Asymmetric Synthesis of Hydroindoles via Desymmetrizing [3+2] Annulation of p-Quinamines and Arylalkylketenes

The asymmetric desymmetrizing [3+2] annulation reaction of p-quinamines and arylalkylketenes to synthesize hydroindoles was realized. Catalyzed by chiral bisguanidinium hemisalt via multiple hydrogen bond interactions, enantiomerically enriched products with reversal of diastereoselectivity in comparison with the racemic version were afforded in good yields under mild reaction conditions. Diaryl-substituted hydroindoles could also perform the Friedel-Crafts type of addition to give more complicated multicycles. Density functional theory calculations revealed that the enantio- and diastereoselectivity stem from varied hydrogen-bonding manners.

Catalytic asymmetric de novo construction of 4-pyrrolin-2-ones via intermolecular formal [3+2] cycloaddition of azoalkenes with azlactones

The chiral phosphoric acid-catalyzed asymmetric intermolecular formal [3+2] cycloaddition of azoalkenes with azlactones has been established. This convergent protocol leads to a facile and enantioselective de novo construction of a wide range of fully substituted 4-pyrrolin-2-ones bearing a fully substituted carbon atom in good yields and with excellent enantioselectivities (26 examples, 72-95% yields and 87-99% ee).

Site-selective and stereoselective transformations on p-quinols & p-quinamines

The intermolecular transformation of simple substrates into highly functionalized scaffolds with multiple stereogenic centers is an attractive strategy in modern organic synthesis. Prochiral 2,5-cyclohexadienones, being stable and easily accessible, are privileged key building blocks for the synthesis of complex molecules and bioactive natural products. In particular, p-quinols and p-quinamines are important subclasses of cyclohexadienones, having both nucleophilic and electrophilic sites, and can undergo various intermolecular cascade annulations via formal cycloadditions and other transformations. This article highlights the recent developments of intermolecular transformations on p-quinols and p-quinamines along with plausible reaction mechanisms. We hope that this review will inspire the readers to explore the new potential applications of these unique prochiral molecules.

Synthesis and Antimicrobial Activity of the Pathogenic E. coli Strains of p-Quinols: Additive Effects of Copper-Catalyzed Addition of Aryl Boronic Acid to Benzoquinones

A mild and efficient protocol for the synthesis of p-quinols under aqueous conditions was developed. The pivotal role of additives in the copper-catalyzed addition of aryl boronic and heteroaryl boronic acids to benzoquinones was observed. It was found that polyvinylpyrrolidone (PVP) was the most efficient additive used for the studied reaction. The noteworthy advantages of this procedure include its broad substrate scope, high yields up to 91%, atom economy, and usage of readily available starting materials. Another benefit of this method is the reusability of the catalytic system up to four times. Further, the obtained p-quinols were characterized on the basis of their antimicrobial activities against E. coli. Antimicrobial activity was further compared with the corresponding 4-benzoquinones and 4-hydroquinones. Among tested compounds, seven derivatives showed an antimicrobial activity profile similar to that observed for commonly used antibiotics such as ciprofloxacin, bleomycin, and cloxacillin. In addition, the obtained p-quinols constitute a suitable platform for further modifications, allowing for a convenient change in their biological activity profile.

Asymmetric organocatalytic sulfenylation for the construction of a diheteroatom-bearing tetrasubstituted carbon centre

Catalytic enantioselective sulfenylation to construct diheteroatom-bearing carbon centres was achieved by employing chiral guanidine organocatalysts. This protocol provided a facile route towards the synthesis of α-fluoro-α-sulfenyl-β-ketoamides, azlactone adducts and α-sulfur-substituted amino acid derivatives in high yields with good to excellent enantioselectivities. A possible working mode was proposed to elucidate the chiral control of the process.

Synthesis of functionalized 3,2′-pyrrolidinyl spirooxindoles via domino 1,6-addition/annulation reactions of para-quinone methides and 3-chlorooxindoles

A highly efficient diastereoselective [4 + 1] cycloaddition of ortho-tosylaminophenyl-substituted p-QMs with 3-chlorooxindoles has been developed to afford 3,2′-pyrrolidinyl spirooxindoles.

Asymmetric catalytic alkynylation of thiazolones and azlactones for synthesis of quaternary α-amino acid precursors

Asymmetric alkynylation of thiazolones and azlactones with alkynylbenziodoxolones as the electrophilic alkyne source catalyzed by thiourea phosphonium salt is described. By using thiazolones as nucleophiles, the desired alkyne functionalized thiazolones were obtained in 55-89% yields with 31-86% ee. Azlactones gave the desired products in comparable yields with lower enantioselectivities. Ring-opening of the alkynylation products led to α,α-disubstituted α-amino acid derivatives efficiently without loss of enantioselectivity.

Organocatalytic asymmetric synthesis of benzothiazolopyrimidines via a [4 + 2] cycloaddition of azlactones with 2-benzothiazolimines

A chiral guanidine-catalyzed [4 + 2] cycloaddition of 2-benzothiazolimines with azlactones characterized by a high yield, ee, and dr and a broad substrate scope has been developed. Gram-scale synthesis and derivatization of the product revealed the potential of utility.

Catalytic asymmetric formal [3+2] cycloaddition of isatogens with azlactones to construct indolin-3-one derivatives

A number of enantioenriched indolin-3-one derivatives were readily obtained by chiral guanidine-catalyzed [3+2] cycloaddition of isatogens with azlactones.

Enantioselective [3 + 2] annulation of 3-hydroxymaleimides with quinone monoimines

Enantioselective [3 + 2] annulation of 3-hydroxymaleimides with quinone monoimines provided a large variety of succinimide fused indolines in moderate to good yields with moderate to good enantioselectivities.

Recent advances in the asymmetric transformations of achiral cyclohexadienones

This review describes recent developments in the asymmetric transformations of achiral cyclohexadienones, including enantioselective desymmetrization of prochiral cyclohexadienones and kinetic resolution of racemic cyclohexadienones.

[4 + 2] Annulation Reaction of In Situ Generated Azoalkenes with Azlactones: Access to 4,5-Dihydropyridazin-3(2H)-Ones

An unprecedented [4 + 2] annulation reaction between in situ formed azoalkenes and azlactones has been developed. This reaction provides a facile access to an array of 4,5-dihydropyridazin-3(2H)-one derivatives, which are very promising in medicinal applications as potential biologically active candidates. Notably, these dihydropyridazinones could also be synthesized via a one-pot reaction protocol by using the in situ formed azlactones from N-acyl amino acids and in situ generated azoalkenes from α-halogeno hydrazones. The potential applications of the methodology were also demonstrated by gram-scale experiments and the versatile conversions of the products into other nitrogen-containing compounds.

Stereocontrolled transformations of cyclohexadienone derivatives to access stereochemically rich and natural product-inspired architectures

The last two decades or so have witnessed an upsurge in defining the art of designing complex natural products and nature-inspired molecules. Throughout these decades, fundamental insights into stereocontrolled, step-economic and atom-economical synthesis principles were achieved by the numerous synthetic accomplishments particularly in diversity-oriented synthesis (DOS). This has empowered the visualization of the third dimension in synthetic design and thus has resulted in a dramatic increase with today's diversity-oriented synthesis (DOS) at the forefront enabling access to diverse scaffolds with a high degree of stereochemical and skeletal complexity. To this end, a starting material-based approach is one of the powerful tools utilized in DOS that allows rapid access to molecular architectures with a high sp3 content. Skeletal and stereochemical diversity is often paramount for the selective modulation of the biological function of a complementary protein in the biological space. In this context, stereocontrolled transformation of cyclohexadienone scaffolds has positioned itself as a powerful platform for the rapid generation of stereochemically enriched and natural product-inspired compound collections. In this review, we cover multidirectional synthetic strategies that utilized cyclohexadienone derivatives as pluripotent building blocks en route for the construction of novel chemical space.

An asymmetric hydrocyanation/Michael reaction of α-diazoacetates via Cu(i)/chiral guanidine catalysis

An asymmetric one-pot hydrocyanation/Michael reaction of α-aryl diazoacetates with trimethylsilyl cyanide, tert-butanol, and N-phenylmaleimides has been realized using a chiral guanidinium salt/CuBr catalyst.

Synthesis of chiral [2,3]-fused indolines through enantioselective dearomatization inverse-electron-demand Diels–Alder reaction/oxidation of indoles with 2-(2-nitrovinyl)-1,4-benzoquinone

Enantioselective dearomatization inverse-electron-demand Diels–Alder reaction/oxidation of indoles with 2-(2-nitrovinyl)-1,4-benzoquinone provided various novel enantioenriched [2,3]-fused indolines.

Organocatalytic asymmetric [4+2] cyclization of 2-benzothiazolimines with azlactones: access to chiral benzothiazolopyrimidine derivatives

A squaramide catalyzed regiospecific and stereoselective [4+2] cyclization of 2-benzothiazolimines with azlactones has been established.

Enantioselective dearomative [3+2] annulation of 5-amino-isoxazoles with quinone monoimines

Enantioselective dearomative [3+2] annulation of 5-amino-isoxazoles with quinone monoimines provided various (bridged) isoxazolines fused dihydrobenzofurans with moderate to good yields in moderate to good enantioselectivities.

Enantioselective synthesis of 3-amino-hydrobenzofuran-2,5-diones via Cu(i)-catalyzed intramolecular conjugate addition of imino esters

The copper-catalyzed enantioselective intramolecular conjugate addition of imino esters for desymmetrization of cyclohexadienones was described, providing access to enantioenriched 3-amino-hydrobenzofuran-2,5-diones.