Transition-metal-catalyzed rearrangement of allenyl sulfides: a route to furan derivatives

Site-Selective Direct γ-Difunctionalization of Diazoenals: Application to the Synthesis of Enal-Functionalized Allenes and Furans

A new approach for the synthetically important γ-functionalized enals has been developed. The strategy involves rhodium-catalyzed direct C-C & C-S bond forming site-selective γ-difunctionalization of diazoenals with aryl propargyl sulfides via sulfur ylide [2,3] sigmatropic rearrangement, resulting in the highly functionalized γ-allenyl(sulfanyl)enals in excellent yield at ambient temperature. This highly versatile approach constitutes a viable alternative to the remote carbonyl-directed γ-functionalization of unmodified enals which suffer from competitive side reactions. The synthetic utility of the γ-allenyl(sulfanyl)enals was demonstrated by the InCl3-catalyzed cycloisomerization to the trisubstituted furanyl-enals via an unusual 1,4-sulfanyl migration. In addition, an operationally simple and efficient one-pot [3+2] annulation of diazoenals and propargyl sulfides, involving combined γ-difunctionalization and cycloisomerization was successfully developed for the diverse enal-functionalized furans.

Direct access to furan and cyclopropane derivatives via palladium-catalyzed C-H activation/alkene insertion/annulation

A practical and effective palladium-catalyzed C-H activation/alkene insertion/annulation has been reported for the synthesis of furans and cyclopropanes from cyclic 1,3-diketones or 1,3-indandione and diverse alkenes, resulting in moderate to good yields. This protocol demonstrates excellent selectivity and is well-compatible with a wide range of alkene substrates, exhibiting exceptional regioselectivities, high efficiency, and good functional group tolerance.

Rhodium(II)-Catalyzed Hinsberg Dearomatization Using Trimethylsilyldiazomethane

Rhodium(II) catalyzes carbene transfer from trimethylsilyldiazomethane to arylmethyl thioethers, generating sulfonium ylides that undergo [2,3]-sigmatropic rearrangement, punching quaternary centers into aromatic rings. The reaction works well with naphthalene, indole, and benzofuran ring systems, but the reaction is unsuccessful with the monocyclic benzene homologue. For aryl thioethers, Rh2(OAc)4 gives good results. For alkyl thioethers, the yields improve with Rh2(cap)4. Surprisingly, thioesters and thiocarbamates are also competent substrates for the reaction.

Syntheses of 4-allyl-/4-allenyl-4-(arylthio)-1,4-dihydroisoquinolin-3-ones via the photochemical Doyle-Kirmse reaction

Facile synthesis of 4-allyl-/4-allenyl-4-(arylthio)-1,4-dihydroisoquinolin-3-ones via the visible-light-induced Doyle-Kirmse reaction of 4-diazo-1,4-dihydroisoquinolin-3-ones with allyl-/propargyl sulfides is reported. The reaction proceeds via the generation of free carbenes from cyclic diazo compounds followed by in situ formation of sulfonium ylide intermediates, which subsequently undergo [2,3-sigmatropic rearrangement] to give highly functionalized dihydroisoquinolinones in moderate to good yields. Broad substrate scope, and catalyst-free and mild conditions are the merits of this reaction.

ZnCl2 -Catalyzed [4+1] Annulation of Alkylthio-Substituted Enaminones and Enaminothiones with Sulfur Ylides

Concise construction of furan and thiophene units has played an important role in the synthesis of potentially bioactive compounds and functional materials. Herein, an efficient Lewis acid ZnCl₂ catalyzed [4+1] annulation of alkylthio-substituted enaminones is reported, that is, α-oxo ketene N,S-acetals with sulfur ylides to afford 2-acyl-3-aminofuran derivatives. In a similar fashion, [4+1] annulation of the corresponding enaminothiones, that is, α-thioxo ketene N,S-acetals, with sulfur ylides efficiently proceeded to give multisubstituted 3-aminothiophenes. This method features wide substrate scopes as well as broad functional group tolerance, offering a concise route to highly functionalized furans and thiophenes.

Recent progress on donor and donor-donor carbenes

Donor and donor-donor carbenes are two important kinds of carbenes, which have experienced tremendous growth in the past two decades. This review provides a comprehensive overview of the recent development of donor and donor-donor carbene chemistry. The development of this chemistry offers efficient protocols to construct a wide variety of C-C and C-X bonds in organic synthesis. This review is organized based on the different types of carbene precursors, including diazo compounds, hydrazones, enynones, cycloheptatrienes and cyclopropenes. The typical transformations, the reaction mechanisms, as well as their subsequent applications in the synthesis of complex natural products and bioactive molecules are discussed. Due to the rapidly increasing interest in this area, we believe that this review will provide a timely and comprehensive discussion of recent progress in donor and donor-donor carbene chemistry.

Combined inorganic base promoted N-addition/[2,3]-sigmatropic rearrangement to construct homoallyl sulfur-containing pyrazolones

The first sequentially combined inorganic base promoted N-addition/[2,3]-sigmatropic rearrangement reaction of α-alkylidene pyrazolinones and propargyl sulfonium salts has been reported to construct homoallyl sulfur-containing pyrazolones with moderate to excellent yields. α-Alkylidene pyrazolinones function as N-nucleophilic agents distinguished from the reported C-addition reactions. Propargyl sulfonium salts were first involved in the [2,3]-sigmatropic rearrangement protocol differentiated from the well-established annulation reactions. The excellent regioselectivity, the broad scope of substrates, gram-scale synthesis and convenient transformation embody the synthetic superiority of this cascade process.

Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Hetero-bimetallic cooperative catalysis for the synthesis of heteroarenes

Review covering the synthesis of 5- and 6-membered as well as condensed heteroarenes, focussing on the combinations in cooperative catalytic systems in strategies used to achieve selectivity and also highlights the mode of action for the cooperative catalysis leading to the synthesis of commercially and biologically relevant heteroarenes.

Palladium-Catalyzed C-S Bond Cleavage with Allenoates: Synthesis of Tetrasubstituted 2-Alkenylfuran Derivatives

Palladium-catalyzed C-S cleavage of tetrasubstituted internal alkene α-oxo ketene dithioacetals was realized with allenoates as the coupling partners, efficiently affording tetrasubstituted 2-alkenylfuran derivatives with excellent regioselectivity under mild conditions. Allenoates acted as C1 synthons in the desulfurative [4 + 1] annulation.

Sulfur-Based Ylides in Transition-Metal-Catalysed Processes

Traditionally employed in the synthesis of small ring systems and rearrangement chemistry, sulfur-based ylides occupy a unique position in the toolbox of the synthetic organic chemist. In recent years a number of pioneering researchers have looked to expand the application of these unorthodox reagents through the use of transition metal catalysis. The strength and flexibility of such a combination have been shown to be of key importance in developing powerful novel methodologies. This chapter summarises recent developments in transition metal-catalysed sulfonium/sulfoxonium ylide reactions, as well as providing a historical perspective. In overviewing the successes in this area, the authors hope to encourage others into this growing field.

Triflic acid promoted solvent free synthesis of densely functionalized furans

A simple, efficient and novel methodology has been developed for the synthesis of substituted furans mediated by triflic acid. In the reaction initial step involves the Friedel–Crafts arylation, followed by the dehydrative cyclization.

Copper/P(tBu)3 -Mediated Regiospecific Synthesis of Fused Furans and Naphthofurans

A novel [3+2] cycloaddition between a variety of cyclic ketones and diverse olefins or alkynes can be effectively promoted by copper in combination with the tri-tert-butylphosphine [P(tBu)₃ ] ligand. This protocol exhibits excellent selectivity and provides an exemplary set of fused heterocycles in good to excellent yields. Present strategy also represents an extremely simple and atom-economic way to construct substituted fused furans and naphthofurans from readily available starting materials under mild reaction conditions. The utility of the method is further demonstrated by the synthesis of chiral furans from (R)-(-)-carvone and (S)-(+)-carvone. A plausible mechanism involving the oxidative radical cyclization has been suggested based on experimental observations.

Direct access to 2-amino-5-azidomethylfurans through palladium-catalyzed azidative cycloisomerization of homoallenyl amides

A room temperature Pd-catalyzed azidative cycloisomerization of homoallenyl amides using TMSN₃ as the azidation reagent and PhI(O₂CCF₃)₂ as the oxidant is described, producing various 2-amino-5-azidomethylfurans in moderate to excellent yields.

The Rh(ii)-catalyzed formal N–S bond insertion reaction of aryldiazoacetates into N-phenyl-sulfenyl phthalimide

The Rh(ii)-catalyzed sulfur ylide [1,2]-rearrangement of carbenoids generated from aryldiazoacetates has been realized via N–S bond insertion under low catalyst loading.

Divergent Synthesis of 2-Aminofurans via Palladium-Catalyzed Acetoxylative, Alkoxylative, and Hydroxylative Cycloisomerization of Homoallenyl Amides

A fine-tunable transformation, including Pd-catalyzed acetoxylative, alkoxylative, and hydroxylative cycloisomerization of homoallenyl amides, has been realized with hypervalent iodine organic compounds as the oxidants, giving polysubstituted 2-aminofurans in promising yields at room temperature. The selective formation of three different types of products from the same starting materials makes this reaction particularly attractive and useful for organic synthesis.

Rh(II)-Catalyzed [2,3]-Sigmatropic Rearrangement of Sulfur Ylides Derived from Cyclopropenes and Sulfides

A new type of Rh2(OAc)4-catalyzed [2,3]-sigmatropic rearrangement of sulfur ylides is reported. A series of cyclopropenes were successfully employed for [2,3]-sigmatropic rearrangement by a reaction with either allylic or propargylic sulfides. Under the optimized conditions, the reaction afforded the products in moderate to excellent yields. In these transformations, the vinyl metal carbenes generated in situ from the cyclopropenes were effectively trapped by sulfides, resulting in the formation of corresponding products upon [2,3]-sigmatropic rearrangements.

K2S2O8/I2 promoted syntheses of α-thio-β-dicarbonyl compounds via oxidative C–S coupling reactions under transition metal-free and solvent-free conditions

K₂S₂O₈/I₂ promoted C–S coupling reaction of β-diketone with disulfide has been described. The resulting α-thio-β-diketones compounds were obtained in good to excellent yields.

Ru(ii)-catalyzed rearrangement of the allenic sulfide bearing propargyl moiety: efficient formation of benzene derivatives

[RuCl₂(p-cymene)]₂ efficiently catalyzes the rearrangement of allenic sulfides bearing the propargyl moiety to afford 1,2,4-trisubstituted benzene derivatives.

Selective syntheses of Δ(α,β) and Δ(β,γ) butenolides from allylic cyclopropenecarboxylates via tandem ring expansion/[3,3]-sigmatropic rearrangements

Allylic cyclopropenecarboxylates undergo ring expansion reactions to give 2-allyloxyfuran intermediates, which subsequently rearrange to Δ(β,γ) butenolides via a Claisen rearrangement or to the corresponding Δ(α,β) butenolides via further Cope rearrangement. Also described are methods for chirality transfer in the rearrangement of nonracemic allylic esters.