Stereoselective Sequential Spirocyclopropanation/Cloke-Wilson Rearrangement Reactions for Synthesis of trans-β,γ-Disubstituted γ-Butyrolactones Using Alkylidene Meldrum's Acid and Benzyl Halides

The stereoselective sequential spirocyclopropanation/Cloke-Wilson rearrangement reactions have been developed to synthesize γ-butyrolactones using alkylidene Meldrum's acids and benzyl halides. The DBU-promoted spirocyclopropanation was carried out efficiently at room temperature to generate trans-isomeric spirocyclopropyl Meldrum's acid, and the following stereospecific thermal decarboxylative Cloke-Wilson rearrangement afforded trans-γ-butyrolactones. A variety of aromatic and aliphatic Meldrum's acid derived olefins and benzyl halides were tolerated. Various trans-β,γ-disubstituted γ-butyrolactones were produced with moderate to good overall yields from 46 to 96% and excellent diastereoselectivities.

Direct cyclopropanation of activated N-heteroarenes via site- and stereoselective dearomative reactions

A divergent cyclopropanation reaction has been accomplished via the dearomative addition of sulfur ylides to activated N-heteroarenes.

A divergent cyclopropanation reaction has been accomplished via the dearomative addition of sulfur ylides to activated N-heteroarenes. A series of biologically significant molecular skeletons was obtained by the direct cyclopropanation of quinolinium zwitterions. Furthermore, a straightforward synthetic route to optically enriched cyclopropane-fused heterocycles was developed using sulfur ylides as chiral nucleophiles in the 1,4-dearomative reaction.

An umpolung reaction of α-iminothioesters possessing a cyclopropyl group

An umpolung N-alkylation reaction of α-cyclopropyl α-iminothioesters with diethylaluminum chloride or ethylmagnesium bromide affords the corresponding N-ethylated α-aminothioesters in good yields. Subsequent oxidation and reaction of the N-ethylated product with a thiolate or a chloride anion proceed effectively to give the ring-opened products in good yields. In contrast, relatively “hard” nucleophiles did not give the ring-opened products but gave the addition products to the iminium carbon.

Tandem N-alkylation/oxidation/second addition reaction to α-cyclopropyl α-imino(thio)esters gave N-alkylated ring-opened products in good yields.

Recent developments in cyclopropene chemistry

As readily accessible strained carbocycles, cyclopropenes show a diverse range of reactivities, and a lot of novel and useful transformations have been developed.

Iron catalysts with N-ligands for carbene transfer of diazo reagents

This review provides an overview of the catalytic activity of iron complexes of nitrogen ligands in driving carbene transfers towards CC, C–H and X–H bonds. The reactivity of diazo reagents is discussed as well as the proposed reaction mechanisms.

gem-Dichlorocyclopropanation of Dicarbonyl Derivatives

A novel methodology for the 1,1-dichlorocyclopropanation of dicarbonyl conjugated olefins was described. The developed protocol is simple and uses readily accessible starting materials, allowing the isolation of the desired adducts in moderate to excellent yields (up to 99 %). Furthermore, the reaction tolerated scale up to the gram scale; thus highlighting the synthetic potential of this transformation. Control experiments and DFT studies revealed that the reaction proceeded through a Michael-initiated ring-closure process, in which reaction temperature played a crucial role. Finally, these gem-dichlorocyclopropanes were also employed in the preparation of a trisubstituted naphthyl derivative and a diastereoselective reduction was also demonstrated.

Beyond the Corey-Chaykovsky Reaction: Synthesis of Unusual Cyclopropanoids via Desymmetrization and Thereof

Desymmetrization-based protocols for the synthesis of highly functionalized indeno-spirocyclopropanes and cyclopropa-fused indanes have been established through unexpected reactions triggered by the Corey-Chaykovsky reagent. These structures were further elaborated in one step to privileged scaffolds such as fluorenones, indenones, and naphthaphenones. For instance, an acid-catalyzed transformation of indeno-spirocyclopropanes provided fluorenones via a homo-Nazarov-type cyclization, and naphthaphenones were obtained via an acid-catalyzed cyclopropane ring-opening/retro-Michael sequence.

Cu-Catalyzed Hydroboration of Benzylidenecyclopropanes: Reaction Optimization, (Hetero)Aryl Scope, and Origins of Pathway Selectivity

The copper-catalyzed hydroboration of benzylidenecyclopropanes, conveniently accessed in one step from readily available benzaldehydes, is reported. Under otherwise identical reaction conditions, two distinct phosphine ligands grant access to different products by either suppressing or promoting cyclopropane opening via β-carbon elimination. Computational studies provide insight into how the rigidity and steric environment of these different bis-phosphine ligands influence the relative activation energies of β-carbon elimination versus protodecupration from the key benzylcopper intermediate. The method tolerates a wide variety of heterocycles prevalent in clinical and pre-clinical drug development, giving access to valuable synthetic intermediates. The versatility of the tertiary cyclopropylboronic ester products is demonstrated through several derivatization reactions.

Diastereoselective ring opening of fully-substituted cyclopropanes via intramolecular Friedel-Crafts alkylation

We herein disclose a diastereoselective ring opening of non-donor-acceptor cyclopropanes via an intramolecular Friedel-Crafts alkylation en route to functionalized dihydronaphthalene scaffolds possessing quaternary carbon stereocentres. The transformation proceeds through a selective bond breaking at the most alkylated carbon centre with a pure retention of configuration. Mechanistic investigations and computational studies revealed that alkoxy functionality is the key for selective bond breaking leading to a complete retention of configuration.

Co(II)-salen catalyzed stereoselective cyclopropanation of fluorinated styrenes

Three cis-selective Co(II)-salen complexes have been developed for the asymmetric cyclopropanation of para-fluorinated styrenes with ethyl diazoacetate. Increasing the steric reach of the C₂ -symmetric ligand side chains improved the enantiomeric ratio of the reaction from 28:1 to 66:1. The methodology was exemplified by the gram-scale synthesis of a lead compound for the treatment of castration-resistant prostate cancer (CRPC), as well as a structurally related analog.

Synthesis of Bicyclo[4.1.0]tetrahydropyridazines by a Sequential [4 + 2] and [1 + 2] Annulation Reaction of Azoalkenes and Crotonate-Derived Sulfur Ylides

The base-induced unprecedented tandem [4 + 2] and [1 + 2] annulation reaction of in situ formed 1,2-diaza-1,3-dienes and crotonate-derived sulfur ylides is reported. This protocol provides a novel and practical method for the synthesis of cyclopropane-fused tetrahydropyridazines with a quaternary carbon center in synthetically useful yield. In this tandem reaction, three new bonds were formed in one pot, and the crotonate-derived sulfur ylide serves as a C3 synthon.

Rh(III)-Catalyzed Cyclopropanation of Unactivated Olefins Initiated by C-H Activation

We have developed a Rh(III)-catalyzed cyclopropanation of unactivated olefins initiated by an alkenyl C-H activation. A variety of 1,1-disubstituted olefins undergo efficient cyclopropanation with a slight excess of alkene stoichiometry. A series of mechanistic interrogations implicate a metal-carbene as an intermediate.

Green pseudo-multicomponent synthesis of some new spirocyclopropane derivatives via electro-catalyzed reaction

Due to the diverse applications of cyclopropane analogs in bioorganic, medicinal, and pharmaceutical chemistry, a clean and efficient procedure was established to synthesize spirocyclopropane via an electrochemical reaction which involves a sequence of Michael addition, halogenation, and intramolecular ring-closing reaction. In this study, an environmentally benign synthesis of spirocyclopropane was carried out through the condensation of indan-1,3-dione by aromatic aldehydes or 2-benzylidenemalononitrile derivatives. Constant current electrosynthesis was applied to a mixture of propanol containing sodium bromide as an electrolyte and a brominating agent at room temperature, respectively.

Diastereoselective synthesis of α-dicarbonyl cyclopropanes via a lanthanide amide-catalysed reaction

Lanthanide bis(trimethylsilyl)amides, [(Me₃Si)₂N]₃Ln(μ-Cl)Li(THF)₃, were used as efficient catalysts for a one-pot reaction of α-ketoesters, dialkyl phosphite, and activated alkenes to produce α-dicarbonyl cyclopropanes in moderate to high yields. The reaction was stereoselective and the two adjacent carbonyls linked to the cyclopropane were in the cis-configuration. The high efficiency of the lanthanide amide in catalysing the reaction is the result of the cooperation between the lanthanide metal centre and the N(SiMe₃)₂ anion.

Gold(iii)-catalyzed chemoselective annulations of anthranils with N-allylynamides for the synthesis of 3-azabicyclo[3.1.0]hexan-2-imines

We herein report the gold(iii)-catalyzed selective annulation of anthranils with N-allylynamides under mild conditions. By trapping the in situ-generated α-imino gold carbenes, 3-azabicyclo[3.1.0]hexan-2-imines were obtained in high synthetic efficiency. The reaction, which can be conducted in the gram scale, tolerates electron-rich and electron-deficient anthranils as well as a diverse set of functionalized ynamides (aryl- and alkyl-substituted terminal).

Chromium-Mediated Stannylcyclopropanation of Alkenes with (Diiodomethyl)stannanes

A stannyl-group-substituted gem-dichromiomethane species, generated in situ from CrCl₂, TMEDA, and tributyl(diiodomethyl)stannane, worked as an efficient stannylcarbene equivalent to promote cyclopropanation of alkenes. The reaction provided synthetically useful stannylcyclopropanes directly from commercially available unactivated alkenes without using potentially flammable alkylzinc and diazo compounds. Structural characterization of stannyl- and germyl-group-substituted gem-dichromiomethane complexes and the effect of group 14 elements containing substituents for cyclopropanation are also described.

Rh(III)-Catalyzed C-H Activation-Initiated Directed Cyclopropanation of Allylic Alcohols

We have developed a Rh(III)-catalyzed diastereoselective [2+1] annulation onto allylic alcohols initiated by alkenyl C-H activation of N-enoxyphthalimides to furnish substituted cyclopropyl-ketones. Notably, the traceless oxyphthalimide handle serves three functions: directing C-H activation, oxidation of Rh(III), and, collectively with the allylic alcohol, in directing cyclopropanation to control diastereoselectivity. Allylic alcohols are shown to be highly reactive olefin coupling partners leading to a directed diastereoselective cyclopropanation reaction, providing products not accessible by other routes.

Introduction of Cyclopropyl and Cyclobutyl Ring on Alkyl Iodides through Cobalt-Catalyzed Cross-Coupling

A cobalt-catalyzed cross-coupling between alkyl iodides and cyclopropyl, cyclobutyl, and alkenyl Grignard reagents is disclosed. The reaction allows the introduction of strained rings on a large panel of primary and secondary alkyl iodides. The catalytic system is simple and nonexpensive, and the reaction is general, chemoselective, and diastereoconvergent. The alkene resulting from the cross-coupling can be transformed to substituted cyclopropanes using a Simmons-Smith reaction. The formation of radical intermediates during the coupling is hypothesized.

Recent advances in the synthesis of cyclopropanes

Cyclopropanes, one of the most important strained rings, have gained much attention for more than a century because of their interesting and unique reactivity. They not only exist in many natural products, but have also been widely used in the fields of organic synthesis, medicinal chemistry and materials science as versatile building blocks. Based on the sustainable development in this area, this review mainly focuses on the recent advances in the synthesis of cyclopropanes classified by the type of catalytic system, including regio-, diastereo-, and enantio-selective reactions.

Stereospecific Synthesis of α-Hydroxy-Cyclopropylboronates from Allylic Epoxides

Herein, we report a catalytic and stereospecific method for the preparation of enantioenriched α-hydroxy cyclopropylboronates with control in four contiguous stereocenters. The reaction involves the borylation of readily available allylic epoxides using an inexpensive Cu(I) salt and a commercially available phosphine ligand. High diastereocontrol is achieved and different diastereomers can be selectively prepared. Functionalization of the carbon-boron bond provides access to different enantiomerically enriched trisubstituted cyclopropanes from a common intermediate.

Synthesis of 1,2-divinylcyclopropanes by metal-catalyzed cyclopropanation of 1,3-dienes with cyclopropenes as vinyl carbene precursors

The synthesis of 1,2-divinylcyclopropanes by the reaction of cyclopropenes with 1,3-dienes is reported. The process relies on the ability of ZnCl₂ or [Rh₂(OAc)₄] to generate metal-vinyl carbene intermediates from cyclopropenes, which effect cyclopropanation of 1,3-dienes. Most of the reactions proceeded in reasonable yields while the diastereoselectivity strongly depends on the structure of the diene. An example of an intramolecular process as well as the use of furan and 1,4-cyclohexadiene as dienes are also reported.

General Cyclopropane Assembly by Enantioselective Transfer of a Redox-Active Carbene to Aliphatic Olefins

Asymmetric cyclopropane synthesis currently requires bespoke strategies, methods, substrates, and reagents, even when targeting similar compounds. This approach slows down discovery and limits available chemical space. Introduced herein is a practical and versatile diazocompound and its performance in the first unified asymmetric synthesis of functionalized cyclopropanes. The redox-active leaving group in this reagent enhances the reactivity and selectivity of geminal carbene transfer. This effect allowed the asymmetric cyclopropanation of various olefins, including unfunctionalized aliphatic alkenes, that enables the three-step total synthesis of (-)-dictyopterene A. This unified synthetic approach delivers high enantioselectivities that are independent of the stereoelectronic properties of the functional groups transferred. Our results demonstrate that orthogonally differentiated diazocompounds are viable and advantageous equivalents of single-carbon chirons.

Oxidative radical ring-opening/cyclization of cyclopropane derivatives

The ring-opening/cyclization of cyclopropane derivatives has drawn great attention in the past several decades. In this review, recent efforts in the development of oxidative radical ring-opening/cyclization of cyclopropane derivatives, including methylenecyclopropanes, cyclopropyl olefins and cyclopropanols, are described. We hope this review will be of sufficient interest for the scientific community to further advance the application of oxidative radical strategies in the ring-opening/cyclization of cyclopropane derivatives.