A substrate-dependent reaction of 1-aryl-2-alkyl-1,2-diketones with 2-aroyl-1-chlorocyclopropanecarboxylates: selective access to 2',5'-dicyclopropoxy-1,1':4',1''-teraryls and pentafulvenes

An interesting substrate-controlled one-pot approach to highly substituted 2',5'-dicyclopropoxy-1,1':4',1''-teraryls and 6-hydroxypentafulvenes involving various 1,2-diketones and 2-aroyl-1-chlorocyclopropanecarboxylates as 3C Michael acceptors triggered by Cs₂CO₃ has been developed. We noticed that 1,2-diketones play a decisive role in this reaction to determine the product's selectivity. For example, aryl rings having electron-poor functionalities at the para and meta-positions of 1,2-diketones led to 2,5-diarylhydroquinones selectively via a cyclodimerization/double oxa-Michael process with highly strained cyclopropenes. However, when 1-naphthyl/electron-donating aryl/ortho-aryl-substituted 1,2-diketones were chosen, the Michael-initiated ring expansion reaction (C-C and CC bonds) took place under the same conditions that gave the corresponding pentafulvenes predominately. Moreover, this reaction has several imperative features such as good to high diastereoselectivities, wide substrate scope, good functional group tolerance, transition metal-free process, etc.

Phosphine-Catalyzed Enantioselective (3+2) Annulation of Vinylcyclopropanes with Imines for the Synthesis of Chiral Pyrrolidines

A phosphine-catalyzed highly enantioselective and diastereoselective (up to 98 % ee and >20 : 1 dr) (3+2) annulation between vinylcyclopropanes and N-tosylaldimines has been developed, which allows facile access to a range of highly functionalized chiral pyrrolidines. Notably, this method makes use of vinylcyclopropanes as a synthon for phosphine-mediated asymmetric annulation reaction, which will offer new opportunities for potential applications of cyclopropanes substrates in phosphine-catalyzed organic transformations.

Simmons-Smith Cyclopropanation of Alkenyl 1,2-Bis(boronates): Stereoselective Access to Functionalized Cyclopropyl Derivatives

A Simmons-Smith stereodefined procedure for the synthesis of cyclopropyl-1,2-bis(boronates) has been developed starting from the corresponding alkenes. The resulting compounds were then subjected to regioselective Suzuki-Miyaura couplings to produce diversely tri- or tetra-substituted arylcyclopropanes in good yields. Further functionalization with 2-lithiothiophene provided 1,2-bis(aryl)cyclopropanes.

Catalytic Reductive Carbene Transfer Reactions

Efforts to develop catalytic carbene transfer reactions have largely relied on the use of diazo precursors. However, diazoalkanes are susceptible to undergoing violent exothermic decomposition unless they contain stabilizing substituents. Consequently, most synthetic methods are restricted to diazoacetates or related derivatives. In this Perspective, we describe an alternative approach to carbene transfer catalysis based on the generation of metal carbenoids from gem-dihaloalkanes and gem-dihaloalkenes. These precursors are readily available and stable in unsubstituted form or with a variety of donor and acceptor substituents. Using this approach, it is possible to design cyclopropanation reactions with non-stabilized carbenes, such as methylene, isopropylidene, and vinylidene. Furthermore, due to the distinct mechanistic pathways of these reactions, novel modes of cycloaddition can be carried out, including [4 + 1]-cycloadditions.

DFT Mechanistic Study of the Cyclopropanation of Styrene and Aryldiazodiacetate Catalyzed by Tris(pentafluorophenyl)borane

Metal-free boron Lewis acids, tris(pentafluorophenyl)borane B(C₆F₅)₃, have the advantages of low toxicity and low cost and are a promising catalyst. A density functional theory (DFT) calculation was used to clarify the mechanism and the origin of the diastereoselective cyclopropanation of aryldiazodiacetate and styrene derivatives catalyzed by B(C₆F₅)₃. Four pathways were calculated: B(C₆F₅)₃-catalyzed N-, C-, and O-bound boron-activated aryldiazodiacetate and without B(C₆F₅)₃ catalysis. By calculating and comparing the energy barriers, the most possible reaction mechanism was proposed, that is, first, B(C₆F₅)₃ catalyzed O-bound boron to activate aryldiazodiacetate, followed by the removal of a N₂ molecule, and finally, styrene nucleophilic attack occurred to produce [2+1] cyclopropane products. N₂ removal is the rate-limiting step, and this step determines the preference of a given mechanism. The calculated results are in agreement with experimental observations. The origin of diastereoselectivity is further explained on the basis of the favorable mechanism. The steric hindrance interference between the styrene aryl group and the large tri(pentafluorophenyl)borane B(C₆F₅)₃ and the favorable π-π stacking interaction between the benzene rings combined to cause the high diastereoselectivity, which resulted in lower energy of the transition state (TS) corresponding to the reaction mechanism. The calculated results not only provide a more detailed explanation of the mechanism for the experimental study but also have certain reference and guiding significance for other catalytic cyclopropanation reactions.

Composite Corelated Molecular Orbital Theory Calculations of Ring Strain for Use in Predicting Polymerization Reactions

Strained ring systems play an important role in synthesis and can be characterized by the ring strain energy (RSE). The RSE of 3, 4, 5, and 6 membered saturated and unsaturated ring systems containing N, O, P, and S heteroatoms and H, F, Si(CH 3 ) 3 , and SO 2 (CH 3 ) substituents were calculated at the G3(MP2) composite correlated molecular orbital theory level using up to 5 models to predict the RSE. Generally, the RSE decreased as ring size increased with a substantial decrease from 4 to 5 membered rings. Replacement of a ring CH 2 with P or S reduced the RSE, consistent with less angle strain. The RSE for unsaturated systems were generally greater than for saturated systems due to increased angle strain. No general trends were found with respect to substituent effects. The RSE values suggest that 3-pyrroline and 2-pyrroline may be able to support ring opening metathesis polymerization and warrant further study.

Successes and challenges in multiscale modelling of artificial metalloenzymes: the case study of POP-Rh2 cyclopropanase

Molecular modelling applications in metalloenzyme design are still scarce due to a series of challenges. On top of that, the simulations of metal-mediated binding and the identification of catalytic competent geometries require both large conformational exploration and simulation of fine electronic properties. Here, we demonstrate how the incorporation of new tools in multiscale strategies, namely substrate diffusion exploration, allows taking a step further. As a showcase, the enantioselective profiles of the most outstanding variants of an artificial Rh₂-based cyclopropanase (GSH, HFF and RFY) developed by Lewis and co-workers (Nat. Commun., 2015, 6, 7789 and Nat. Chem., 2018, 10, 318-324) have been rationalized. DFT calculations on the free-cofactor-mediated process identify the carbene insertion and the cyclopropanoid formation as crucial events, the latter being the enantiodetermining step, which displays up to 8 competitive orientations easily altered by the protein environment. The key intermediates of the reaction were docked into the protein scaffold showing that some mutated residues have direct interaction with the cofactor and/or the co-substrate. These interactions take the form of a direct coordination of Rh in GSH and HFF and a strong hydrophobic patch with the carbene moiety in RFY. Posterior molecular dynamics sustain that the cofactor induces global re-arrangements of the protein. Finally, massive exploration of substrate diffusion, based on the GPathFinder approach, defines this event as the origin of the enantioselectivity in GSH and RFY. For HFF, fine molecular dockings suggest that it is likely related to local interactions upon diffusion. This work shows how modelling of long-range mutations on the catalytic profiles of metalloenzymes may be unavoidable and software simulating substrate diffusion should be applied.

Photoredox/Nickel Dual Catalysis Enables the Synthesis of Alkyl Cyclopropanes via C(sp3)-C(sp3) Cross Electrophile Coupling of Unactivated Alkyl Electrophiles

A facile synthesis of mono-, 1,1- and 1,2-disubstituted cyclopropanes via visible light-mediated photoredox/nickel dual catalysis is demonstrated. The challenging intramolecular C(sp³)-C(sp³) cross-electrophile coupling of readily available unactivated 1,3-dialkyl electrophiles was performed under mild conditions that allowed traditionally reactive functional groups to be included. Mechanistic inspection and control experiments revealed the importance of dual catalysis and that the reaction proceeds via a stepwise oxidative addition followed by an intramolecular S_N2 reaction.

Catalytic Net Oxidative C-C Activation and Silylation of Cyclopropanols with a Traceless Acetal Directing Group

Redox-neutral carbon-carbon (C-C) bond activation and functionalization strategies of cyclopropanols that give metallo homoenolate have offered merits to construct a range of useful β-functionalized ketones in an inverse-polarity fashion. Discovery and identification of oxidative C-C activation reactions of cyclopropanols that generate metallo enolate-homoenolate would provide an opportunity to afford α,β-difunctionalized ketones. We report catalytic, net oxidative C-C activation, and silylation of cyclopropanols with traceless acetal directing groups under consecutive Ir and Rh catalysis in regio-, stereo-, and chemo-selective fashion. In detail, Ir-catalyzed hydrosilylation of cyclopropyl acetates provides the acetal directing group in quantitative yield. Rh-catalyzed proximal C-C silylation of the resulting cyclopropyl silyl acetal produces the metallo enolate-homoenolate equivalent, dioxasilepine, which uniquely holds an interconnected β-silyl moiety and Z-vinyl acetal. Upon sequential treatment of a silaphile that removes the acetal directing group and electrophile, the seven-membered silicon-containing heterocycle, serving as the ketone α,β-dianion equivalent, delivers α,β-difunctionalized ketones. Scope of the hitherto unexplored reactivity of cyclopropanols toward net oxidative C-C silylation and the versatility of the resulting dioxasilepines were demonstrated. These include late-stage, net oxidative C-C silylation of biologically relevant molecules and facile production of a range of α,β-difunctionalized ketones. Preliminary mechanistic studies suggest that the C-C activation harnessing the electron-rich Wilkinson-type catalyst is likely the turnover-determining step and a Rh-π interaction is the key to the efficient metal insertion to the proximal C-C bond in cyclopropanols.

Structures of ganorbifates C-I, seven previously undescribed lanostanoids from the mushroom Ganoderma orbiforme, and insights of computed biosynthesis with DFT

Ganorbifates C-I, seven undescribed biosynthetically related polyoxygenated 3,4-seco-27-norlanostanoid congeners, were isolated from the edible mushroom, Ganoderma orbiforme. Ganorbifate C features a unique cyclobutene ring constructed at C19/C11, and both D and E incorporate an unusual cyclopropane ring formed by C-19/C-9 linkage. Their structures, including the absolute configurations, were determined by spectroscopic methods and ECD calculations. The proposed Norrish-Yang cyclization-based key biosynthetic pathway for ganorbifates C-E is revealed by density functional theory (DFT) calculations. The computational studies uncover the formation of both cyclobutene and cyclopropane rings in the isolates and the stereoselectivity centers of these steps are consistent with those in the natural products. All compounds exhibited NO generation inhibition in LPS-induced BV-2 microglial cells, among them ganorbifate C was the most promising one with the IC₅₀ values of 4.37 μM.

Organoelectrocatalysis Enables Direct Cyclopropanation of Methylene Compounds

Cyclopropane is a prevalent structural unit in natural products and bioactive compounds. While the transition metal-catalyzed alkene cyclopropanation of functionalized compounds such as α-diazocarbonyl derivatives has been well established and provides straightforward access to cyclopropanes, cyclopropanation directly from the more stable and simpler methylene compounds has remained an unsolved challenge despite the highly desirable benefits of minimal prefunctionalization and increased operational safety. Herein we report an electrocatalytic strategy for the cyclopropanation of active methylene compounds, employing an organic catalyst. The method shows a broad substrate scope and excellent scalability, requires no metal catalyst or external chemical oxidant, and provides convenient access to several types of cyclopropane-fused heterocyclic and carbocyclic compounds. Mechanistic investigations suggest that the reactions proceed through a radical-polar crossover process to form the two new carbon-carbon bonds in the nascent cyclopropane ring.

Reductive Ring Opening of Arylcyclopropanecarboxamides Accompanied by Borylation and Enolate Formation

Treatment of arylcyclopropanecarboxamides with a sodium dispersion in the presence of methoxypinacolborane as a reduction-resistant electrophile leads to reductive cleavage of the cyclopropane ring followed by instant trapping with the boron electrophile to yield the enolates of γ-aryl-γ-borylalkanamides. The enolates react further with a different electrophile to yield the corresponding α-substituted amides with anti selectivity.

1,3-Oxyalkynylation of Aryl Cyclopropanes with Ethylnylbenziodoxolones Using Photoredox Catalysis

Alkynes and cyclopropanes are vital motifs in chemistry. Herein, a photoredox catalyzed 1,3-oxyalkynylation of aryl cyclopropanes with ethylnylbenziodoxolones (EBXs) in an atom-economic fashion is described. This cascade comprises single-electron oxidation of the aryl cyclopropane and nucleophilic ring opening followed by radical alkynylation at the benzylic position. The EBX compounds act as bifunctional reagents providing the nucleophilic acid as well as the alkynyl entity. The introduced method features mild conditions and wide substrate scope.

Insights into gold-catalyzed formation of aza-heterocycles using benzofuroxans as nitrene transfer reagents: mechanism and origins of chemoselectivity

Density functional theory (DFT) calculations have been performed to reveal the mechanism of gold(i)-catalyzed annulation of N-allylynamides and benzofuroxans as nitrene transfer reagents to construct azaheterocyclic compounds. The calculated results revealed that the reaction mechanism mainly undergoes eight processes. Among the reaction steps, intramolecular nucleophilic attack of the imino N atom on the α-position of activated gold keteniminium is a rate-determining process, which is different from that proposed previously by experiment. The chemoselectivity of the products is controlled by competition between the cyclopropanation of α-imino gold carbenes and intramolecular nucleophilic attack of the phenyl ring on α-imino gold carbenes, and could be explained by NPA charge. The different yields of cyclopropanated product in different solvents are dictated by the relative polarity leading to the different energy barriers of the rate-determining steps. The present work expounds the experimental observation at the molecular level and is informative for exploring efficient syntheses of 3-azabicyclo[3.1.0]hexanes.

DFT calculations were employed to study the mechanism and origin of chemoselectivity of gold-catalyzed annulation of N-allylynamides and benzofuroxans as nitrene transfer reagents.

Intramolecular Photochemical [2+1]-Cycloadditions of Nucleophilic Siloxy Carbenes

Visible light induced singlet nucleophilic carbenes undergo rapid [2 + 1]-cycloaddition with tethered olefins to afford unique bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane scaffolds. This cyclopropanation process requires only visible light irradiation to proceed, circumventing the use of exogenous (photo)catalysts, sensitisers or additives and showcases a vastly underexplored mode of reactivity for nucleophilic carbenes in chemical synthesis. The discovery of additional transformations including a cyclopropanation/retro-Michael/Michael cascade process to afford chromanones and a photochemical C–H insertion reaction are also described.

Visible light induced singlet nucleophilic carbenes undergo rapid [2 + 1]-cycloaddition with tethered olefins to afford unique bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane scaffolds.

Dearomative Cyclopropanation of Naphthols via Cyclopropene Ring-Opening

The dearomatization of 2-naphthols represents a simple method for the construction of complex 3D structures from simple planar starting materials. We describe a cyclopropanation of 2-naphthols that proceeds via cyclopropene ring-opening using rhodium and acid catalysis under mild conditions. The vinyl cyclopropane molecules were formed with high chemoselectivity and scalability, which could be further functionalized at different sites. Both computational and experimental evidence were used to elucidate the reaction mechanism.

Access to Bromo-γ-butenolides via Zwitterion-Catalyzed Rearrangement of Cyclopropene Carboxylic Acids

γ-Butenolides are useful structural motifs in many pharmaceutically relevant compounds. In particular, halogenated γ-butenolides are attractive building blocks because the halogen handles can readily be manipulated to give various functional molecules. In this study, a catalytic synthesis of halogenated γ-butenolides from cyclopropene carboxylic acids was developed using zwitterionic catalysts and N-haloamides as the halogen sources. The catalytic protocol could also be applied to the synthesis of halogenated pyrrolones by using cyclopropene amides as the starting materials.

Directed Regioselective Carbometallation of 1,2-Dialkyl-Substituted Cyclopropenes

A regio- and diastereoselective copper-catalyzed carbomagnesiation of 1,2-dialkylated cyclopropenes is reported. The regioselectivity is controlled by a subtle tethered Lewis basic moiety. The chelating moieties allow the differentiation between two electronically tantamount organometallic intermediates. Further functionalization grants access to polysubstituted stereodefined cyclopropanes bearing up to five alkyl groups.

Biosynthesis of cyclopropane in natural products

Covering: 2012 to 2021Cyclopropane attracts wide interests in the fields of synthetic and pharmaceutical chemistry, and chemical biology because of its unique structural and chemical properties. This structural motif is widespread in natural products, and is usually essential for biological activities. Nature has evolved diverse strategies to access this structural motif, and increasing knowledge of the enzymes forming cyclopropane (i.e., cyclopropanases) has been revealed over the last two decades. Here, the scientific literature from the last two decades relating to cyclopropane biosynthesis is summarized, and the enzymatic cyclopropanations, according to reaction mechanism, which can be grouped into two major pathways according to whether the reaction involves an exogenous C1 unit from S-adenosylmethionine (SAM) or not, is discussed. The reactions can further be classified based on the key intermediates required prior to cyclopropane formation, which can be carbocations, carbanions, or carbon radicals. Besides the general biosynthetic pathways of the cyclopropane-containing natural products, particular emphasis is placed on the mechanism and engineering of the enzymes required for forming this unique structure motif.

Cooperative NHC/Photoredox Catalyzed Ring-Opening of Aryl Cyclopropanes to 1-Aroyloxylated-3-Acylated Alkanes

Cyclopropanes are an important class of building blocks in organic synthesis. Herein, a ring‐opening/arylcarboxylation/acylation cascade reaction for the 1,3‐difunctionalization of aryl cyclopropanes enabled by cooperative NHC and organophotoredox catalysis is reported. The cascade works on monosubstituted cyclopropanes that are in contrast to the heavily investigated donor–acceptor cyclopropanes more challenging to be difunctionalized. The key step is a radical/radical cross coupling of a benzylic radical generated in the photoredox catalysis cycle with a ketyl radical from the NHC catalysis cycle. The transformation features metal‐free reaction conditions and tolerates a diverse range of functionalities.

Modular Enantioselective Synthesis of cis-Cyclopropanes through Self-Sensitized Stereoselective Photodecarboxylation with Benzothiazolines

Chiral cis-cyclopropanes are strained rigid analogues of alkyl chains, whose study and application are limited by their difficult synthesis. A modular approach from olefin materials is enabled by the discovery of the electron donor-acceptor (EDA) interaction between 2-substituted benzothiazolines and N-hydroxyphthalimide esters. These complexes are activated by visible light without photocatalysts, and the benzothiazoline reagent plays a triple role as a photoreductant, a stereoselective hydrogen-atom donor, and a Brønsted acid. Beyond the enantioselective synthesis of cis-cyclopropanes, these results introduce benzothiazolines as accessible and easily tunable self-sensitized photoreductants.

Mild Intermolecular Synthesis of a Cyclopropane-Containing Tricyclic Skeleton: Unusual Reactivity of Isobenzopyryliums

Cyclopropanes embedded in a polycyclic bridged architecture are a versatile structural motif, but such complex frameworks often impose substantial synthetic challenges. Herein we introduce a new approach for the expedient access to such spring-loaded strained systems via an exceptionally mild intermolecular convergent process between the readily available isobenzopyryliums and vinyl boronic acids. Different from the typical conventional approaches, our protocol does not involve the highly active carbenoid intermediates or strong conditions in order to overcome the disfavored kinetic and thermodynamic problems. Instead, the key cyclopropane ring was formed between the well-positioned nucleophile and electrophile in the adduct from the regioselective [4+2] cycloaddition. Thus, this unusual process also represents a new reactivity of the versatile isobenzopyryliums. The choice of a Brønsted acid catalyst with proper acidity is crucial to the high efficiency and selectivity for this multiple bond-forming process. The strained products are precursors to other useful synthetic building blocks.

Combined Experimental and Computational Study of Ruthenium N-Hydroxyphthalimidoyl Carbenes in Alkene Cyclopropanation Reactions

A combined experimental-computational approach has been used to study the cyclopropanation reaction of N-hydroxyphthalimide diazoacetate (NHPI-DA) with various olefins, catalyzed by a ruthenium-phenyloxazoline (Ru-Pheox) complex. Kinetic studies show that the better selectivity of the employed redox-active NHPI diazoacetate is a result of a much slower dimerization reaction compared to aliphatic diazoacetates. Density functional theory calculations reveal that several reactions can take place with similar energy barriers, namely, dimerization of the NHPI diazoacetate, cyclopropanation (inner-sphere and outer-sphere), and a previously unrecognized migratory insertion of the carbene into the phenyloxazoline ligand. The calculations show that the migratory insertion reaction yields an unconsidered ruthenium complex that is catalytically competent for both the dimerization and cyclopropanation, and its relevance is assessed experimentally. The stereoselectivity of the reaction is argued to stem from an intricate balance between the various mechanistic scenarios.

Practical Enzymatic Production of Carbocycles

The Pd-catalyzed carbon-carbon bond formation pioneered by Heck in 1969 has dominated medicinal chemistry development for the ensuing fifty years. As the demand for more complex three-dimensional active pharmaceuticals continues to increase, preparative enzyme-mediated assembly, by virtue of its exquisite selectivity and sustainable nature, is poised to provide a practical and affordable alternative for accessing such compounds. In this minireview, we summarize recent state-of-the-art developments in practical enzyme-mediated assembly of carbocycles. When appropriate, background information on the enzymatic transformation is provided and challenges and/or limitations are also highlighted.

Functionalized Cyclopropanes as Versatile Intermediates for the Diversity-Oriented Synthesis of γ-Lactones, γ-Lactams and δ-Lactams

A two-step procedure for the preparation of cyclopropanecarboxaldehyde-1,1-diester from a γ,δ-epoxyester and its synthetic versatility are described herein. The epoxide ring-opening/cyclopropanation process occurs in the presence of Mg(ClO4)2 under heating, resulting in cyclopropanemethanol-1,1-diester in 65% yield. A mild TEMPO-mediated oxidation of this substrate readily generated the corresponding aldehyde in 75% yield, which was applied in the one-pot synthesis of four cyclopropylidene-γ-lactams and three δ-lactams. In addition, vinylcyclopropanes were obtained through the Wittig reaction of the aldehyde with phosphonium salts and used as precursors for tetrahydrofurans.

Molybdenum-Catalyzed Deoxygenative Cyclopropanation of 1,2-Dicarbonyl or Monocarbonyl Compounds

The transition-metal-catalyzed cyclopropanation of alkenes by the decomposition of diazo compounds is a powerful and straightforward strategy to produce cyclopropanes, but is tempered by the potentially explosive nature of diazo substrates. Herein we report the Mo-catalyzed regiospecific deoxygenative cyclopropanation of readily available and bench-stable 1,2-dicarbonyl compounds, in which one of the two carbonyl groups acts as a carbene equivalent upon deoxygenation and engages in the subsequent cyclopropanation process. The use of a commercially available Mo catalyst afforded an array of valuable cyclopropanes with exclusive regioselectivity in up to 90 % yield. The synthetic utility of this method was further demonstrated by gram-scale syntheses, late-stage functionalization, and the cyclopropanation of a simple monocarbonyl compound. Preliminary mechanistic studies suggest that phosphine (or silane) acts as both a mild reductant and a good oxygen acceptor that efficiently regenerates the catalytically active Mo catalyst through reduction of the Mo-oxo complexes.

Reversal of polarity by catalytic SET oxidation: synthesis of azabicyclo[m.n.0]alkanes via chemoselective reduction of amidines

A one-pot catalytic method has been developed for the stereoselective synthesis of cyclopropane-fused cyclic amidines using CuBr2/K2S2O8 as an efficient single electron transfer (SET) oxidative system. The generality of this mild method is demonstrated with a wide variety of substrates to furnish pharmaceutically important amidines containing aza-bicyclic and novel aza-tricyclic frameworks in very good yields. A chemoselective reduction of cyclic amidines to 2-/3-azabicyclo[m.n.0]alkanes and octahydroindoles has been developed using a NaBH4/I2 reagent system. The synthetic scope of the chemoselective reduction of the amidine functionality has been exemplified in the stereoselective synthesis of an iminosugar based (±)-epiquinamide analogue.

Enantioselective Catalytic Cyclopropanation-Rearrangement Approach to Chiral Spiroketals

A highly enantioselective synthesis of chiral heterobicyclic spiroketals is reported via a "one-pot" cyclopropanation-rearrangement (CP-RA) cascade reaction that is sequentially catalyzed by a chiral Rh(II) catalyst and tetrabutylammonium fluoride (TBAF). Exocyclic vinyl substrates form spirocyclopropanes with tert-butyldimethylsilyl-protected enoldiazoacetates in excellent yields and with excellent enantioselectivities when catalyzed by chiral dirhodium(II) carboxylates, and following desilylation with simultaneous rearrangement in the presence of TBAF, they give (S)-spiroketals in high yields with excellent chirality retention (>95% ee).

Rhodaelectro-catalyzed chemo-divergent C-H activations with alkylidenecyclopropanes for selective cyclopropylations

Herein, we report on selectivity control in C-H activations with alkylidenecyclopropanes (ACPs) for the chemo-selective assembly of cyclopropanes or dienes. Thus, unprecedented rhodaelectro-catalyzed C-H activations were realized with diversely decorated ACPs with a wide substrate scope and electricity as the sole oxidant.

Reaction of 3-Oxa-2-oxobicyclo[4.2.0]oct-4-ene-1-carboxylate with Dimethylsulfoxonium Methylide

We have been interested in the reactivities of small-ring compounds and have reported reactions that proceed through cyclopropane intermediates starting from coumarin derivatives bearing an electron-withdrawing group at the 3-position or 2-oxo-2H-pyran-3-carboxylate derivatives and dimethylsulfoxonium methylide. This time, the reaction between 3-oxa-2-oxobicyclo[4.2.0]oct-4-ene-1-carboxylate and dimethylsulfoxonium methylide has been investigated. 3a,4,5,7a-Tetrahydro-7-hydroxybenzofuran-6-carboxylate and/or 2-hydroxybicyclo[4.1.0]hept-2-ene-3-carboxylate were obtained. The compounds were characterized using various spectral and X-ray crystallographic techniques. A plausible reaction mechanism has been discussed. This reaction was applied to some 3-oxa-2-oxobicyclo[4.2.0]oct-4-ene-1-carboxylate derivatives to clarify the generality.