Metal-Catalyzed Oxa-[4+2] Cyclizations of Quinone Methides with Alkynyl Benzyl Alcohols

Synthesis of fluorinated spiro-1,3-oxazines and thiazines via Selectfluor-mediated intramolecular cyclization

A fluorination-induced intramolecular cyclization for the synthesis of fluoro-substituted spiro-1,3-oxazine and spiro-1,3-thiazine derivatives is described. N-(2-(Cyclohex-1-en-1-yl)ethyl)benzamide and benzothioamide are used as the substrates, and the cationic fluorinating agent Selectfluor works as the fluoride source. The reaction yields a single diastereomer. The scope of this regioselective fluorination reaction is established by preparing thirty different spirooxazine and spirothiazine derivatives.

Brønsted acid catalyzed [4 + 2] cycloaddition for the synthesis of bisbenzannulated spiroketals with antifungal activities

The intermolecular [4 + 2] cycloaddition of o-hydroxy benzyl alcohols with isochroman ketals was realized by CF3CO2H catalysis. A broad range of bisbenzannulated [6,6]-spiroketals were formed under the metal-free mild conditions in moderate to excellent yields (45-98%) with mostly excellent diastereoselectivities (up to >20 : 1 dr). Furthermore, the enantioselective version was also preliminarily investigated and the bisbenzannulated [6,6]-spiroketal was obtained with 61% ee in the presence of Sc(OTf)3/Feng's chiral N,N'-dioxide ligand. Some of the bisbenzannulated [6,6]-spiroketal products showed good in vitro antifungal activities against Sclerotinia sclerotiorum and Rhizoctonia solani.

Recent advances in [4 + 4] annulation of conjugated heterodienes with 1,4-dipolar species for the synthesis of eight-membered heterocycles

Numerous eight-membered heterocycles are of significance in biological chemistry, the pharmaceutical industry, agrochemistry, and materials science. However, the assembly of eight-membered heterocycles is usually challenging due to the unfavorable enthalpic and entropic barriers of the transition states during the ring formation. Tremendous efforts have been devoted to the development of synthetic routes to eight-membered heterocycles. Despite these developments, the exploration of more strategies for the facile and effective assembly of eight-membered heterocyclic molecules in a single vessel under mild conditions is still highly desirable. The conjugated heterodiene-participating [4 + 4] annulation serves as a convenient and robust strategy for the synthesis of eight-membered heterocycles from easily accessible starting materials. In recent years, great progress has been achieved in this attractive field. In this short review, we highlighted the recent advances in the synthesis of eight-membered heterocycles via cascade reactions based on [4 + 4] annulation of conjugated heterodienes with 1,4-dipolar species. The brief backgrounds, the general reactions, the proposed mechanisms and their features are summarized. The prospects and challenges of this field are also outlined at the end of this review. In addition, to highlight the importance and practicality of these reactions, the properties of several series of eight-membered heterocycles have also been introduced briefly.

Facile construction of dibenzodioxo[3.3.1]nonanes bearing spirocyclohexadienones via domino [4 + 2] cycloaddition/C(sp3)-H oxidative dehydrogenation coupling reactions

A novel palladium catalyzed homodimerization of ortho-hydroxyphenyl substituted p-QMs has been developed via [4 + 2] cycloaddition/oxidative dehydrogenation coupling domino reactions. An interesting palladium catalyzed intramolecular benzyl C-H oxidation dehydrogenation to form a transannular C(sp³)-O bond was found. This protocol provided an efficient method to construct various dibenzodioxo[3.3.1]nonanes bearing spirocyclohexadienones.

Enantioselective Synthesis of Spiroketals and Spiroaminals via Gold and Iridium Sequential Catalysis

The enantioselective cascade reaction between racemic 2-(1-hydroxyallyl)phenols and alkynols/alkynamides was realized by using a gold and iridium sequential catalytic system. In this procedure, the in situ generated exocyclic vinyl ethers or enamides undergo the asymmetric allylation/spiroketalization with π-ally-Ir amphiphilic species, which provides an efficient and straightforward access to spiroketals and spiroaminals with excellent enantioselectivities. Moreover, racemic 2-(1-hydroxyallyl)anilines were also suitable in this reaction along with a kinetic resolution process, affording enantioenriched spiroaminals and 2-(1-hydroxyallyl)anilines in good yields. The synthetic utility of this method has been demonstrated by efficient enantioselective synthesis of the analogue of Paecilospirone.

Diastereoselective synthesis of chroman bearing spirobenzofuranone scaffolds via oxa-Michael/1,6-conjugated addition of para-quinone methides with benzofuranone-type olefins

A simple and convenient cyclization of ortho-hydroxyphenyl-substituted para-quinone methides with benzofuran-2-one type active olefins via oxa-Michael/1,6-conjugated addition has been developed, which afforded an easy access to enriched functionalized chroman-spirobenzofuran-2-one scaffolds with good to excellent yields (up to 90%) and diastereoselectivities (up to >19 : 1 dr). This reaction provided an efficient method for constructing desired spirocyclic compounds combining both well-known heterocyclic pharmacophores chroman and benzofuran-2-one.

Highly diastereoselective synthesis of spirocyclic compounds combining both well-known heterocyclic pharmacophores chroman and benzofuran-2-one.

Synthesis of Spiroindolenine Bearing Chroman Scaffolds Based on Cylization Reaction to para-Methylated Methides

An efficient diastereoselective cyclization reaction of ortho-hydroxyphenyl-substituted para-quinone methides with α,β-unsaturated imines generated in situ from aryl sulfonyl indoles was developed, which afforded spiroindolenine bearing chroman scaffolds in good to...

Construction of chiral chroman skeletons via catalytic asymmetric [4 + 2] cyclization of ortho-hydroxyphenyl-substituted para-quinone methides catalyzed by a chiral-at-metal rhodium complex

Construction of chiral chroman skeletons via catalytic asymmetric [4 + 2] cyclization of ortho-hydroxyphenyl-substituted p-QMs catalyzed by a chiral-at-metal rhodium complex.

[3 + 2] Coupling of Quinone Monoacetals with Vinyl Ethers Effected by Tetrabutylammonium Triflate: Regiocontrolled Synthesis of 2-Oxygenated Dihydrobenzofurans

The synthesis of 2-oxygenated dihydrobenzofurans involving the [3 + 2] coupling of quinone monoacetals with vinyl ethers has been realized by tetrabutylammonium triflate catalysis. The reaction involves a new activation method of the acetal moiety in quinone monoacetals under acid-free conditions affording the highly oxygenated dihydrobenzofurans. This new activation mode was achieved by using the triflate anion catalyst for stabilization of the highly reactive cationic intermediate.

Phosphine-catalyzed sequential (2+3)/(2+4) annulation of γ-vinyl allenoates: access to the synthesis of chromeno[4,3-b]pyrroles

A phosphine-catalyzed cascade (2+3)/(2+4) cyclization reaction of γ-vinyl allenoates with aldimine esters has been developed to provide a series of chromeno[4,3-b]pyrrole derivatives that contain three contiguous stereogenic centers. The method gives a good yield, excellent chemoselectivity and diastereoselectivity under mild conditions.

Biomimetic approach to the catalytic enantioselective synthesis of tetracyclic isochroman

Polyketide oligomers containing the structure of tetracyclic isochroman comprise a large class of natural products with diverse activity. However, a general and stereoselective method towards the rapid construction of this structure remains challenging due to the inherent instability and complex stereochemistry of polyketide. By mimicking the biosynthetic pathway of this structurally diverse set of natural products, we herein develop an asymmetric hetero-Diels–Alder reaction of in-situ generated isochromene and ortho-quinonemethide. A broad range of tetracyclic isochroman frameworks are prepared in good yields and excellent stereoinduction (up to 95% ee) from readily available α-propargyl benzyl alcohols and 2-(hydroxylmethyl) phenols under mild conditions. This direct enantioselective cascade reaction is achieved by a Au(I)/chiral Sc(III) bimetallic catalytic system. Experimental studies indicate that the key hetero-Diels-Alder reaction involves a stepwise pathway, and the steric hindrance between in-situ generated isochromene and t-Bu group of Sc(III)/N,N’-dioxide complex is responsible for the enantioselectivity in the hetero-Diels–Alder reaction step.

General and stereoselective synthesis of tetracyclic isochroman-containing polyketide oligomers is challenging. Here, the authors report on an Au(I)/chiral Sc(III) bimetallic catalyst for a biomimetic asymmetric hetero-Diels–Alder reaction for in-situ generation of isochromene and ortho-quinonemethide.

Construction of Oxygen- and Nitrogen-based Heterocycles from p-Quinone Methides

In the last few years, there has been an explosive growth in the area of para-quinone methide (p-QM) chemistry. This boom is actually due to the unique reactivity pattern of p-QMs, and also their remarkable synthetic applications. In fact, p-QMs serve as synthons for unsymmetrical diaryl- and triarylmethanes, and also for the construction of diverse range of carbocycles and heterocycles. In the last few years, a wide range of structurally complex heterocyclic frameworks could be accessed through the synthetic transformations of structurally modified stable p-QMs. Therefore, the main focus of this review article is to cover the recent advancements in the transition-metal, Lewis acid and base-catalyzed/mediated synthetic transformations of the stable p-quinone methides (p-QMs) to oxygen- and nitrogen-containing heterocycles.

Tandem (2 + 2) Annulation/Retro-4π Electrocyclization/Imino-Nazarov Cyclization Reaction of p-Quinone Methides with Ynamides: Expeditious Construction of Functionalized Aminoindenes

A new tandem annulation of p-quinone methides (p-QMs) with ynamides is described. This cascade reaction features a unique combination of (2 + 2) annulation, retro-4π electrocyclization, and imino-Nazarov cyclization, wherein vinyl p-quinone methides (p-VQMs) as one of the key intermediates have been identified chemically. Significantly, an unusual structural reconstruction of p-QMs involving the cleavage of the C5-C6 bond and the late-stage formation of the C4-C6 bond is involved, leading to a methodology development for the construction of functionalized aminoindenes.

Au(I)/(R)-BINOL-Ti(IV) Concerted Catalyzed Asymmetric Cascade Cycloaddition Reaction of Arylalkynols

An efficient catalytic asymmetric cascade cycloaddition reaction of arylalkynols with dioxopyrrolidines was developed. This reaction was achieved using Au(I) and (R)-BINOL-Ti(IV) bimetallic catalysts and exclusively delivered a series of chiral oxo-bridged bicyclic benzooxacine compounds in up to 86% yield with 96% ee as well as >33:1 dr. Meanwhile, three new σ bonds and three new stereogenic centers were formed in a one-pot process.

Divergent Asymmetric Reactions of ortho-Quinone Methides with α-Thiocyanato Indanones for the Synthesis of Spiro- and Fused-Indanones

Reported in this work is a water triggered chemo-divergent enantioselective spiro-annulation and cascade reaction of ortho-quinone methides (o-QMs) with α-thiocyanato indanones catalyzed by a chiral organic base. In the case of spiro-annulation, the use of trace amount of water as additive is critical to achieve high enantioselectivity (up to 96 % ee). We found that a cascade reaction was enabled by just tuning the ratio of water in solvent. Accordingly, two new highly efficient asymmetric reactions for the divergent synthesis of spiro- and fused-indanone scaffolds with excellent enantioselectivities (up to 99 % ee) were developed. Mechanistic investigations suggest that interfacial hydrogen bonding may play an important role in achieving the switchable reaction pathways.

Asymmetric cycloisomerization/[3 + 2] cycloaddition for the synthesis of chiral spiroisobenzofuran-1,3′-pyrrolidine derivatives

An asymmetric tandem cycloisomerization/[3 + 2] cycloaddition reaction of 2,2′-diester aziridine and 2-ethynyl benzyl alcohol with Au(i)/chiral N,N′-dioxide−Dy(iii) as a relay catalyst system was developed.

Gold(I)-Catalyzed Domino Reaction for Furopyrans Synthesis

We report herein an efficient synthesis of furopyran derivatives through a gold(I)-catalyzed domino reaction. The cascade reaction starts with two regioselective cyclizations, a 5-endo-dig and a 8-endo-dig, followed with a Grob-type fragmentation and a hetero Diels–Alder. The obtained furopyran derivatives contain fused and spiro-heterocycles. During this one-pot process, four bonds and four controlled stereogenic centers including a quaternary center are formed.

Substituted Benzothietes: Synthesis and a Quantum Chemical Investigation of Their Cycloreversion Properties

A flexible synthesis for highly substituted benzothietes that does not require flash-vacuum pyrolysis was developed. This allows for the use of a number of functional groups and nonvaporizable molecules. Highly stabilized derivatives were isolated. The molecular orbital properties of various benzothietes were evaluated by density functional methods. The mechanism of the cycloreversion of the four-membered ring was compared to that of the oxygen-containing analogues.

Total Synthesis of Caesalpinnone A

Total synthesis of caesalpinnone A was achieved in 12 steps starting from resorcinol. Key features of the synthesis include BINOL-phosphoric acid catalyzed [4 + 2] cycloaddition, trans-selective nucleophilic substitution, deallylation/oxa-Michael addition cascade, and late-stage photo-Fries rearrangement.

Gold-catalyzed domino reactions of alkynol and p-quinone methides: divergent synthesis of fused- and spiro-ketals

A gold-catalyzed domino reaction of alkynyl alcohols and p-quinone methides to obtain divergent fused- and spiro-ketals has been developed.

Cocatalyst-controlled divergent cascade cycloaddition reaction of arylalkynols and dioxopyrrolidienes: access to spiroketals and oxa-bridged eight-membered cyclic ethers

A cocatalyst-controlled divergent cascade cycloaddition reaction was developed for the synthesis of two different complex oxygen-containing heterocyclic compounds from arylalkynols and dioxopyrrolidienes in the presence of Au(i) catalyst.

Enantioselective one-pot synthesis of 4H-chromene derivatives catalyzed by a chiral Ni(ii) complex

A Ni(ii)–bis(oxazoline) complex and p-TSOH are used to form enantioenriched 4H-chromenes from ortho-quinone methides (o-QMs) and dicarbonyls, providing the desired products in up to 95% ee. The method is compatible with various β-ketoester substrates, and the products obtained could be converted into biologically active 4H-chromene derivatives.

A Ni(ii)–bis(oxazoline) complex and p-TSOH are used to form enantioenriched 4H-chromenes from ortho-quinone methides (o-QMs) and dicarbonyls, providing the desired products in up to 95% ee.

Recent developments in 1,6-addition reactions of para-quinone methides (p-QMs)

In this review, we provide a comprehensive overview of recent progress in this rapidly growing field by summarizing the 1,6-conjugate addition and annulation reactions of p-QMs with consideration of their mechanisms and applications.

Organocatalytic and enantioselective [4+2] cyclization between hydroxymaleimides and ortho-hydroxyphenyl para-quinone methide-selective preparation of chiral hemiketals

A highly effective and enantioselective organocatalytic [4+2] cyclization has been disclosed, and a series of new chiral hemiketals containing chromane and succinimide frameworks have been firstly prepared in excellent results with 100% atom efficacy.

Cu(ii)-Catalyzed formal [4 + 2] cycloaddition between quinone methides (QMs) and electron-poor 3-vinylindoles

A new Cu(ii)-catalyzed formal [4 + 2] cycloaddition between quinone methides (QMs) including para-quinone methides (p-QMs) and ortho-quinone methides (o-QMs) and electron-poor 3-vinylindoles was established.

Palladium-catalyzed enantioselective [5 + 4] annulation of ortho-quinone methides and vinylethylene carbonates

Highly enantioselective [5 + 4] annulations of ortho-quinone methides with vinylethylene carbonates are enabled by asymmetric palladium catalysis for the synthesis of chiral nine-membered benzo-heterocycles.

Phosphine-catalyzed [4 + 2] cyclization of para-quinone methide derivatives with allenes

The first [4 + 2] cyclization of para-quinone methide derivatives with allenes has been established via phosphine catalysis, which afforded a series of chroman derivatives in high yields (up to 97%) and excellent (E/Z)-selectivities (all >95 : 5 E/Z).

Organocatalytic [4 + 2] cyclizations of para-quinone methide derivatives with isocyanates

The first [4 + 2] cyclizations of o-hydroxyphenyl-substituted p-quinone methide derivatives with isocyanates have been established under the catalysis of Brønsted acids or Brønsted bases, which efficiently constructed benzoxazin-2-one frameworks in generally high yields.

An organic-base catalyzed asymmetric 1,4-addition of tritylthiol to in situ generated aza-o-quinone methides at the H2O/DCM interface

Highly enantioselective organocatalytic Michael addition of tritylthiol to N-o-QM intermediates generated in situ at the H₂O/DCM interface is presented.