-Azaallyl Anions to Epoxides

We report the direct preparation of 1,3-amino alcohols that contain up to three contiguous stereogenic centers by the umpolung coupling of imines and epoxides. Nucleophilic 2-azaallyl anions, generated from imines, are stereoselectively added to epoxides to furnish 1,3-amino alcohols after hydrolysis of the product imine. Transformations afford amino alcohols with >98% site selectivity with respect to both reaction partners and in up to >98% yield and >20:1 dr.

Catalytic asymmetric synthesis of CF3-substituted tertiary propargylic alcohols via direct aldol reaction of α-N3 amide

Organofluorine compounds are found in several important classes of chemicals, such as pharmaceuticals, agrochemicals, and functional materials. Chemists have been immensely interested in the development of methodologies for expeditious access to fluorine containing building blocks. In this study, we report a new method for the catalytic asymmetric synthesis of CF₃-substituted tertiary propargylic alcohols with two contiguous stereogenic centers via the direct aldol reaction of an α-N₃ amide to trifluoromethyl ketones. The key to the success of this method is the identification of a catalyst comprising Cu(ii)/chiral hydroxamic acid to promote the desired aldol reaction, constructing a tetrasubstituted carbon in a highly stereoselective fashion. Despite substantial prior advances in asymmetric catalysis, this class of catalysts has not been utilized for the formation of carbon-carbon bond-forming reactions. Our mechanistic study sheds light on the unique profile of this catalytic system, where the Cu(ii) complex plays a bifunctional role of serving as a Lewis acid and a Brønsted base. Furthermore, the densely functionalized aldol adducts undergo chemoselective transformations, affording a series of fluorine containing chiral building blocks with widespread application.

Concise, Protecting-Group-Free Synthesis of (+)-Nemonapride via Eu(OTf)3-Catalyzed Aminolysis of 3,4-Epoxy Alcohol

A concise, protecting-group-free synthesis of the antipsychotic agent (+)-nemonapride has been achieved featuring a europium(III) trifluoromethanesulfonate (Eu(OTf)₃)-catalyzed C4 selective aminolysis of a 3,4-epoxy alcohol by benzylamine and an expedient use of the resulting 4-benzylamino-1,3-diol product for constructing the pyrrolidine skeleton.

Metal-Catalyzed Directed Regio- and Enantioselective Ring-Opening of Epoxides

Control of regio- and stereoselectivity of chemical reactions is the central theme in synthetic chemistry. Regioselective and enantiospecific ring opening of readily available enantioenriched epoxides precursors provides a straightforward access to diverse highly functionalized molecules which can serve as chiral building blocks for synthesis of biologically active compounds. However, prior to our research work, the scope of catalytic highly regioselective ring opening of epoxides is limited to structurally or electronically biased epoxides, such as terminal and aromatic epoxides. Regioselective ring-opening of epoxides with substituents on both sides demonstrating similar steric and electronic effects is still a formidable challenge for organic chemists. To address this challenge, our approach is to use the readily available functional moiety incorporated in an epoxide as a directing group to realize the regioselective nucleophilic attack on the oxirane ring. Alternatively, asymmetric ring-opening of epoxides can also provide the ring-opening products in highly enantioenriched form. However, excellent results are usually obtained in the case of the kinetic resolution of terminal epoxides or the desymmetrization of meso-epoxides. In these cases, the issue of regiocontrol of the ring-opening is circumvented or minimized. Based on our successful results of regioselective ring-opening of functionalized epoxides by implementing the directed ring-opening strategy, we also investigated the enantioselective ring-opening of internal epoxides bearing a functional moiety as directing group. This Account summarizes our research on metal-catalyzed directed ring-opening reactions of epoxides, which encompasses the following breakthroughs: (1) highly regioselective ring-opening of various substrates including epoxy allylic alcohols, epoxy homoallylic alcohols and epoxy allylic sulfonamides with a variety of N-, O-, and halide-nucleophiles catalyzed by W-, Mo-, or Ni-salt; (2) first kinetic resolutions of epoxy allylic alcohols, epoxy homoallylic alcohols, and epoxy allylic sulfonamides with various amines as nucleophiles, which were catalyzed by W-bishydroxamic acid (W-BHA), nickel-BINAM, and Gd-N,N'-dioxide catalytic system, respectively; (3) successful implementation of the strategy of combined asymmetric syntheses by the combination of the enantioselective epoxidation and the enantioselective ring-opening of 2,3-epoxy alcohols establishing a new entry to prepare amino alcohols in regio-, diastereo-, and enantiomerically pure form.

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.

Synthesis of virtually enantiopure aminodiols with three adjacent stereogenic centers by epoxidation and ring-opening

A virtually complete enantioselective synthesis of 3-amino-1,2-diols with three consecutive stereocenters was accomplished by a sequential cascade of two kinetic resolutions, which features a Sharpless or Hafnium-catalyzed asymmetric epoxidation and a subsequent W-catalyzed aminolysis. Enantiopure products with up to >99.9% ee and >99.9 : 0.1 dr were obtained and could serve as potential building blocks for pharmaceutical or biological significant molecules.

Gadolinium-Catalyzed Regio- and Enantioselective Aminolysis of Aromatic trans-2,3-Epoxy Sulfonamides

The first enantioselective aminolysis of aromatic trans-2,3-epoxy sulfonamides has been accomplished, which was efficiently catalyzed by a Gd-N,N'-dioxide complex. Under the directing effect of the sulfonamide moiety the ring-opening reaction proceeded selectively at the C-3 position in a highly enantioselective manner furnishing various Ts- and SES-protected 3-amino-3-phenylpropan-2-olamines as products.