Hydride Transfer-Enabled Dearomative Spirocyclization of Isoxazoles with O-Alkyl ortho-Oxybenzaldehydes

The TfOH-catalyzed dearomative [5 + 1] annulations were developed for the construction of chromane-fused spiroisoxazolines from readily available 5-amino-isoxazoles and O-alkyl ortho-oxybenzaldehydes. The "two-birds-with-one-stone" strategy featuring dearomatization of 5-amino-isoxazoles and α-C(sp3)-H bond functionalization of oxygen was disclosed via a cascade condensation/[1,5]-hydride transfer/dearomative spirocyclization process. In addition, the unprecedented direct dearomative spirocyclization of isoxazoles was achieved, which introduced a new family member of dearomative spirocyclization for dearomatization chemistry.

Asymmetric electrophilic functionalization of amino-substituted heteroaromatic compounds: a convenient tool for the enantioselective synthesis of nitrogen heterocycles

The catalytic asymmetric electrophilic functionalization of the less reactive N-heteroaromatic compounds has been reported using the approach of the introduction of an exocyclic amino substituent. This strategy has allowed enantioselective Friedel-Crafts alkylation in pyrazoles, isoxazoles and isothiazoles, as well as in aminoindoles, aminobenzofurans and aminobenzothiophenes. Several stereoselective methods have been used for the 1,4-addition or 1,2-addition of these heteroaromatic compounds to different electrophiles employing organocatalysts or chiral metal complexes. The activating exocyclic amino substituent has also been used as a nucleophile in tandem reactions, including formal cycloadditions ([3+2] and [3+3]), for the synthesis of highly functionalized chiral nitrogen heterocycles.

Mechanisms and Origins of Regio- and Stereoselectivities in NHC-Catalyzed [3 + 3] Annulation of α-Bromoenals and 5-Aminoisoxazoles: A DFT Study

Density functional theory (DFT) calculations were conducted to explore the mechanisms and origins of regio- and stereoselectivities underlying the [3 + 3] annulation reaction between α-bromoenals and 5-aminoisoxazoles with N-heterocyclic carbene (NHC) as the catalyst. The reaction occurs in nine steps: (1) nucleophilic addition of NHC to α-bromoenal, (2) Breslow intermediate formation through 1,2-proton transfer, (3) debromination, (4) α,β-unsaturated acyl azolium intermediate formation via 1,3-proton transfer, (5) addition of α,β-unsaturated acyl azolium intermediate to 5-aminoisoxazole, (6) deprotonation, (7) protonation, (8) ring closure, and (9) elimination of NHC. For the fifth step, 1,2-addition suggested in the experiment was not supported by our results. Instead, we found that Michael addition is energetically the most feasible pathway and the stereo-controlling step that preferentially provides the S-configuration product. DFT-computed results and experimental findings agree well. Analysis of distortion/interaction reveals that lower distortion energy leads to stability of the transition state corresponding to the S-configuration product. Global reactivity index analysis indicates that the behavior of the NHC catalyst differs significantly before and after the Breslow intermediate debromination. Before debromination, the nucleophilicity of α-bromoenal is enhanced by addition to NHC. However, after debromination, the α,β-unsaturated acyl azole generates and acts as an electrophilic reagent.

Phosphine-catalyzed dearomative [3+2] cycloaddition of 4-nitroisoxazoles with allenoates or Morita-Baylis-Hillman carbonates

An efficient phosphine-catalyzed dearomative [3+2] annulation of 4-nitroisoxazoles with allenoates or Morita-Baylis-Hillman carbonates has been established for the convenient synthesis of bicyclic isoxazoline derivatives. This reaction approach showed a broad substrate scope, high functional group compatibility, and excellent regioselectivity and diastereoselectivity. Furthermore, the success at the gram-scale and synthetic applications of the obtained compound 3a demonstrate the great potential of this methodology for practical applications in organic synthesis.

Recent Advances in the Domino Annulation Reaction of Quinone Imines

Quinone imines are important derivatives of quinones with a wide range of applications in organic synthesis and the pharmaceutical industry. The attack of nucleophilic reagents on quinone imines tends to lead to aromatization of the quinone skeleton, resulting in both the high reactivity and the unique reactivity of quinone imines. The extreme value of quinone imines in the construction of nitrogen- or oxygen-containing heterocycles has attracted widespread attention, and remarkable advances have been reported recently. This review provides an overview of the application of quinone imines in the synthesis of cyclic compounds via the domino annulation reaction.

Domino Michael/Oxa-Michael Reactions of the Unsymmetric Double Michael Acceptor for Access to Bicyclic Furo[2,3-b]pyrrole

By creating an unsymmetric double Michael acceptor 1, we were able to synthesize the nonaromatic-fused bicyclic furo[2,3-b]pyrrole nucleus using a domino Michael/oxa-Michael reaction. Adopting benzoyl acetonitrile 2d (CN as the electron-withdrawing group) as a substrate, we discovered a (DHQ)2AQN-catalyzed method for high diastereo- and enantioselectivity of those products. The reaction path has been determined by isolating the reaction intermediates, and density functional theory calculations support these findings. Beyond providing a synthetic approach, this work illustrated the compounds' possible use in antitumor activity.

Organocatalytic Atroposelective Construction of Pentatomic Heterobiaryl Diamines through Arylation of 5-Aminoisoxazoles with Azonaphthalenes

An efficient catalytic asymmetric Michael-type reaction of azonaphthalenes with 5-aminoisoxazoles has been developed. The reaction was based on the utilization of a chiral phosphoric acid as the catalyst, delivering a large panel of axially chiral heterobiaryl diamines in generally good yields with excellent enantioselectivities. The gram-scale reaction and postmodification of the chiral product demonstrated their potentials in the synthesis of chiral catalysts and ligands. This approach not only provides a useful method for the construction of pentatomic heterobiaryl scaffolds but also offers new members to the axially chiral diamine family with promising applications in synthetic and medicinal chemistry.

Enantioselective Synthesis of 2,3,3a,8a-Tetrahydrofuro[2,3-b]benzofuran Scaffolds Enabled by Cu(II)/SPDO-Catalyzed [3+2] Cycloaddition of 2,3-Dihydrofuran and Quinone Esters

An asymmetric [3+2] cycloaddition of quinone esters with 2,3-dihydrofuran has been realized via a newly developed Cu(II)/SPDO complex. It provides straightforward access to 2,3,3a,8a-tetrahydrofuro[2,3-b]benzofurans (TFB) with high enantioselectivity (up to 97.5:2.5 er) and diastereoselectivity (all >20:1 dr). The resulting adducts contain two adjacent stereocenters and a continuously functionalized benzene ring. Additionally, this transformation could be easily performed on a gram scale, allowing for expedient synthesis of natural dihydroaflatoxin D2 and aflatoxin B2.

Anodic dearomatization of 2-alkynylanilines for the synthesis of multi-functionalized indoles

An anodic oxidative dearomatization reaction of 2-alkynylanilines was developed. The formed dearomatized compounds were used as versatile precursors in the synthesis of a variety of benzenoid ring multi-functionalized indoles through simple conversions.

Dinuclear Zinc Catalyzed Enantioselective Dearomatization [3+2] Annulation of 2-Nitrobenzofurans and 2-Nitrobenzothiophenes

The application of dinuclear zinc catalysts in a dearomatization reaction has been developed. Catalytic asymmetric dearomatization [3+2] annulations of 2-nitrobenzofurans or 2-nitrobenzothiophenes with CF₃ -containing N-unprotected isatin-derived azomethine ylides catalyzed by dinuclear zinc catalysts are realized with excellent diastereomer ratios (dr) of >20 : 1 and enantiomeric excess (ee) of up to 99 %. This protocol provides a practical, straightforward access to structurally diverse pyrrolidinyl spirooxindoles containing a 2,3-fused-dihydrobenzofuran (or dihydrobenzothiphene) moiety, and four contiguous stereocenters. Reactions can be performed on a gram scale. The absolute configuration of products is confirmed by X-ray single crystal structure analysis, and a possible mechanism is proposed.

Silver-Catalyzed Asymmetric Dearomatization of Electron-Deficient Heteroarenes via Interrupted Barton-Zard Reaction

Herein we report a catalytic asymmetric dearomatization reaction of electron-deficient heteroarenes with α-substituted isocyanoacetates through an interrupted Barton-Zard reaction. A range of optically active pyrrolo[3,4-b]indole derivatives was obtained in good yields (up to 97 %) with high stereoselectivities (up to >20:1 dr and 97 % ee), using a catalytic system consisting of a cinchona-derived amino-phosphine and silver oxide. This reaction features wide substrate scope and mild conditions, and provides a new strategy for developing asymmetric dearomatization reactions.

Regiodivergent Organocatalytic Reactions

Organocatalysts are abundantly used for various transformations, particularly to obtain highly enantio- and diastereomeric pure products by controlling the stereochemistry. These applications of organocatalysts have been the topic of several reviews. Organocatalysts have emerged as one of the very essential areas of research due to their mild reaction conditions, cost-effective nature, non-toxicity, and environmentally benign approach that obviates the need for transition metal catalysts and other toxic reagents. Various types of organocatalysts including amine catalysts, Brønsted acids, and Lewis bases such as N-heterocyclic carbene (NHC) catalysts, cinchona alkaloids, 4-dimethylaminopyridine (DMAP), and hydrogen bond-donating catalysts, have gained renewed interest because of their regioselectivity. In this review, we present recent advances in regiodivergent reactions that are governed by organocatalysts. Additionally, we briefly discuss the reaction pathways of achieving regiodivergent products by changes in conditions such as solvents, additives, or the temperature.

A solvent-free and efficient synthesis of bicyclic 2-pyridone derivatives for endoplasmic reticulum imaging

A solvent-free method was developed for the synthesis of bicyclic 2-pyridone (DHIP) derivatives, which demonstrated excellent endoplasmic reticulum (ER) targeting and antibacterial activity after a slight structure regulation.

An enantioselective aza-Friedel-Crafts reaction of 5-aminoisoxazoles with isatin-derived N-Boc ketimines

By employing a chiral phosphoric acid as a catalyst, an enantioselective aza-Friedel-Crafts reaction of 5-aminoisoxazoles with isatin-derived N-Boc ketimines was realized. The reaction provided a wide variety of novel 3-isoxazole 3-amino-oxindoles with good yields (up to 99%) and moderate to good enantioselectivities (up to 99%). The absolute configuration of one product was assigned by X-ray crystal structural analysis and a plausible reaction mechanism was proposed. In addition, a scale-up reaction was performed successfully. Finally, one product was subjected to Suzuki-Miyaura coupling with phenylboronic acid to afford the product in a moderate yield without erosion of the enantioselectivity.

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.