[4 + 1 + 1] Annulations of α-Bromo Carbonyls and 1-Azadienes toward Fused Benzoazaheterocycles

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.

DBU-catalyzed annulation strategy for modular assembly of 2,3-difunctionalized dihydrobenzofurans

An organocatalytic approach for the construction of 2,3-dihydrobenzofuran scaffold through a formal [4 + 1] annulation of 2-(2-nitrovinyl)phenols and α-bromoacetophenones in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) has been developed. This protocol could be easily performed in one mmol scale, giving a broad range of 2,3-dihydrobenzofuran derivatives in moderate to excellent yields and remarkable diastereoselectivity (>20 : 1 dr in general) with good functional group tolerance.

Cascade Reaction of Enyne-Amides with Sulfur-Ylides for the Synthesis of Indole-Tethered 5-Oxaspiro[2.4]hept-6-ene Derivatives

An unexpected cascade reaction of enyne-amides with sulfur-ylides has been developed. This cascade reaction involves cycloisomerization, dearomatic cyclopropanation, ring-opening rearomatization, and subsequent cyclopropanation, differing from the common [2 + n] cyclization of enyne-amides. A variety of (spirocyclopropane)dihydrofuran derivatives have been efficiently and conveniently synthesized in a single vessel, exhibiting excellent diastereoselectivity and good functional group tolerance.

Pd-catalyzed decarboxylative 1,4-addition reactions of benzofuran-based azadienes with allyl phenyl carbonates

Under the catalysis of Pd(OAc)2/dppf/Na2CO3, the decarboxylative 1,4-addition reaction of benzofuran-based azadienes with allyl phenyl carbonates took place easily and delivered the desired products in reasonable chemical yields. The chemical structure of the target compounds was clearly identified by single crystal X-ray structural analysis.

Chemodivergent Synthesis of Aza-Heterocycles with a Quarternary Carbon Center via [4 + 1] Annulation between Azoalkenes and α-Bromo Carbonyl Compounds

An efficient [4 + 1] annulation reaction between in situ generated azoalkene intermediates and α-bromocarbonyls has been established. A series of skeletally diverse aza-heterocycles with a functionalized quaternary center were obtained in up to 89% yield under mild conditions.

Phosphine-Catalyzed (4 + 2) Annulation of Allenoates with Benzofuran-Derived Azadienes and Subsequent Thio-Michael Addition

A phosphine-catalyzed (4 + 2) annulation of tetrahydrobenzofuranone-derived allenoates and benzofuran-derived azadienes (BDAs) has been achieved to construct the decahydro-2H-naphtho[1,8-bc]furan derivatives, which were subsequently treated with 4-methylbenzenethiol and trimethylamine to produce thio-Michael addition products in high to excellent yields with good diastereoselectivities.

Substrate-Switched Chemodivergent Pyrazole and Pyrazoline Synthesis: [3 + 2] Cycloaddition/Ring-Opening Rearrangement Reaction of Azadienes with Nitrile Imines

By virtue of a fundamentally new reaction model of benzofuran-derived azadienes (BDAs), an unprecedented synthesis of biologically important pyrazoles has been achieved through a tandem [3 + 2] cycloaddition/ring-opening rearrangement reaction of BDAs with nitrile imines. The nature and type of substrates are found to act as a chemical switch to trigger two distinct reaction pathways. A minor modification to the substrates allows the access to spiro-pyrazolines.

Feeding sequence-regulated divergent (4 + 1 + 1’) annulations of α-bromo carbonyls and 1-azadienes via computational calculations-based mechanism elucidation

By investigating the mechanism of our previously reported (4 + 1 + 1) annulations of α-bromo carbonyls and 1-azadienes via density functional theory (DFT) calculations, the formation of a carbon...

Cascade Cyclization of Azadienes with Difluoroenoxysilanes: A One-Pot Formal [4 + 2] Approach to Fluorinated Polyfused Heterocycles

A TfOH-promoted synthesis of fluorinated polyfused heterocycles via the cascade cyclization of azadienes and difluoroenoxysilanes has been developed, leading to the facile construction of fluorinated benzofuro[3,2-b]pyridines, 5H-indeno[1,2-b]pyridines, and 5,6-dihydrobenzo[h]quinolines. This one-pot formal [4 + 2] approach involves 1,4-difluoroalkylation, desulfonylation, cyclization, and dehydrated and dehydrofluorinated aromatization and represents the first application of difluoroenoxysilane in cascade transformations. Furthermore, this methodology is highlighted by the synthesis of three fluoro analogues of bioactive molecules with potent topoisomerase inhibitory activities.

Base-Catalyzed Sequential 1,4-Addition/Intramolecular Cyclization/Aromatization Reaction: Synthesis of Benzofuro[3,2-b]pyridines

An efficient K₂CO₃-catalyzed 1,4-addition/intramolecular cyclization/aromatization reaction of aurone-derived 1-azadienes with trisubstituted allenoates has been developed, giving a series of benzofuro[3,2-b]pyridines in moderate to excellent yields. The reaction proceeded efficiently under an air atmosphere without the use of transition metal catalysts. This protocol provides a concise approach to benzofuro[3,2-b]pyridines.

Diversity-oriented synthesis of benzofuro[3,2-b]pyridine derivatives from aurone-derived α,β-unsaturated imines and activated terminal alkynes

An efficient annulation reaction of aurone-derived α,β-unsaturated imines and activated terminal alkynes mediated by triethylamine is described, which enables the facile synthesis of 1,4-dihydrobenzofuro[3,2-b]pyridines in high yields. When the nucleophile of triethylamine was replaced with triphenylphosphine, another class of 1,4-dihydrobenzofuro[3,2-b]pyridines tethered with an additional acrylate motif were obtained instead. These two types of 1,4-dihydrobenzofuro[3,2-b]pyridines could be aromatized in the presence of DBU to afford benzofuro[3,2-b]pyridines, which could also be accessed via a one-pot procedure.

An unusual formal migrative cycloaddition of aurone-derived azadienes: synthesis of benzofuran-fused nitrogen heterocycles

Aurone-derived azadienes are well-known four-atom synthons for direct [4 + n] cycloadditions owing to their s-cis conformation as well as the thermodynamically favored aromatization nature of these processes. However, distinct from this common reactivity, herein we report an unusual formal migrative annulation with siloxy alkynes initiated by [2 + 2] cycloaddition. Unexpectedly, this process generates benzofuran-fused nitrogen heterocyclic products with formal substituent migration. This observation is rationalized by less common [2 + 2] cycloaddition followed by 4π and 6π electrocyclic events. DFT calculations provided support to the proposed mechanism.

Access to 5,6-Spirocycles Bearing Three Contiguous Stereocenters via Pd-Catalyzed Stereoselective [4 + 2] Cycloaddition of Azadienes

We present herein a highly diastereo- and enantioselective Pd-catalyzed [4 + 2] cycloaddition of benzofuran-derived azadienes with vinyl benzoxazinanones, which represents a rare highly stereoselective cycloaddition of this class of fused azadienes as a two-atom synthon. The use of a phosphoramidite ligand bearing a chiral secondary amine with a simple biphenyl backbone proved to be the key to construct the novel spirocyclic tetrahydroquinoline scaffold containing three contiguous stereocenters as a single diastereomer in high enantioselectivity.

Diastereoselective Synthesis of 1,2-Dihydrobenzofuro[3,2-b]pyridines via a Carbon-Carbon Double-Bond Cleavage/Rearrangement Cascade

A new Lewis acid-catalyzed [2 + 2] cycloaddition/retroelectrocyclization (CA-RE)/1,6-addition relay of aurone-derived 1-azadienes and 1-alkynylnaphthalen-2-ols has been reported, leading to the regio- and diastereoselective synthesis of 1,2-dihydrobenzofuro[3,2-b]pyridine with a chiral carbon center and an axial chirality in good yields. This protocol enables the C-C double-bond scission/recombination to rapidly construct aza-heterocyclic architectures and features 100% atom utilization, a wide substrate scope, good compatibility with substituents, and excellent diastereoselectivity.

Hydride Transfer Initiated Redox-Neutral Cascade Cyclizations of Aurones: Facile Access to [6,5] Spirocycles

Reported herein is the hydride transfer initiated redox-neutral cascade cyclizations of aurones, providing a variety of [6,5] spiro-heterocycles in satisfactory yields and good diastereoselectivities.

Domino synthesis of fully substituted pyridines by silver-catalyzed chemoselective hetero-dimerization of isocyanides

A silver-catalyzed hetero-dimerization of various vinyl isocyanides with isocyanoacetamides has been developed for the efficient and practical synthesis of fully substituted pyridines in a single operation.

Effective neutralization of chemical warfare agents (HD, VX) by Me-DABCOF: a small molecule with dual action

Me-DABCOF, a mild universal, non-corrosive, water-soluble decontamination agent that effectively neutralizes chemical warfare agents (HD, VX).