Chiral ion-pair organocatalyst promotes highly enantioselective 3-exo iodo-cycloetherification of allyl alcohols

Organocatalytic Enantioselective Chloroiminocyclization for the Synthesis of Imidazoline

Imidazoline is an important scaffold in organic synthesis and a pharmacophore in medicinal chemistry. We apply basic imines as nucleophiles for the catalytic asymmetric chloroiminocyclization to furnish tetrasubstituted stereocenter-containing imidazolines in excellent yields and enantioselectivities. The reaction can be conducted in the polar solvent acetonitrile under concentrated reaction conditions.

A Domino Radical Amidation/Semipinacol Approach to All-Carbon Quaternary Centers Bearing an Aminomethyl Group

A photoredox-mediated radical amidation ring-expansion sequence that enables the generation of all-carbon quaternary centers bearing a protected aminomethyl substituent is described. The methodology can be applied to both styrene and unactivated alkene substrates generating structurally diverse sp3 -rich amine derivatives in a concise manner.

Additive-controlled asymmetric iodocyclization enables enantioselective access to both α- and β-nucleosides

β-Nucleosides and their analogs are dominant clinically-used antiviral and antitumor drugs. α-Nucleosides, the anomers of β-nucleosides, exist in nature and have significant potential as drugs or drug carriers. Currently, the most widely used methods for synthesizing β- and α-nucleosides are via N-glycosylation and pentose aminooxazoline, respectively. However, the stereoselectivities of both methods highly depend on the assisting group at the C2' position. Herein, we report an additive-controlled stereodivergent iodocyclization method for the selective synthesis of α- or β-nucleosides. The stereoselectivity at the anomeric carbon is controlled by the additive (NaI for β-nucleosides; PPh₃S for α-nucleosides). A series of β- and α-nucleosides are prepared in high yields (up to 95%) and stereoselectivities (β:α up to 66:1, α:β up to 70:1). Notably, the introduced iodine at the C2' position of the nucleoside is readily functionalized, leading to multiple structurally diverse nucleoside analogs, including stavudine, an FDA-approved anti-HIV agent, and molnupiravir, an FDA-approved anti-SARS-CoV-2 agent.

A Combined DFT, Energy Decomposition, and Data Analysis Approach to Investigate the Relationship Between Noncovalent Interactions and Selectivity in a Flexible DABCOnium/Chiral Anion Catalyst System

Developing strategies to study reactivity and selectivity in flexible catalyst systems has become an important topic of research. Herein, we report a combined experimental and computational study aimed at understanding the mechanistic role of an achiral DABCOnium cofactor in a regio- and enantiodivergent bromocyclization reaction. It was found that electron-deficient aryl substituents enable rigidified transition states via an anion-π interaction with the catalyst, which drives the selectivity of the reaction. In contrast, electron-rich aryl groups on the DABCOnium result in significantly more flexible transition states, where interactions between the catalyst and substrate are more important. An analysis of not only the lowest-energy transition state structures but also an ensemble of low-energy transition state conformers via energy decomposition analysis and machine learning was crucial to revealing the dominant noncovalent interactions responsible for observed changes in selectivity in this flexible system.

Asymmetric Intermolecular Iodinative Difunctionalization of Allylic Sulfonamides Enabled by Organosulfide Catalysis: Modular Entry to Iodinated Chiral Molecules

Electrophilic halogenation of alkenes is a powerful transformation offering a convenient route for the construction of valuable functionalized molecules. However, as a highly important reaction in this field, catalytic asymmetric intermolecular iodinative difunctionalization remains a formidable challenge. Herein, we report that an efficient Lewis basic chiral sulfide-catalyzed approach enables this reaction. By this approach, challenging substrates such as γ,γ-disubstituted allylic sulfonamides and 1,1-disubstituted alkenes with an allylic sulfonamide unit undergo electrophilic iodinative difunctionalization to give a variety of iodine-functionalized chiral molecules in good yields with excellent enantio- and diastereoselectivities. A series of free phenols as nucleophiles are successfully incorporated into the substrates. Aside from phenols, primary and secondary alcohols, fluoride, and azide also serve as efficient nucleophiles. The obtained iodinated products are a good platform molecule, which can be easily transformed into various chiral compounds such as α-aryl ketones, chiral secondary amines, and aziridines via rearrangement or substitution. Mechanistic studies revealed that the chiral sulfide catalyst displays a superior effect on control of the reactivity of electrophilic iodine and the enantioselective construction of the chiral iodiranium ion intermediate and catalyst aggregates might be formed as a resting state in the reactions.

Intermolecular Catalytic Asymmetric Iodoetherification of Unfunctionalized Alkenes

A newly prepared trinuclear Zn₃-(R,S,S)-aminoiminobinaphthoxide complex (triZn-II) catalyzed the first general intermolecular asymmetric iodoetherification of unfunctionalized alkenes. Using triZn-II, the iodoetherification reaction of unfunctionalized alkenes with o-nitrophenols proceeded smoothly to give the products with up to 92.5:7.5 er, and diene substrates were converted to the products with up to 99:1 er with the formation of a meso-isomer (dl/meso = 78/22). The chiral iodoethers gave a new platform for the synthesis of chiral morpholines.

The applications of catalytic asymmetric halocyclization in natural product synthesis

Catalytic asymmetric halocyclization of olefinic substrate has evolved rapidly and been well utilized as a practical strategy for constructing enantioenriched cyclic skeletons in natural product synthesis.

A concise synthesis of herbertenolide

A concise synthesis of (±)-herbertenolide has been accomplished herein. The strategy relies on a H2O2-mediated oxidative ring contraction of all-substituted cyclic α-formyl ketones for the stereospecific construction of contiguous quaternary...

Accessing Chiral 2,2-Disubstituted Morpholines via Organocatalytic Enantioselective Chlorocycloetherification

Chiral morpholine is an important scaffold in organic synthesis and a pharmacophore in medicinal chemistry. However, catalytic enantioselective procedure for the construction of morpholine remains sparse. We report herein a...

Asymmetric Bromoaminocyclization and Desymmetrization of Cyclohexa-1,4-dienes through Anion Phase-Transfer Catalysis

The catalytic enantioselective desymmetrizing bromoaminocyclization of prochiral cyclohexa-1,4-dienes has been achieved by using chiral anion phase-transfer catalysis, providing a range of enantioenriched cis-3a-arylhydroindoles bearing an all-carbon quaternary stereocenter in good yields (up to 78%) and excellent enantioselectivities (up to 97% ee). Furthermore, the potential application of this methodology to natural product total synthesis was demonstrated by the asymmetric synthesis of (+)-Mesembrane.

Asymmetric Halocyclizations of 2-Vinylbenzyl Alcohols with Chiral FLPs

By the use of a chiral frustrated Lewis pair (FLP) consisting of a chiral-diene-derived borane and ^tBu₃P as the catalyst, an asymmetric halocyclization of 2-vinylbenzyl alcohols with NBS or NIS was successfully realized. A variety of optically active 1,3-dihydroisobenofuran derivatives were obtained in high yields with up to 87% ee and could be conveniently converted to other useful chiral compounds.

Catalytic Dynamic Kinetic Resolutions in Tandem to Construct Two-Axis Terphenyl Atropisomers

The defined structure of molecules bearing multiple stereogenic axes is of increasing relevance to materials science, pharmaceuticals, and catalysis. However, catalytic enantioselective approaches to control multiple stereogenic axes remain synthetically challenging. We report the catalytic synthesis of two-axis terphenyl atropisomers, with complementary strategies to both chlorinated and brominated variants, formed with high diastereo- and enantioselectivity. The chemistry proceeds through a sequence of two distinct dynamic kinetic resolutions: first, an atroposelective ring opening of Bringmann-type lactones produces a product with one established axis of chirality, and second, a stereoselective arene halogenation delivers the product with the second axis of chirality established. In order to achieve these results, a class of Brønsted basic guanidinylated peptides, which catalyze an efficient atroposelective chlorination, is reported for the first time. In addition, a complementary bromination is reported, which also establishes the second stereogenic axis. These bromo-terphenyls are accessible following the discovery that chiral anion phase transfer catalysis by C₂-symmetric phosphoric acids allows catalyst control in the second stereochemistry-determining event. Accordingly, we established the fully catalyst-controlled stereodivergent synthesis of all possible chlorinated stereoisomers while also demonstrating diastereodivergence in the brominated variants, with significant levels of enantioselectivity in all cases.

Regio- and Enantioselective Bromocyclization of Difluoroalkenes as a Strategy to Access Tetrasubstituted Difluoromethylene-Containing Stereocenters

Difluoromethylene-containing compounds have attracted substantial research interest over the past decades for their ability to mimic biological functions of traditional functional groups while providing a wide variety of pharmacological benefits bestowed by the C-F bond. We report a novel strategy to access RCF2Br-containing heterocycles by regio- and enantioselective bromocyclization of difluoroalkenes enabled by chiral anion phase-transfer catalysis. The utility of this methodology was highlighted through a synthesis of an analogue of efavirenz, a drug used for treating HIV. Additionally, the synthetic versatility of the CF2Br intermediates was showcased through functionalization to a variety of enantioenriched α,α-difluoromethylene-containing products.

Enantioselective Syntheses of 4H-3,1-Benzoxazines via Catalytic Asymmetric Chlorocyclization of o-Vinylanilides

The catalytic asymmetric halocyclization of alkene is a powerful and straightforward strategy for the synthesis of chiral heterocyclic compounds. Herein, an effective approach to chiral benzoxazine derivatives through organocatalyzed chlorocyclization of o-vinylanilides was reported. This method provides facile access to a series of chiral benzoxazines in good to excellent yields (up to 99% yield) and with high-level enantiocontrol (up to 92% ee).

Enantioselective Kinetic Resolution/Desymmetrization of Para-Quinols: A Case Study in Boronic-Acid-Directed Phosphoric Acid Catalysis

A chiral phosphoric acid-catalyzed kinetic resolution and desymmetrization of para-quinols operating via oxa-Michael addition was developed and subsequently subjected to mechanistic study. Good to excellent s-factors/enantioselectivities were obtained over a broad range of substrates. Kinetic studies were performed, and DFT studies favor a hydrogen bonding activation mode. The mechanistic studies provide insights to previously reported chiral anion phase transfer reactions involving chiral phosphate catalysts in combination with boronic acids.

[1,3]/[1,4]-Sulfur atom migration in β-hydroxyalkylphosphine sulfides

β-Hydroxyalkylphosphine sulfides undergo [1,3]- or [1,4]-sulfur atom phosphorus-to-carbon migration in the presence of Lewis or Brønsted acids. The direction of sulfur atom migration depends on the type of acid used for the reaction. In the presence of a Brønsted acid, mainly [1,3]-rearrangement is observed, whereas a Lewis acid catalyzes the [1,4]-sulfur migration. To gain insight into the mechanism of these transformations, the stereochemistry of these rearrangements have been tested, along with the conduction of some control experiments and DFT calculations.

Carboxylate phosphabetaine as a bifunctional organocatalyst for the intramolecular ring opening of oxetane

Carboxylate phosphabetaine acts as a competent organocatalyst for promoting the intramolecular ring opening of oxetanes.

Enantioselective Bromolactonization of Trisubstituted Olefinic Acids Catalyzed by Chiral Pyridyl Phosphoramides

Enantioselective bromolactonization of trisubstituted olefinic acids producing synthetically useful chiral lactones with two contiguous asymmetric centers has remained mainly unexplored except for the 6-exo cyclization mode. In this work, the 5-exo- and 6-endo modes of bromocyclization of trisubstituted olefinic acids were enabled for the first time using N-bromosuccinimide and a pyridyl phosphoramide catalyst. The utility of the resulting bromolactones was demonstrated by transformations harnessing reactive alkyl bromide moieties without losing stereochemical information. Optimization studies and control experiments revealed that the basicity of pyridine moieties and presence of N-H protons in the phosphoramide species strongly affected both the reactivity and enantioselectivity parameters.

Synthesis of Enantioenriched Bromohydrins via Divergent Reactions of Racemic Intermediates from Anchimeric Oxygen Borrowing

We report a chiral phosphoric acid catalyzed bromocyclization/regiodivergent reaction of racemic intermediates sequence, which is enabled by anchimeric oxygen borrowing. Different types of alkenes are applicable, and both enantiomers of the bromohydrin products were obtained in generally excellent yields and enantioselectivities. In addition, an example of enantioconvergent synthesis from the two isomeric products is presented.

One-step pathway to selenoisobenzofuran-1(3H)-imine derivatives through highly selective selenocyclization of olefinic amides with benzeneselenyl chloride

A 1,4-diazabicyclo[2.2.2]octane (DABCO) catalyzed selenocyclization of olefinic amides was achieved under mild reaction conditions. The reaction formed various benzeneselenyl substituted isobenzofuran-1(3H)-imine derivatives in good yields. The product was determined using single-crystal X-ray analysis. For compound 2u, the relative stereochemistry was established on the basis of NOESY NMR studies.

Iron-catalyzed or iodine-induced intramolecular halocyclization of N-vinyl-tethered methylenecyclopropanes: facile access to halogenated 1,2-dihydroquinolines

Iron(iii) catalyst or I₂ acts as a Lewis acid to activate enamines, affording the corresponding halogenated 1,2-dihydroquinolines in good yields.

Biomimetic Approach to the Catalytic Enantioselective Synthesis of Flavonoids

Herein is reported the direct asymmetric addition of phenol nucleophiles to benzopyrylium salts as a means to produce enantioenriched flavonoid-like compounds. This enantioselective C-C bond construction was achieved through a chiral anion phase-transfer strategy that mimics the proposed biosynthesis of this structurally diverse set of natural products. The utility of this methodology was demonstrated in enantioselective synthesis of a 2,8-dioxabicyclo[3.3.1]nonane and a 2,4-diarylbenzopyran.

Acceleration of Acid-Catalyzed Hydrolysis in a Biphasic System by Sodium Tetracyanocyclopentadienides

The hydrolysis of tert-butyldimethylsilyl L-menthyl ether (3) in a CH2Cl2-1 M HCl biphasic solvent system was accelerated by the addition of sodium tetracyanocyclopentadienides 1. Particularly, the reaction rate was enhanced using sodium salt 1a-c with a lipophilic substituent on the cyclopentadienide ring. From the results obtained by a triphasic experiment, hydrolysis proceeds via the formation of hydronium ion 2 in the aqueous phase by ion exchange, followed by the transfer of 2 to the CH2Cl2 phase.

Enantioselective bromocyclization of 2-geranylphenols induced by chiral phosphite-urea bifunctional catalysts

Chiral phosphite-urea bifunctional catalysts have been developed for the enantioselective bromocyclization of 2-geranylphenols with N-bromophthalimide (NBP) for the first time. The chiral triaryl phosphite moiety activates NBP to generate a bromophosphonium ion. On the other hand, the urea moiety interacts with a hydroxyl group of the substrate through hydrogen bonding interactions. Enantioselectivity is effectively induced through two-point attractive interactions between the catalyst and the substrate.