Organocatalytic 1,4-Addition of Azadienes with 3-Homoacyl Coumarins toward Highly Enantioenriched Benzofuran Coumarin Skeletons

Acid-Regulated Selective Synthesis of Benzofuran Derivatives via Single-Component BDA Retro-Aldol/Michael Addition Cascade and [4 + 2] Cycloaddition Reactions

The acid-controlled single-component retro-aldol/Michael addition cascade reaction and [4 + 2] cycloaddition of benzofuran-derived azadienes (BDAs) are reported for the first time. Under the conditions of trifluoromethanesulfonic acid as the catalyst and with the addition of water, BDAs initiate the retro-aldol reaction, followed by a 1,4-Michael addition, yielding (arylmethylene)bis(dibenzofuran) products with excellent yields and broad substrate applicability. This represents the first application of BDAs in a retro-aldol reaction. In contrast, in the absence of water and with boron trifluoride etherate as the catalyst, BDAs undergo a [4 + 2] cycloaddition reaction, constructing the spiro[benzofuran-2,3'-benzofuro[3,2-b]pyridine] framework with high yields and diastereoselectivity. The method features mild conditions and high atom economy, and provides a new approach for constructing benzofuran scaffold derivatives.

Enantio- and Diastereoselective Copper-Catalyzed Borylative Coupling of Styrenes and Azadienes

Here we disclose a CuB-catalyzed reaction between aurone-derived α,β-unsaturated imines and styrenes to produce 2-substituted benzofuran derivatives bearing both the γ-boryl functionality and α,β-unsymmetric stereogenic centers. The reaction represents the first transition-metal-catalyzed unsymmetric 1,4-Michael additions of azadienes, which would enrich the arsenal of CuB catalysis in organic synthesis. In addition, the synthetically versatile boron-alkylated products can be elaborated by chemical transformations to useful optically active benzofuran heterocycles.

Copper-Catalyzed Enantioselective 1,2-Allylation of Azadienes with Allylboronates

Catalytic asymmetric 1,2-allylation of aurone-derived azadienes is very difficult to achieve due to the driving force for aromatization and the greater steric hindrance of 1,2-addition compared with 1,4-addition. By taking advantage of the ability of nitrogen ligated metal complexes, we successfully demonstrated the first example of copper-catalyzed 1,2-allylation of azadienes with allylboronates for the highly enantioselective synthesis of homoallylic amines. Meanwhile, the enantioenriched 1,4-addition products could also be obtained through a subsequent 3,3-sigmatropic rearrangement of the 1,2-addition products. Extensive DFT calculations were carried out to elucidate the origins of high regioselectivity (1,2- vs 1,4-) and enantioselectivity.

Chitosan grafting coumarin-3-carboxylic acid fluorescent fiber with enhanced strength prepared by in-situ wet-spinning

Chitosan grafting coumarin-3-carboxylic acid (CS-g-CCA) fiber with fluorescent function and enhanced tensile strength was successfully prepared by in-situ wet-spinning. FTIR and NMR results demonstrate that CCA is successfully grafted onto the CS molecule chains. As the grafting rate increases from 4.2 to 15.8 %, the spinning solution viscosity increases from 22 to 54 Pa·s. SEM observations show that the CS and CS-g-CCA fiber surfaces and cross-sections exhibit homogeneity and smoothness. Likewise, as the drawing ratio increases from 1.0 to 1.4, 2D WAXS patterns illustrate the molecular chain oriented significantly along the drawing direction. The CS-g-CCA fiber (the grafting rate of 15.8 %) exhibits a maximum breaking strength of 1.06 cn/dtex, increasing by 20 % more than the CS fiber. Meanwhile, it has a peak fluorescence intensity of around 480 nm. In addition, the continuous preparation process simplifies the technological route and improves the preparation efficiency of CS-g-CCA fiber. As-prepared CS-g-CCA fiber with enhanced tensile strength and elevated fluorescence efficiency lays the foundation for potential application in fluorescent probes, anti-counterfeiting, and biomedicine fields.

Three-component acyloxylation of diazo compounds with carboxylic acids and azadienes

Despite many studies on benzofuran-derived azadiene being reported, multi-component studies have scarcely been reported on this heterocyclic skeleton. The first cascade three-component acyloxylation of diazos with acids and azadienes has been reported under mild conditions. The reaction is applicable to various (A/A) diazo compounds, generating diverse complex benzofuran derivatives.

New bifunctional 1,3-diamine organocatalysts derived from (+)-camphoric acid for asymmetric Michael addition of 1,3-dicarbonyl compounds to nitroolefins

Novel 1,3-diamine-derived bifunctional thiourea and squaramide organocatalysts were synthesized from (+)-camphoric acid. These catalysts were easily obtained in up to two to five synthetic steps, in a flexible approach that facilitates their structure variation. Their catalytic activity was examined in the asymmetric Michael addition of 1,3-dicarbonyl compounds to several trans-β-nitrostyrenes. Yields up to 98% and enantiomeric excesses up to 74% and high diastereoselectivities when applicable (dr up to 93:7) were obtained in these reactions showing that 1,3-diamine-derived bifunctional thioureas are efficient organocatalysts.

Asymmetric Michael reaction of 3-homoacyl coumarins with chromone-fused dienes toward enantioenriched coumarin chromone skeletons

Asymmetric Michael reaction of 3-homoacyl coumarins and chromone-fused dienes was developed by employing a chiral squaramide, and a series of coumarin chromone skeletons were furnished in moderate to high yields (up to 99%) and stereoselectivities (up to 98 : 2 dr, 99% ee).

Asymmetric 1,4-Michael Addition Reaction of Azadienes with α-Thiocyanoindanones Catalyzed by Bifunctional Chiral Squaramide

In this paper, the organocatalytic asymmetric 1,4-Michael addition reaction of azadienes and α-thiocyanoindanones was investigated. A series of chiral benzofuran compounds containing thiocyano group and quaternary carbon center were synthesized in moderate yields with good enantioselectivities (up to 90:10 er) and high diastereoselectivities (up to >95:5 dr). This is the first case of 1,4-Michael addition reaction using α-thiocyanoindanones to obtain a series of chiral thiocyano compounds and further broaden the scope of application of azadiene substrates. In addition, a possible reaction mechanism is also described in the article.

Asymmetric organocatalyzed synthesis of coumarin derivatives

Coumarin derivatives are essential scaffolds in medicinal and synthetic chemistry. Compounds of this class have shown important activities, such as anticancer and antiparasitic, besides the commercially available drugs. These properties led to the development of efficient and greener synthetic methods to achieve the 2H-chromen-2-one core. In this context, the advances in asymmetric organocatalyzed synthesis of coumarin derivatives are discussed in this review, according to the mode of activation of the catalyst.

Construction of cyclopentane-fused coumarins via DBU-catalyzed [3+2] cycloaddition of 3-homoacyl coumarins with cyclic 1-azadienes

The metal-free DBU catalyzed [3+2] cycloaddition of 3-homoacyl coumarins with cyclic 1-azadienes proceeded smoothly to furnish the corresponding highly functionalized cyclopentane-fused coumarins with excellent diastereoselectivity and complete chemoselectivity and in good yields under mild conditions.

A metal-free DBU catalyzed [3+2] cycloaddition of 3-homoacyl coumarins with cyclic 1-azadienes has been developed for the synthesis of cyclopentane-fused coumarins with excellent diastereoselectivity and complete chemoselectivity.

Chiral Phosphoric Acid-Catalyzed C6 Functionalization of 2,3-Disubstituted Indoles for Synthesis of Heterotriarylmethanes

The direct regio- and enantioselective C6 functionalization of 2,3-disubstituted indoles with azadienes has been developed using chiral phosphoric acid as catalyst, providing a convenient approach to synthesize the optically active heterotriarylmethanes with excellent yields, broad substrate scope, and up to 98% ee. Mechanistic studies revealed that N-alkylation of 2,3-disubstituted indoles with azadienes would be reversible, and enantioselective C6 functionalization could be enabled.

Reactivity of steroidal 1-azadienes toward enamines: an approach to novel chiral penta- and hexacyclic steroids

The chemical behavior of steroidal N-sulfonyl-1-azadienes toward carbonyl compounds, in the presence of pyrrolidine, is described. With aldehydes, these azadienes participate in hetero-Diels-Alder reactions with the in situ generated enamines. The stereoselectivity results from the approach of the dienophiles from the less hindered α-face of the steroid, with the pyrrolidine moiety endo and retention of the enamine trans geometry. This diastereoselective synthetic methodology led to a new class of chiral pentacyclic steroids. Interestingly, the studied steroidal scaffolds follow a different mechanistic pathway with cyclic ketones. They undergo a diastereoselective annulation reaction, under enamine catalysis, affording chiral hexacyclic steroids.

Facile access to benzofuran-fused tetrahydropyridines via catalytic asymmetric [4 + 2] cycloaddition of aurone-derived 1-azadienes with 3-vinylindoles

A highly enantioselective [4 + 2] cycloaddition reaction of 1-azadienes with 3-vinylindoles, catalyzed by chiral phosphoric acid has been developed to furnish a range of benzofuran-fused tetrahydropyridines with three contiguous stereogenic centers.