Enantioselective Vinylogous Amination of 5-Alkyl-4-nitroisoxazoles with a Dipeptide-Based Guanidinium Phase-Transfer Catalyst

Borane-Catalyzed Enantioselective α-Alkylation of Unactivated 2-Alkylbenzoxazoles with Electron-Deficient Olefins

Chiral borane-catalyzed reactions have recently emerged as a powerful tool for the enantioselective production of chiral scaffolds. In this study, we demonstrated for the first time that a chiral bisborane catalyst can be used for the α-functionalization of 2-alkylazaarenes; specifically, we accomplished unprecedented highly enantioselective α-alkylation of unactivated 2-alkylbenzoxazoles with electron-deficient olefins. The strong Lewis acidity and the steric bulk of the bisborane catalyst were essential to the observed reactivity and selectivity.

Catalytic Enantioselective Nucleophilic Addition to Arynes by a New Quaternary Guanidinium Salt-Based Phase-Transfer Catalyst

Owing to the mild generation methods, arynes have been widely used in synthetic chemistry. However, achieving asymmetric organocatalytic reactions with arynes remains a formidable and infrequent challenge, primarily because these neutral and transient species tend to spontaneously quench. To address this issue, a solid-liquid phase-transfer strategy is devised in which the generation speed of arynes could be controlled by the in situ generated fluoride-based chiral phase-transfer catalyst. In this study, we present a catalytic enantioselective nucleophilic addition reaction involving arynes, utilizing an amino amide-based guanidinium salt QG•X. Furthermore, we demonstrate the broad compatibility of this reaction with various arynes and cyclic/acyclic β-keto amides, leading to the creation of diverse α-aryl quaternary stereocenters with good stereoselectivity. Mechanistic investigations have uncovered the potential involvement of a chiral intramolecular cationic-anionic pair and HF during the ion exchange between QG•X and CsF for nucleophile activation and aryne generation. Additionally, DFT calculations suggested that the observed high levels of enantioselectivity can be attributed to steric repulsion and the cumulative noncovalent interactions between the catalysts and substrates.

Application of Biobased Solvents in Asymmetric Catalysis

The necessity of more sustainable conditions that follow the twelve principles of Green Chemistry have pushed researchers to the development of novel reagents, catalysts and solvents for greener asymmetric methodologies. Solvents are in general a fundamental part for developing organic processes, as well as for the separation and purification of the reaction products. By this reason, in the last years, the application of the so-called green solvents has emerged as a useful alternative to the classical organic solvents. These solvents must present some properties, such as a low vapor pressure and toxicity, high boiling point and biodegradability, and must be obtained from renewable sources. In the present revision, the recent application of these biobased solvents in the synthesis of optically active compounds employing different catalytic methodologies, including biocatalysis, organocatalysis and metal catalysis, will be analyzed to provide a novel tool for carrying out more ecofriendly organic processes.

Iridium-Catalyzed Asymmetric Allylic Substitution of Methyl Azaarenes

Herein, an Ir-catalyzed asymmetric allylic substitution reaction of methyl azaarenes is described. Azaarenes such as (benzo)thiazole, oxazole, benzoimidazole, pyridine, and (iso)quinoline are all tolerated. The corresponding chiral azaarene derivatives are obtained in good yields with high enantioselectivity (up to 96 % yield and 99 % ee). The utilization of the Knochel reagent TMPZnBr⋅LiBr warrants the in situ formation of benzylic nucleophiles without additional activating reagents. ¹ H NMR studies suggested a two-fold function of the Knochel reagent in this reaction. The synthetic utility of this method has been showcased by a concise enantioselective synthesis of an allosteric protein kinase modulator.

Amide phosphonium salt catalyzed enantioselective Mannich addition of isoxazole-based nucleophiles to β,γ-alkynyl-α-ketimino esters

An enantioselective Mannich addition of 3,5-disubstituted 4-nitroisoxazoles to β,γ-alkynyl-α-ketimino esters promoted by an amide phosphonium salt-based catalyst has been developed. N-Cbz-protected ketimino esters with various aryl substituents attached to the alkyne unit were reacted with a series of isoxazoles with different substitution patterns. Chiral tertiary propargylic amine products were obtained with moderate to good yields and enantioselectivities. TIPS- and cyclopropyl-substituted alkynyl ketimines were also examined in the current system and the desired products were obtained with moderate yields and enantioselectivities. The potential scalability and utility of the current protocol were demonstrated by carrying out a relatively larger scale reaction followed by further transformations.

Asymmetric Brønsted base-catalyzed aza-Michael addition and [3 + 2] cycloaddition reactions of N-ester acylhydrazones and enones

We reported the first enantioselective Brønsted base-catalyzed aza-Michael addition and stepwise cascade [3+2] cycloaddition between acylhydrazones and unsaturated ketones. The enantio-enriched aza-Michael adducts can be transformed into 2-pyrazolines.

Vinylogous and Arylogous Stereoselective Base-Promoted Phase-Transfer Catalysis

Vinylogous enolate and enolate-type carbanions, generated by deprotonation of α,β-unsaturated compounds and characterized by delocalization of the negative charge over two or more carbon atoms, are extensively used in organic synthesis, enabling functionalization and C–C bond formation at remote positions. Similarly, reactions with electrophiles at benzylic and heterobenzylic position are performed through generation of arylogous and heteroarylogous enolate-type nucleophiles. Although widely exploited in metal-catalysis and organocatalysis, it is only in recent years that the vinylogy and arylogy principles have been translated fruitfully in phase-transfer catalyzed processes. This review provides an overview of the methods developed to date, involving vinylogous and (hetero)arylogous carbon nucleophiles under phase-transfer catalytic conditions, highlighting main mechanistic aspects.

Enantioselective Bifunctional Ammonium Salt-Catalyzed Syntheses of 3-CF3S-, 3-RS-, and 3-F-Substituted Isoindolinones

We herein report the ammonium salt-catalyzed synthesis of chiral 3,3-disubstituted isoindolinones bearing a heteroatom functionality in the 3-position. A broad variety of differently substituted CF3S- and RS-derivatives were obtained with often high enantioselectivities when using Maruoka's bifunctional chiral ammonium salt catalyst. In addition, a first proof-of-concept for the racemic synthesis of the analogous F-containing products was obtained as well, giving access to one of the rare examples of a fairly stable α-F-α-amino acid derivative.

Enantioselective amination of 4-alkylisoquinoline-1,3(2H,4H)-dione derivatives

A mild and efficient enantioselective amination of 4-alkylisoquinoline-1,3(2H,4H)-dione derivatives was established, which is compatible with a broad range of substrates and delivers the final products in excellent yields (up to 99%) and ee values (up to 99%) with low catalyst loading (down to 1 mol%). The synthetic potential of this methodology was also demonstrated in the gram scale level.

Difluoroisoxazolacetophenone: A Difluoroalkylation Reagent for Organocatalytic Vinylogous Nitroaldol Reactions of 1,2-Diketones

Difluoroisoxazolacetophenone (DFIO) is developed as a new difluoroalkylation reagent that can be easily prepared from inexpensive starting materials. In situ remote C-C bond cleavage of DFIO affords γ,γ-difluoroisoxazole nitronate that undergoes base-catalyzed vinylogous nitroaldol additions to isatins, benzothiophene-2,3-dione, unsaturated-α-ketoesters, and cyclic 1,2-diketones. This organocatalytic debenzoate vinylogous nitroaldol reaction provides a new and mild approach for the preparation of various difluoroisoxazole-substituted 3-hydroxy-2-oxindoles.

Organocatalytic Asymmetric Tandem Cyclization/Michael Addition via Oxazol-5(2H)-One Formation: Access to Perfluoroalkyl-Containing N,O-Acetal Derivatives

Herein, we report a convenient organocatalytic asymmetric tandem cyclization/Michael addition protocol for the synthesis of diastereomerically pure and highly enantioenriched perfluoroalkyl-containing N,O-acetal derivatives starting from racemic N-perfluoroacyl amino acids under mild conditions. This efficient atom economic reaction leads to highly enantioselective and diastereoselective construction of N,O-acetal derivatives containing oxazolone and perfluoroalkyl moieties containing vicinal quaternary and tertiary stereocenters (up to 97% yield, up to 96% ee, and up to >20:1 dr).

Phase-transfer catalysis and the ion pair concept

This review outlines the recent advances in the field of asymmetric phase-transfer catalysis and the ion-pair concept including alkylation of amino acids and peptides, oxyindoles and other substrates, conjugate additions, fluorinations, photo-induced phase-transfer catalysis, Nitro-Mannich reactions, heterocyclizations and cycloadditions for the preparation of heterocycles, derivatization of isoxazoles, umpolung conjugate addition of imines and other three asymmetric reactions.

Allylic-Allylic Alkylation with 3,5-Dimethyl-4-nitroisoxazole: A Route to Dicarboxylic Acid Derivatives

In this work, the first application of 3,5-dimethyl-4-nitroisoxazole as a vinylogous pronucleophile in the allylic–allylic alkylation of Morita–Baylis–Hillman (MBH) carbonates is described. The reaction has been realized under nucleophilic catalysis conditions with dimeric cinchona alkaloids, providing excellent enantiocontrol of the process. The usefulness of the products thus obtained has been confirmed in selected chemoselective reactions. The most important one involves the transformation of the isoxazole moiety into a carboxylic acid group, thus opening access to dicarboxylic acid derivatives.

Recent trends in catalytic sp3 C-H functionalization of heterocycles

Heterocycles are a ubiquitous substructure in organic small molecules designed for use in materials and medicines. Recent work in catalysis has focused on enabling access to new heterocycle structures by sp³ C-H functionalization on alkyl side-chain substituents-especially at the heterobenzylic position-with more than two hundred manuscripts published just within the last ten years. Rather than describing in detail each of these reports, in this mini-review we attempt to highlight gaps in existing techniques. A semi-quantitative overview of ongoing work strongly suggests that several specific heterocycle types and bond formations outside of C-C, C-N, and C-O have been almost completely overlooked.

Direct organocatalytic asymmetric Michael reaction of fluorine hemiaminal-type nucleophile to 4-nitro-5-styrylisoxazoles

Herein, we report a chiral bifunctional thiourea catalyzed asymmetric Michael addition reaction between 2-(trifluoromethyl)oxazol-5(2H)-one as a direct C-2-position nucleophile to 4-nitro-5-styrylisoxazoles for the first time.

Tandem grinding reactions involving aldol condensation and Michael addition in sequence for synthesis of 3,4,5-trisubstituted isoxazoles

A one-pot, base-catalyzed, tandem grinding process involving carrying out aldol condensation and Michael addition in sequence to produce 3,4,5-trisubstituted isoxazoles from 3,5-dimethyl-4-nitroisoxazole, aromatic aldehydes and activated methylene compounds has been developed. In the presence of 10 mol% of pyrrolidine, aldol condensations of 3,5-dimethyl-4-nitroisoxazole with various aromatic aldehydes were performed with 3-10 minutes of grinding to provide 5-styryl-3-methyl-4-nitroisoxazoles in good to quantitative yields without further purification. Then, Michael additions between 5-styryl-3-methyl-4-nitroisoxazoles and activated methylene compounds (including ethyl 2-nitroacetate and alkyl 2-cyanoacetates) were carried out in the presence of 10 mol% of Et3N in the same mortar with 3-5 minutes of continuous grinding to produce 3,4,5-trisubstituted isoxazoles in good to excellent yields.

Cyclopentyl Methyl Ether: An Elective Ecofriendly Ethereal Solvent in Classical and Modern Organic Chemistry

Solvents represent one of the major contributions to the environmental impact of fine-chemical synthesis. As a result, the use of environmentally friendly solvents in widely employed reactions is a challenge of vast real interest in contemporary organic chemistry. Within this Review, a great variety of examples showing how cyclopentyl methyl ether has been established as particularly useful for this purpose are reported. Indeed, its low toxicity, high boiling point, low melting point, hydrophobicity, chemical stability towards a wide range of conditions, exceptional stability towards the abstraction of hydrogen atoms, relatively low latent heat of vaporization, and the ease with which it can be recovered and recycled enable its successful employment as a solvent in a wide range of synthetic applications, including organometallic chemistry, catalysis, biphasic reactions, oxidations, and radical reactions.

A chiral-at-metal asymmetric catalyzed vinylogous Michael addition of ortho-methyl aromatic nitro compounds for isoxazole derivative synthesis

A chiral-at-metal rhodium(iii) complex catalyzed asymmetric vinylogous Michael addition of 5-methyl 4-nitroisoxazoles with α,β-unsaturated 2-acyl imidazoles has been developed, delivering the corresponding adducts with up to 96% yield and 97% ee.

Direct Asymmetric Vinylogous Mannich Addition of 3,5-Disubstituted-4-nitroisoxazoles to Isatin-Derived Imines Catalyzed by a Bifunctional Phase-Transfer-Catalyst

Asymmetric Mannich-type addition of 3,5-disubstituted-4-nitroisoxazoles to isatin-derived Boc-protected imines has been realized by using 0.01 equiv of amide phosphonium salt as a phase transfer catalyst. Our current methodology allows for the formation of 3-isoxazolylmethyl-substituted 3-aminooxindoles in excellent yields with good to excellent enantioselectivities. The practical value of this methodology was exemplified by a gram-scale synthesis of 5an, a key intermediate for the formal synthesis (+)-AG-041R.

Chiral guanidines and their derivatives in asymmetric synthesis

This article reviews current achievements of chiral guanidines and their derivatives in organocatalysis, and updates versatile guanidine–metal salt combinations in asymmetric catalytic reactions.