Dearomatization through Halofunctionalization Reactions

Electrochemical Synthesis of Spiro[4.5]trienones through Radical-Initiated Dearomative Spirocyclization

A novel and green route has been developed for the electrochemical synthesis of spiro[4.5]trienones through radical-initiated dearomative spirocyclization of alkynes with diselenides. This metal-free and oxidant-free electrosynthesis reaction was performed in an undivided cell under mild conditions. A variety of selenation spiro[4.5]trienones products were prepared in moderate-to-good yields, showing a broad scope and functional group tolerance. Moreover, the developed continuous-flow system combined with electrosynthesis possesses the potential to achieve scaled-up reactions, overcoming the low efficiency of conventional electrochemical scaled-up reactions.

Dearomative Allylation of Naphthyl Cyanohydrins by Palladium Catalysis: Catalyst-Enhanced Site Selectivity

A dearomative allylation of naphthyl cyanohydrins with allyl borates and allyl stannanes under palladium catalysis was developed. At the initial stage of this study, the dearomative reaction (C4 substitution of the aromatics) was competing with benzyl substitution. To circumvent this issue, the use of palladium and meta-disubstituted triarylphosphine as the catalyst in a 1:1 ratio was found to enhance the site selectivity, furnishing the desired dearomatized products. Further derivatizations of products were also successful.

Enantioselective Syntheses of 4H-3,1-Benzoxazines via Catalytic Asymmetric Chlorocyclization of o-Vinylanilides

The catalytic asymmetric halocyclization of alkene is a powerful and straightforward strategy for the synthesis of chiral heterocyclic compounds. Herein, an effective approach to chiral benzoxazine derivatives through organocatalyzed chlorocyclization of o-vinylanilides was reported. This method provides facile access to a series of chiral benzoxazines in good to excellent yields (up to 99% yield) and with high-level enantiocontrol (up to 92% ee).

Progresses in organocatalytic asymmetric dearomatization reactions of indole derivatives

This review summarizes the progresses in organocatalytic asymmetric dearomatization reactions of indole derivatives and their applications in total synthesis of natural products, and gives some insights into challenging issues in this research field.

Asymmetric acyl-Mannich reaction of isoquinolines with α-(diazomethyl)phosphonate and diazoacetate catalyzed by chiral Brønsted acids

α-Diazo-β-isoquinoline derivatives were obtained in excellent yields and enantioselectivities by asymmetric acyl-Mannich reaction of (diazomethyl)phosphonate or diazoacetate with isoquinolines.

Catalytic asymmetric multiple dearomatizations of phenols enabled by a cascade 1,8-addition and Diels-Alder reaction

A direct catalytic asymmetric multiple dearomatization reaction of phenols was disclosed, which provides expedient access to a series of architecturally complex polycyclic compounds bearing four stereogenic centers in high enantiopurity. The key to achieve such a transformation is the combination of a dearomative 1,8-addition of β-naphthols to para-quinone methides generated in situ from propargylic alcohols and a subsequent intramolecular dearomative Diels–Alder reaction. Noteworthily, this protocol enrichs not only the diversity of dearomatized products but also the toolbox of dearomatization strategies.

The first chiral phosphoric acid catalyzed asymmetric multiple dearomatizations of phenols for the synthesis of bridged polycyclic compounds are reported.

Elucidation of Spirodactylone, a Polycyclic Alkaloid from the Sponge Dactylia sp., and Nonenzymatic Generation from the Co-metabolite Denigrin B

Spirodactylone (1), a hexacyclic indolizidone alkaloid possessing a novel spiro ring system, was isolated from the marine sponge Dactylia sp. The structure was elucidated by extensive spectroscopic methods including application of the LR-HSQMBC NMR pulse sequence. Oxidative cyclization of denigrin B (2), an aryl-substituted 2-oxo-pyrroline derivative that was also isolated from the sponge extract, provided material identical to spirodactylone (1). This confirmed the assigned structure and provides insight into the probable biogenesis of 1.

Multicatalytic dearomatization of phenols into epoxyquinols via a photooxygenation process

A multicatalytic photooxygenation of substituted phenols in the presence of rose bengal and cesium carbonate under green LED light is reported. This transformation enabled the introduction of both atoms of singlet oxygen and led to the one-pot synthesis of epoxyquinols in a stereoselective way.

Anthracene-Based Cyclophanes with Selective Fluorescent Responses for TTP and GTP: Insights into Recognition and Sensing Mechanisms

Three anthracene-based cyclophanes were synthesized and their binding properties towards nucleoside triphosphates were studied. A new polycyclic amine derived from dearomatized anthracene was identified as a major side product in the cyclization reaction between 9,10-anthracenedicarboxaldehyde and diethylenetriamine. Its structure was determined by single-crystal X-ray analysis. The cyclophanes were found to form 1:1 complexes with all nucleoside triphosphates as well as with pyrophosphate in a buffered aqueous solution at pH 6.2. A turn-on fluorescence response was observed for all nucleotides except for GTP, which demonstrated strong fluorescence quenching. The strongest turn-on fluorescence was observed for the largest receptor 3 in the presence of thymidine triphosphate (TTP). Based on the NMR and fluorescence experiments, two major binding modes for nucleotide complexes were identified.

Controllable synthesis of 3-iodo-2H-quinolizin-2-ones and 1,3-diiodo-2H-quinolizin-2-onesviaelectrophilic cyclization of azacyclic ynones

An efficient approach was developed to divergently synthesize 3-ipdo-2H-quinolizin-2-ones and 1,3-diiodo-2H-quinolizin-2-ones from azacyclic ynones with high regioselectivity under metal-free, room temperature conditions in air.

A DFT study on the mechanism and origins of the ligand-controlled regioselectivity of a palladium-catalyzed dearomatic reaction of 1-(chloromethyl)naphthalene with phenylacetonitrile

DFT calculations demonstrated that the bulky ligand ^tBuPPh₂ generates para-substituted products, while the small ligand Me₂PPh provides ortho-substituted products.

Pd-Catalyzed Dearomative Carboxylation of Indolylmethanol Derivatives

By using a new catalytic system (PdCl₂[P( n-Bu)₃]₂ in combination with ZnEt₂), various 3-indolylmethanol derivatives were successfully carboxylated with CO₂ (1 atm) via dearomatization of the indole nucleus, affording 3-methyleneindoline-2-carboxylates. In contrast, carboxylation of 2-indolylmethanol derivatives afforded unexpected doubly carboxylated products, which are useful synthetic precursors for biologically active compounds.

Catalytic Regio- and Enantioselective Haloazidation of Allylic Alcohols

Herein we report a highly regio- and stereoselective haloazidation of allylic alcohols. This enantioselective reaction uses readily available materials and can be performed on a variety of alkyl-substituted alkenes and can incorporate either bromine or chlorine as the electrophilic halogen component. Both halide and azido groups of the resulting products can be transformed into valuable building blocks with complete stereospecificity. The first example of an enantioselective 1,4-haloazidation of a conjugated diene is reported as well as its application to a concise synthesis of an aza-sugar.

Copper-Catalysed Bromoamination of Maleimides using NBS as the Bromine Source

CuBr2-catalysed bromoamination of maleimides has been achieved in THF with alkylamines and N-bromosuccinimide as nitrogen and bromine sources respectively. The reaction conditions were optimised. A series of bromoamination products such as 3-amino-4-bromomaleimides was synthesised with satisfactory yields. Furthermore, a plausible mechanism of the bromoamination of maleimides was discussed.

Dearomatizing Spiroannulation Reagents: Direct Access to Spirocycles from Indoles and Dihalides

Unfunctionalized indoles can be directly converted into 3,3'-spirocyclic indolenines and indolines upon reaction with electrophilic dihalides in the presence of t-BuOK/BEt₃. This double C-C bond forming reaction, which simultaneously generates a quaternary spirocyclic center, typically proceeds in high yield and has good functional group tolerance. In contrast to existing dearomatizing spirocyclization approaches, there is no need to prepare a prefunctionalized aromatic precursor, enabling faster access to valuable spirocyclic products from simple, commercially available aromatics in one step.

Asymmetric fluorinative dearomatization of tryptamine derivatives

An asymmetric fluorinative dearomatization reaction of tryptamine derivatives was developed by using a chiral anion phase transfer catalyst (PTC) system, and the preliminary results of the reaction mechanistic study were achieved. This method is characterized by a simple operation, facile introduction of a fluorine atom in a highly enantioselective manner and construction of two contiguous quaternary stereogenic centers.

Palladium(0)-Catalyzed Intermolecular Asymmetric Allylic Dearomatization of Polycyclic Indoles

An intermolecular Pd-catalyzed allylic dearomatization reaction of polycyclic indoles with substituted allylic carbonates was realized in the presence of a newly synthesized chiral phosphoramidite ligand. Various polycyclic indoline and indolenine derivatives were successfully synthesized in excellent yields (up to 99%) with excellent enantioselectivity (up to 98% ee). The obtained products could undergo versatile transformations, increasing the application potential of the method in organic synthesis.

Chemoselective N–H functionalization of indole derivatives via the Reissert-type reaction catalyzed by a chiral phosphoric acid

An asymmetric N-alkylation of indole derivatives via the Reissert-type reaction was realized in the presence of 10 mol% chiral phosphoric acid.