Asymmetric Organocatalytic Sequential Reaction of Structurally Complex Cyclic Hemiacetals and Functionalized Nitro-olefins To Synthesize Diverse Heterocycles

Triethylamine-Promoted Henry Reaction/Elimination of HNO2/Cyclization Sequence of Functionalized Nitroalkanes and 2-Oxoaldehydes: Diversity-Oriented Synthesis of Oxacycles

The triethylamine-promoted cascade Henry reaction/elimination of HNO₂/cyclization reaction of 2-oxoaldehydes with nitroalkanes bearing various remote functionalities is described. Both chiral and achiral nitroalkanes were applicable to this protocol, leading to a variety of oxacycles, such as chromenes, chromanes, cyclic hemiacetals, and polycyclic acetals. An unexpected regioselective photooxygenation occurred without sensitizer during derivatization to convert a derived diene product into a dioxetane by reaction with singlet oxygen, which provided chromen-2-one and benzaldehyde after fragmentation.

Asymmetric Retro-Claisen Reaction Catalyzed by Chiral Aza-Bisoxazoline-Zn(II) Complex: Enantioselective Synthesis of α-Arylated Ketones

An asymmetric retro-Claisen reaction of α-monosubstituted β-diketones and quinones (or quinone imine) has been developed under the catalysis of a chiral aza-bisoxazoline-Zn(II) complex. The reaction proceeds via a sequence of conjugate addition, arylation, hemiketal anion-initiated C-C bond cleavage, and enantioselective protonation of enolate to provide various functionalized α-arylated ketones bearing a tertiary stereogenic center with high enantioselectivities. Notably, biologically important benzofuran and γ-butyrolactone derivatives could be synthesized by application of the developed protocol.

Synthesis of Chiral Polycyclic Tetrahydrocarbazoles by Enantioselective Aminocatalytic Double Activation of 2-Hydroxycinnamaldehydes with Dienals

An efficient aminocatalytic enantioselective double-activation strategy has been developed that combines several different aminocatalytic modes in a cascade process, such as iminium ion, vinylogous iminium ion, trienamine, and dienamine activations. By using this strategy, 2-hydroxycinnamaldehydes worked well with various dienals via [4 + 2] cycloaddition and the oxa-Michael reaction-initiated cascade, respectively, leading to chiral polycyclic tetrahydrocarbazole and chromane derivatives with excellent diastereo- and enantioselectivities.

Asymmetric organocatalytic multicomponent reactions for efficient construction of bicyclic compounds bearing bisacetal and isoxazolidine moieties

An organocatalytic multicomponent reaction of N-protected hydroxylamines, acrylaldehyde and acetal-containing enones was developed. Bisacetal-containing bicyclic isoxazolidine derivatives bearing four continuous stereocenters were formed with excellent stereoselectivities. A plausible reaction pathway was proposed based on ¹⁸O-labeling control experiments.

An asymmetric multicatalytic reaction sequence of 2-hydroxycinnamaldehydes and enolic 1,3-dicarbonyl compounds to construct bridged bicyclic acetals

2-Hydroxycinnamaldehydes and cyclic 1,3-dicarbonyl nucleophiles were used in an asymmetric organocatalyzed reaction sequence to construct bridged bicyclic acetals via a multicatalytic process involving iminium catalysis and anion-binding catalysis.

The Quinary Catalyst-Substrate Complex Induced Construction of Spiro-Bridged or Cagelike Polyheterocyclic Compounds via a Substrate-Controlled Cascade Process

The asymmetric organocatalytic cascade reaction of cyclic β-oxo aldehydes to 2-hydroxycinnamaldehydes is developed. The bifunctional tertiary amine-thiourea catalyst was used in a rationally designed multiple catalysis where the asymmetric iminium catalysis and thiourea anion-binding catalysis were combined by carboxylate anion as a ternary catalytic system to form a quinary catalyst-substrate complex, providing an efficient protocol for the construction of enantioenriched spiro-bridged or cagelike polyheterocyclic compounds. The reuse of catalysts was also successfully realized.

Asymmetric Organocatalyzed Reaction Sequence To Synthesize Chiral Bridged and Spiro-Bridged Benzofused Aminals via Divergent Pathways

A highly efficient asymmetric organocatalysis-triggered reaction sequence is developed. 2-Hydroxy cinnamaldehydes and cyclic N-sulfonyl ketimines were both used as multisite substrates (more than two reactive sites) to access structurally diverse chiral bridged and spiro-bridged benzofused aminal derivatives, where an inseparable equilibrating mixture of isomers can be regioselectively converted into bridged benzofused aminals with different ring connectivities via divergent pathways. Several stereoselective transformations of the resulted bridged aminals are presented.

Organocatalytic asymmetric synthesis of both cis- and trans-configured pyrano[2,3-b]chromenes via different dehydration pathways

The enamine-catalyzed [3 + 3]-cycloaddition between chroman-2-ols and β,γ-unsaturated α-ketoesters is developed to access both enantiomers of cis- and trans-fused pyrano[2,3-b]chromene derivatives.

Asymmetric organocatalyzed reaction sequence of 2-hydroxy cinnamaldehydes and acyclic N-sulfonyl ketimines to construct diverse chiral bridged polycyclic aminals

2-Hydroxy cinnamaldehydes and acyclic N-sulfonyl ketimines were used as multisite substrates in an iminium catalysis triggered sequential process, leading to diverse chiral bridged polycyclic aminals in a highly regio- and stereoselective manner.

Asymmetric construction of polycyclic indole derivatives with different ring connectivities by an organocatalysis triggered two-step sequence

Organocatalyzed reaction sequences between hemiacetals and indole-containing nitroolefins were developed for the preparation of indole derivatives with different ring connectivities.

Diversified Synthesis of Chiral Chromane-Containing Polyheterocyclic Compounds via Asymmetric Organocatalytic Cascade Reactions

Two different organocatalytic cascade reaction pathways have been developed toward the diversified synthesis of chromane-containing polyheterocyclic compounds from the readily available starting materials. The application of Hantzsch ester is proposed to be the key to achieve the switch between these two different cascade reaction pathways, and then the electron-deficient 1-aza-1,3-butadienes could be used as the four-atom and two-carbon unit, respectively, to react with 2-hydroxy cinnamaldehydes in a highly regio- and stereocontrolled manner. On the basis of larger-scale synthesis, further transformations of the obtained products have also been realized, leading to cycloadducts with high structural and stereogenic complexity bearing five stereogenic centers, and one is a tetrasubstituted stereocenter.

Two Competitive but Switchable Organocatalytic Cascade Reaction Pathways: The Diversified Synthesis of Chiral Acetal-Containing Bridged Cyclic Compounds

The organocatalytic enantioselective synthesis of methanobenzodioxepine derivatives bearing a 6,6,5-bridged ring system is presented. The m-CPBA-triggered in situ α-oxidation of β-oxoesters to provide the required but unstable α-hydroxy-β-dicarbonyl substrates is the key to this three-step sequence, providing the desired cyclic acetals with excellent stereoselectivities containing two bridgehead and one fully substituted stereocenters. It is noteworthy that the absence of m-CPBA furnished the acetal products bearing a 6,6,6-bridged ring system with similar good results from the same starting materials.