Practical,Modular, and General Synthesis of Benzofurans through Extended Pummerer Annulation/Cross‐Coupling Strategy

Operationally simple, efficient, and widely applicable Pummerer annulations of simple phenols with ketene dithioacetal monoxides, with the aid of trifluoroacetic anhydride, have been shown to provide a variety of benzofurans having a methylthio group at the 2‐position. Subsequent and newly developed nickel‐catalyzed arylation at the methylthio group culminates in diversity‐oriented synthesis of multisubstituted benzofurans. Our extended Pummerer annulation/cross‐coupling sequence is powerful enough to synthesize biologically active natural products as well as highly fluorescent benzofuran derivatives.     

Cascades of Interrupted Pummerer Reaction‐Sigmatropic Rearrangement

A new class of Pummerer chemistry has emerged as a powerful tool in organic synthesis. The new technology consists of a beautiful cascade of an interrupted Pummerer reaction and the subsequent [3,3] sigmatropic rearrangement. The interrupted Pummerer reactions of alkenyl or aryl sulfoxides with unsaturated nucleophiles such as allylic silanes, ketones, and phenols provide sulfonium intermediates, which are ready to undergo smooth charge‐accelerated [3,3] sigmatropic rearrangement with excellent to exclusive regioselectivity. Some of the transformations proceed with transient loss of aromaticity. The reactions afforded five‐membered heterocycles, benzofurans, and biaryls of importance, depending on the sulfoxides and nucleophiles used. The reactions are unique and game‐changing because they are efficient, robust, redox‐neutral, regioselective, and metal‐free, which perfectly fits the need of modern organic synthesis. This chemistry also underscores the synthetic potential of organosulfur chemistry.     

Hypervalent Iodine(III) Reagent Mediated Synthesis of 2‐Methylbenzofuran Derivatives by Pummerer‐type Reaction in Ionic Liquid

An efficient route for the synthesis of 2‐methylbenzofuran derivatives utilizing hypervalent iodine(III) reagent and a‐(methylthio)acetone via Pummerer‐type reaction in ionic liquid is described.     

Ene Reaction of Pummerer‐type Reaction Intermediate of (Methylthio)Acetonitrile: Synthesis of Dienenitriles

Pummerer‐type reaction intermediate 2 of (methylthio)acetonitrile ( 1 ) has been found to react with 1‐alkenes to afford ene adducts 3 . Dienenitriles ( 4 ) were synthesized from adducts 3 .     

Synthesis of a Library of Fluorescent 2‐Aryl‐3‐trifluoromethylnaphthofurans from Naphthols by Using a Sequential Pummerer‐Annulation/Cross‐Coupling Strategy and their Photophysical Properties

A library of 2‐aryl‐3‐trifluoromethylnaphthofurans was synthesized with high efficiency from simple naphthols. In this synthesis, the Pummerer‐type annulation of naphthols with 3‐(2,2,2‐trifluoroethylidene)‐2,4‐dithiapentane 2‐oxide was followed by a cross‐coupling of the resulting 2‐methylthio‐3‐trifluoromethylnaphthofurans with a variety of arylzinc reagents. A palladium complex, Pd‐PEPPSI‐IPr, was the most efficient catalyst for the arylation step, which represents the first cross‐coupling of aryl sulfides by using an N‐heterocyclic‐carbene‐ligated palladium complex. This library consists of new π‐expanded molecules, all of which are fluorescent in the solid state as well as in solution. Their photophysical properties, such as absorption and emission, fluorescence quantum yields, and fluorescence lifetimes, were thoroughly investigated. This library was also useful to identify acidochromic molecules.     

Ene Reaction with Pummerer‐type Reaction Intermediate of α‐(Methylthio)‐N‐methoxy‐N‐methyl Acetamide: A New Synthesis of N‐methoxy‐N‐methyl‐(E,E)‐2,4‐dienamides

Pummerer‐type reaction intermediate 2 of α‐(methylthio)‐N‐methoxy‐N‐methyl acetamide (1) has been found to react with 1‐alkenes to afford ene adducts 3 . N‐Methoxy‐N‐methyl‐(E,E)‐2,4‐dienamides were synthesized from the adducts 3b‐f .     

Synthesis of difluoromethylthio imidazole from methylthio imidazole

Demethylation of methylthio imidazole 1 via a Pummerer rearrangement onto the corresponding sulfoxide 2 afforded imidazole thiol 4 which was then difluoromethylated with the sodium chlorodifluoroacetate/NaI system in 71% overall yield.     

An Interrupted Pummerer/Nickel‐Catalysed Cross‐Coupling Sequence

An interrupted Pummerer/nickel‐catalysed cross‐coupling strategy has been developed and used in the elaboration of styrenes. The operationally simple method can be carried out as a one‐pot process, involves the direct formation of stable alkenyl sulfonium salt intermediates, utilises a commercially available sulfoxide, catalyst, and ligand, operates at ambient temperature, accommodates sp‐, sp²‐, and sp³‐hybridised organozinc coupling partners, and delivers functionalised styrene products in high yields over two steps. An interrupted Pummerer/cyclisation approach has also been used to access carbo‐ and heterocyclic alkenyl sulfonium salts for cross‐coupling.     

From Ynamides to Highly Substituted Benzo[b]furans: Gold(I)‐Catalyzed 5‐endo‐dig‐Cyclization/Rearrangement of Alkylic Oxonium Intermediates

A series of arylynamides with alkyloxy groups at the ortho position of the aryl group was prepared through a short alkylation/cross‐coupling/amidation sequence. The gold‐catalyzed conversion of these substrates combined both C? O and C? C formation steps, thus providing benzofurans with amine functionalities at the 2‐position and alkyl groups at the 3‐position. Cross‐over experiments showed that the alkyl‐migration step was an intermolecular process. X‐ray crystal‐structure analysis of two of the products supported our structural assignment. In some cases, the corresponding benzofurans without the alkyl group at the 3‐position were obtained as side‐products, which were formed through a competing protodeauration process.     

Palladium(0)‐Catalyzed Single and Double Isonitrile Insertion: A Facile Synthesis of Benzofurans,Indoles, and Isatins

A palladium(0)‐catalyzed cascade process consisting of isonitrile insertion and α‐Csp³?H cross‐coupling can be achieved for the synthesis of benzofurans and indoles. The construction of isatins by a Pd‐catalyzed cascade reaction incorporating double isonitrile insertion, amination, and hydrolysis has also been achieved. The key features of this work include diverse heterocycle synthesis, phosphine‐ligand‐free reaction conditions, a one‐pot procedure, simple and commercially available starting materials, broad functional‐group compatibility, and moderate to good reaction yields.     

Modification of the Swern Oxidation: Use of Stoichiometric Amounts of an Easily Separable, Recyclable, and Odorless Sulfoxide That Can Be Polymer-Bound

Readily available 6-(methylsulfinyl)hexanoic acid (1) is employed as a substitute for DMSO in Swern oxidation reactions using oxalyl chloride to smoothly convert primary or secondary alcohols to corresponding aldehydes or ketones in high yield. The resulting 6-(methylthio)hexanoic acid (2) is easily separable by aqueous extraction or by filtration through silica gel and can be reoxidized to 1 with sodium metaperiodate in 97% yield. Low temperature (-60 degrees C) (13)C NMR spectrometry is used to examine the intermediates of this Swern process. The results indicate that any residual unoxidized alcohol is generated during Pummerer elimination of the alkoxysulfonium intermediate and can be minimized by extended exposure to triethylamine at -40 degrees C. Reaction of the potassium salt of 1 with cross-linked chloromethyl polystyrene affords a polymer-bound reagent 12 that quantitatively oxidizes borneol to camphor when used in two-fold excess.     

Direct Assembly of 3,4‐Difunctionalized Benzofurans and Polycyclic Benzofurans by Phenol Dearomatization and Palladium‐Catalyzed Domino Reaction

A method to directly convert 2‐alkynylphenols to 3,4‐difunctionalized benzofurans and polycyclic benzofurans was developed. This protocol involves a hypervalent‐iodine‐mediated oxidative dearomatization to break the aromaticity of 2‐alkynylphenols, and a palladium‐catalyzed domino reaction to install two functional groups at the C3 and the C4 positions and restore the aromaticity of benzofurans.     

Interrupted Pummerer Reaction in Latent‐Active Glycosylation: Glycosyl Donors with a Recyclable and Regenerative Leaving Group

Latent O‐glycosides, 2‐(2‐propylthiol)benzyl (PTB) glycosides, were converted into the corresponding active glycosyl donors, 2‐(2‐propylsulfinyl)benzyl (PSB) glycosides, by a simple and efficient oxidation. Treatment of the PSB donor and various acceptors with triflic anhydride provided the desired glycosides in good to excellent yields. The leaving group, which was activated by an interrupted Pummerer reaction, can be recycled (PSB‐OH) and regenerated as the precursor (PTB‐OH). A natural hepatoprotective glycoside, leonoside F, was efficiently synthesized in a convergent [3+1] manner with this newly developed method. The present total synthesis also led to a structural revision of this phenylethanoid glycoside.     

Access to Indoles via Diels–Alder Reactions of 5‐Methylthio‐2‐vinylpyrroles with Maleimides

Diels–Alder reactions of 5‐methylthio‐2‐vinyl‐1H‐pyrroles with maleimides followed by isomerization gave tetrahydroindoles in moderate to good yield. Aromatization using activated MnO₂ in refluxing toluene gave the corresponding 2‐methylthioindoles in good yields, and demethylthioation using Raney nickel gave the 2‐H indoles in excellent yields. The protection of the adducts produced aromatization in improved yield, demonstrating the effectiveness of the methylthio group as a protecting group for pyrroles; however, 5‐methylthio‐2‐vinylpyrrole was shown to perform with slightly less efficiency than 2‐vinylpyrrole in Diels–Alder reactions, indicating the protective group was more deactivating than desired. This route toward indoles offers high convergency and conveniently available starting materials that are easily purified. Bis‐methylthioated vinylpyrroles were shown to have potential as highly activated Diels–Alder dienes.     

Evidence for intermolecular interaction between sulfonium and sulfide sulfur atoms and its application to synthesis of cyclic bis(disulfide) dimer

On the basis of the remote Pummerer reaction of p-bis(alkylthio)-aromatic S-oxides, the intermolecular interaction between the sulfonium and sulfide sulfur atoms is described. (1) In marked contrast to the Pummerer reaction of 1b-d(3) with (CF(3)CO)(2)O (J. Org. Chem. 1999, 64, 3190-3195), the reaction of 3,3',5,5'-tetramesityl-4-(trideuteriomethylsulfinyl)-4'-(methylthio)biphenyl (1a-d(3)) as a sterically hindered analogue of 1b gave only 2a-d(2). (2) Both reactions of the two unsymmetrical regioisomers of 1-(ethylthio)-4-(methylthio)benzene S-oxide (5a and 5b) with (CF(3)CO)(2)O afforded a mixture of the mono-Pummerer products 6a and 6b, the bis-Pummerer product 7, and the bis-sulfide 8 in a similar ratio. The quenching at the initial stage of both reactions produced 5a, 5b, 8, and the bis-sulfoxide 10 in a similar ratio. These results indicate the equilibrium in the intermolecular interaction between the sulfur atoms. (3) The reaction of the p-bis(benzylthio)-aromatic S-oxide 16 with (CF(3)SO(2))(2)O gave the cyclic bis(disulfide) dimer 17 for the diphenyl sulfide and diphenylmethane spacers or the cyclic tetrakis(disulfide) tetramer 19 for the benzene and biphenyl spacers via the debenzylation of an intermolecular dithia dication. The cyclic bis(dithia dication) dimer A resulting from the intermolecular interaction between the sulfonium and sulfide sulfur atoms is proposed as an intermediate throughout the present reactions.     

Palladium‐Catalyzed Amination of Aryl Sulfides with Anilines

A combination of a palladium–NHC catalyst and potassium hexamethyldisilazide enables the amination of aryl sulfides with anilines to afford a wide variety of diarylamines. The reaction conditions are versatile enough for the reaction of even bulky ortho‐substituted aryl sulfides. This amination can be applied to the modular synthesis of N‐aryl carbazoles from the corresponding ortho‐bromothioanisoles. As aryl sulfoxides undergo extended Pummerer reactions to afford ortho‐substituted aryl sulfides, the Pummerer products are thus useful substrates for the amination to culminate in efficient syntheses of a 2‐anilinobenzothiophene and an indole as proof‐of‐principle of the utility of the extended Pummerer reaction/amination cascade.     

Gold(I)‐Catalyzed Rearrangement of 3‐Silyloxy‐1,5‐enynes: An Efficient Synthesis of Benzo[b]thiophenes,Dibenzothiophenes, Dibenzofurans,and Indole Derivatives

With the IPr ligand (IPr=1,3‐bis‐(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) on gold(I) excellent yields in the benzanellation of 2‐substituted thiophenes, benzothiophenes, pyrroles, benzofurans, and indoles were achieved. The 1‐siloxybut‐3‐ynyl side chains, incorporated in the anellation, are easily accessible by the addition of a propargyl metal reagent to a formyl group and silylation of the alcohol. This conveniently allows an anellation at the position of the formyl group under mild conditions. All reactions involve a 2,3‐shift of the side chain in the anellation step and thus, provide an easy access to specific substitution patterns. Only in the case of 2‐substituted indoles with their highly nucleophilic 3‐position a direct hydroarylation without shift is observed. On the other hand, 3‐substituted indoles give the same products as 2‐substituted indoles. Then, a 3,2‐shift in the indole ring system has to be involved.     

Catalytic Asymmetric Dearomatization by Visible‐Light‐Activated [2+2] Photocycloaddition

A novel method for the catalytic asymmetric dearomatization by visible‐light‐activated [2+2] photocycloaddition with benzofurans and one example of a benzothiophene is reported, thereby providing chiral tricyclic structures with up to four stereocenters including quaternary stereocenters. The benzofurans and the benzothiophene are functionalized at the 2‐position with a chelating N‐acylpyrazole moiety which permits the coordination of a visible‐light‐activatable chiral‐at‐rhodium Lewis acid catalyst. Computational molecular modeling revealed the origin of the unusual regioselectivity and identified the heteroatom in the heterocycle to be key for the regiocontrol.     

Tetracyclic phenothiazines. VI. Attempted pummerer cyclization of 10-(2-Keto-3-methylsulfinylpropyl)phenothiazine

The title compound ( 3 ), a β-ketosulfoxide prepared by a Claisen-type condensation of ethyl 2-(10-phenothiazino)acetate ( 4 ) with dimsyl carbanion, was subjected to a variety of catalysts in attempts to achieve a Pummerer cyclization to 1,2-dihydro-3-methylthio-2-oxo-3H-pyrido[3,2,1-kl]phenothiazine ( 2 ). Although the Pummerer rearrangement product, 10-(2-oxo-3-hydroxy-3-methylthiopropyl)phenothiazine ( 5 ), could be obtained in excellent yield under mild conditions, neither it nor the β-ketosulfoxide could be successfully cyclized under any of the conditions attempted. Instead, phenothiazine, 2-hyroxy-3-(10-phenothiazinyl)-2-propen-1-al ( 7 ), 3-(10-phenothiazinyl)-1,1-di(methylthio)propan-2-one ( 8 ), 3-(10-phenothiazinyl)-1-methylthiopropan-2-one ( 9 ) and 10-phenothiazinylformamide ( 10 ) were variously obtained.     

Synthesis of aryl methylthio ethers employing α-oxoketene dithioacetals

Reaction of α-oxoketene dithioacetals with methallyl magnesium chloride followed by acid treatment affords simple and annulated aryl methylthio ethers. The methylthio substituent can be easily removed with Raney Ni, undergo a Ni catalyzed substitution reaction with Grignard reagents, or serve as a directing and protecting group in Friedel-Crafts acylation reactions.