Intramolecular Oxyalkylation of Unactivated Alkenes

The synthesis of cyclopropanes by the cyclization of allylic diazoesters is well-known. In prior studies toward the sesquiterpenoid euonyminol, we attempted to carry out an intramolecular cyclopropanation of an allylic diazoester containing an electronically-unbiased alkene embedded in a 6-oxa-bicyclo[3.2.1]-oct-3-ene skeleton. We obtained exclusively a product arising from 1,2-addition of oxygen and carbon (oxyalkylation) to the alkene. While oxyalkylation products have been reported when electron-rich alkenes (e.g. enol ethers) are employed, examples derived from electronically-unbiased alkenes are rare. Here, we establish that the oxyalkylation is general for a range of 6-oxa-bicyclo[3.2.1]-oct-3-ene substrates and show that these products form competitively in the cyclization of simpler α-diazo β-ketoesters. Our data suggest increasing charge separation in the transition state for the addition promotes the oxyalkylation pathway.

Organocatalysis as an enabling tool for enantioselective ring-opening reactions of cyclopropanes

The rich reactivity profile of cyclopropanes has been extensively explored to trigger new organic transformations that enable unusual disconnective approaches to synthesize molecular motifs that are not easily reached through conventional reactions. In particular, the chemistry of cyclopropanes has received special attention in the last decade, with multiple new approaches that capitalize on the use of organocatalysis for the activation of the cyclopropane scaffold. This situation has also opened the possibility of developing enantioselective variants of many reactions that until now were only carried out in an enantiospecific or diastereoselective manner. Our group has been particularly active in this field, focusing more specifically on the use of aminocatalysis and Brønsted acid catalysis as major organocatalytic activation manifolds to trigger new unprecedented transformations involving cyclopropanes that add to the current toolbox of general methodologies available to organic chemists for the enantioselective synthesis of chiral compounds.

Intramolecular 1,2-Aroyl Migration in Spiro Donor-Acceptor Cyclopropanes: Formation of 1,4-Naphthoquinones and 1-Naphthols as Ring-Expansion Products

Most of the known rearrangement reactions of donor-acceptor cyclopropanes (DACs) involve the migration of cationic carbon atom to anionic carbon or heteroatoms in 1,3- or 1,4-positions. In the present work, we observed that spiro DACs based on 1,3-indanedione or 1-indanone moiety undergo intramolecular 1,2-aroyl migration when treated with titanium(IV) chloride to afford 1,4-naphthoquinones and α-naphthols readily. The reactions take place through the formation of putative 1,3-dipolar intermediates, followed by cleavage and migration of the aroyl group to the adjacent carbon to afford the ring-expansion products.

Lewis acid-catalyzed one-pot thioalkenylation of donor-acceptor cyclopropanes using in situ generated dithiocarbamates and propiolates

Lewis acid-catalyzed one-pot 1,3-thioalkenylation of donor-acceptor (D-A) cyclopropanes has been demonstrated employing in situ generated dithiocarbamates (from amines and CS2) as nucleophilic triggers and alkyl propiolates as electrophiles. This method addresses the limitations of previously known carbothiolation approach, eliminating the need for extra filtration prior to the subsequent trapping with electrophiles. The anticipated thioalkenylated products were obtained in good to excellent yields with a moderate to good E/Z ratio. Three new bonds (C-N, C-S, and C-C) are formed during this 1,3-bisfunctionalization reaction. Notably, employing enantiomerically pure D-A cyclopropanes resulted in enantiopure 1,3-thioalkenylated products, underscoring the stereospecific nature of the developed reaction.

Chalcone-Based Synthesis of Tetrahydropyridazines via Cloke-Wilson-Type Rearrangement-Involved Tandem Reaction between Cyclopropyl Ketones and Hydrazines

A facile and efficient approach for the synthesis of multisubstituted tetrahydropyridazines starting from cyclopropyl ketones and hydrazines has been developed. The transformation is chalcone-based and takes place via a Cloke-Wilson-type rearrangement-involved tandem reaction catalyzed by TfOH in HFIP.

Regio- and Stereospecific Hydrative Cloke-Wilson Rearrangement

The Cloke-Wilson rearrangement of unsymmetrical β-diketone-derived cyclopropanes inevitably yields a mixture of two 4-acylated 2,3-dihydrofuran regiomers. By using alkynes as masked acyls, Tf2NH-promoted Cloke-Wilson rearrangement of polysubstituted 1-(1-alkynyl)cyclopropyl ketones followed by alkyne hydration is described, regioselectively affording 2,3-dihydrofurans bearing 4-acyls nonequivalent to that involved in the Cloke-Wilson rearrangement. The 2,3-dihydrofuran rings with cis 2,3-diaryls are unexpectedly more stable than their trans diastereomers under the reaction conditions, guaranteeing the regiospecificity of this hydrative Cloke-Wilson rearrangement with high fidelity.

Organocatalytic Cloke-Wilson Rearrangement: Carbocation-Initiated Tandem Ring Opening/Cyclization of Cyclopropanes under Neutral Conditions

We report a metal-, acid-, and base-free 2-(bromomethyl)naphthalene (2-BMN)-promoted organocatalytic Cloke-Wilson rearrangement of chain doubly activated cyclopropanes for the construction of 2,3-dihydrofurans via a carbocation-initiated tandem intramolecular ring-opening/recyclization process. The strategy is especially suitable for the construction of furan units in complex molecules, providing a solution to the problem of heavy-metal residues in dihydrofuran-containing drugs synthesized by traditional metal-based protocols. Thus, it is of potential interest in synthetic and medicinal chemistry.

Taming Chiral Quaternary Stereocenters via Remote H-Bonding Stereoinduction in Palladium-Catalyzed (3+2) Cycloadditions

Ring-opening transformations of donor-acceptor (D-A) cyclopropanes enable the rapid assembly of complex molecules. However, the enantioselective formation of chiral quaternary stereocenters using substrates bearing two different acceptors remains a challenge. Herein, we describe the first palladium-catalyzed highly diastereo- and enantioselective (3+2) cycloaddition of vinyl cyclopropanes bearing two different electron-withdrawing groups, a subset of D-A cyclopropanes. The key to the success of this reaction is the remote stereoinduction through hydrogen bond from chiral ligands, which thereby addressed the aforementioned challenge. A variety of chiral five-membered heterocycles were produced in good yields and with high stereoselectivity (up to 99 % yields, 99 : 1 er and >19 : 1 dr). In-depth mechanistic investigations, including control experiments and theoretical calculations, revealed the origin of the stereoselectivity and the importance of H-bonding in stereocontrol.

A facile metal-free one-flask synthesis of multi-substituted furans via a BF3·Et2O mediated formal [4 + 1] reaction of 3-chloro-3-phenyldiazirines and α,β-alkenyl ketones

A facile, efficient and metal free one-flask approach to diversely substituted furans from easily accessible 3-chloro-3-phenyldiazirines and α,β-alkenyl ketones is reported. This protocol integrates three steps of cyclopropanation, Cloke–Wilson rearrangement and elimination of HCl in one-flask to give products in moderate to good yields. It provides a metal and oxidant free approach to multi-substituted furans with the advantages of easy operation, mild reaction conditions and a broad scope of substrates.

A metal and oxidant free approach to multi-substituted furans from easily accessible 3-chloro-3-phenyldiazirines and α,β-alkenyl ketones in one flask.

Insertion of S2 into Donor-Acceptor Cyclopropanes: Access to Dithiolanes and Their Conversion to Thietane Dioxides

A facile and efficient route to dithiolanes starting from donor-acceptor cyclopropanes is reported. Potassium p-toluenethiosulfonate has been established as the reagent of choice for this formal insertion of the disulfide moiety. Using this methodology, dithiolanes have been synthesized in moderate to good yields with high functional group tolerance. Upon treatment with an excess of mCPBA, the corresponding dithiolanes delivered four-membered thietane dioxides, the formal (3+1)-cycloaddition product of D-A cyclopropanes, and sulfur dioxide.

Absence of Intermediates in the BINOL-Derived Mg(II)/Phosphate-Catalyzed Desymmetrizative Ring Expansion of 1-Vinylcyclobutanols

The catalyzed desymmetrizative ring expansion of alkenylcyclobutanols promoted by halofunctionalization of the alkene moiety with N-bromosuccinimide has been experimentally and computationally studied. The reaction yields highly enantioenriched cyclopentanones bearing two all-carbon quaternary stereocenters, one of them being generated in the rearrangement of the cyclobutane ring and the other by enantioselective desymmetrization. The reaction is competitive with the formation of a spiroepoxide, but it turns completely selective toward the cyclopentanone when a chiral bisphosphonium magnesium salt is employed as a catalyst. Mechanistic studies support the formation of an ion pair leading to a complex with only a unit of phosphoric acid, which is the resting state of the catalytic cycle. Calculations reproduce in an excellent way the observed reactivity and predict the effect exerted by the substituents of the aromatic ring linked to the double bond. The computational studies also revealed the reaction as a highly asynchronous concerted process taking place as one kinetic step but in two stages: (i) halogenation of the double bond and (ii) rearrangement of the cyclobutane. No intermediates are present in the reaction as energy minima. The experimental scope of the reaction further confirms the predictions for the observed reactivity and selectivity.

Asymmetric Sequential Corey-Chaykovsky Cyclopropanation/Cloke-Wilson Rearrangement for the Synthesis of 2,3-Dihydrofurans

The first sequential Corey-Chaykovsky cyclopropanation/Cloke-Wilson rearrangement between propargyl sulfonium salts and acrylonitrile derivatives has been developed, affording the tetra-substituted 2,3-dihydrofurans in generally excellent yields (57-98%) with good diastereoselectivities (7:1-18:1). In addition, chiral propargyl sulfonium salt is also suitable for this strategy, giving the optically active 2,3-dihydrofurans with good enantioselectivities. This reaction sequence was designed upon in situ generated 10π-conjugated structures from the dearomatization of indole fragments and subsequent intramolecular 1,6-addition.

Expanding Stereoelectronic Limits of endo-tet Cyclizations: Synthesis of Benz[b]azepines from Donor-Acceptor Cyclopropanes

The importance of intramolecular constraints in cyclic transition-state geometries is especially pronounced in n-endo-tet cyclizations, where the usual backside approach of a nucleophile to the breaking bond is impossible for the rings containing less than eight atoms. Herein, we expand the limits of endo-tet cyclizations and show that donor-acceptor cyclopropanes can provide a seven-membered ring via a genuine 6-endo-tet process. Substrates containing a N-alkyl-N-arylcarbamoyl moiety as an acceptor group undergo Lewis acid-induced cyclization to form tetrahydrobenz[b]azepin-2-ones in high yields. The reaction proceeds with the inversion of the configuration at the electrophilic carbon. In this process, a formally six-membered transition state yields a seven-membered ring as the pre-existing cycle is merged into the forming ring. The stereochemistry of the products can be controlled by the reaction time and by the nature of Lewis acid, opening access to both diastereomers by tuning of the reaction conditions.

Transition-Metal-Free [3+2] Dehydration Cycloaddition of Donor-Acceptor Cyclopropanes With 2-Naphthols

A Brønsted acid-catalyzed domino ring-opening cyclization transformation of donor-acceptor (D-A) cyclopropanes and 2-naphthols has been developed. This formal [3+2] cyclization reaction provided novel and efficient access to the naphthalene-fused cyclopentanes in the absence of any transition-metal catalysts or additives. This robust procedure was completed smoothly on a gram-scale to afford the corresponding product with comparable efficiency. Furthermore, the synthetic application of the prepared product has been demonstrated by its transformation into a variety of synthetically useful molecules.

Computational evidence of glycosyl cations

Glycosyl cations are key intermediates in the glycosylation reactions taking place through a S_N1-type mechanism. To obtain a reliable description of the glycosylation reaction mechanism a combination of computational studies and experimental data such as kinetic isotopic effects is needed. Computational studies have elucidated S_N2-type glycosylation reaction mechanisms, but elucidation of mechanisms in which ion pairs can be formed presents some difficulties because of the recombination of the ions. Recent topological and dynamic studies open the door to the ultimate confirmation of the presence of glycosyl cations in the form of intimate ion pairs during certain glycosylation reactions. This review covers the state-of-the-art tools and applications of computational chemistry mainly developed during the last ten years to understand glycosylation reactions in which an oxocarbenium ion could be involved.

Exploiting Heavier Organochalcogen Compounds in Donor-Acceptor Cyclopropane Chemistry

Donor-acceptor (D-A) cyclopropanes have gained increased momentum over the past two decades. The use of these highly strained three-membered entities paved the way to innovative and original transformations yielding complex cyclic and acyclic architectures that otherwise might be difficult to address. Since the fundamentals were laid by Wenkert and Reissig in the late 1970s, the field has flourished impressively including asymmetric transformations as well as elegant synthetic applications in the construction of natural occurring products. In this Account, we aim to highlight especially our efforts in the context of an efficient access to sulfur- and selenium-containing compounds, of either cyclic or open-chain nature, by exploiting D-A cyclopropane chemistry. Light will be shed on the three fundamental transformations: ring-opening reactions, cycloadditions, and rearrangements.Our synthetic endeavors started back in 2011 guided by quantum chemical studies to obtain 3,3'-linked bisthiophenes along with an unprecedented rearrangement delivering sulfur- and selenium-containing cagelike scaffolds. Inspired by these surprising results, we further deepened our efforts to the construction of new sulfur-carbon and selenium-carbon bonds within the context of D-A cyclopropane chemistry. In the first instance, we capitalized on the great versatility of organosulfur and organoselenium compounds regarding their amphiphilic character to act either as nucleophilic or as electrophilic species. By such an approach, ring-openings via a nucleophilic attack of sulfenyl and selenyl halides furnished 1,3-bishalochalcogenated products. A similar protocol led us to a desymmetrization reaction of meso-cyclopropyl carbaldehydes employing novel chiral imidazolidinone organocatalysts. In contrast, electrophilic sulfur was supplied by N-(arylthio)succinimide substrates to access thiolated γ-amino acid derivatives and their selenium equivalents.Combining the highly reactive thiocarbonyl compounds and vicinal donor-acceptor substituted cyclopropanes opened new vistas in the field of atom-economic cycloaddition reactions to build up sulfur-containing heterocycles of various sizes. The first systematic study of such transformations was made by our group in 2017 leading to highly decorated thiolanes, whereas an intramolecular approach furnished thia-[n.2.1]bicyclic ring systems. Our investigations were then successfully extended to the synthesis of tetrahydroselenophenes by using capricious selenoketones. Recently, we were able to yield the unsaturated analogues, selenophenes, by a (3 + 2)-cycloaddition of D-A cyclopropanes with ammonium selenocyanates followed by oxidation. The formal insertion of thioketenes was realized by employing 3-thioxocyclobutanones as surrogates for disubstituted thioketenes to obtain 2-substituted tetrahydrothiophenes bearing a semicyclic double bond via a (3 + 2) spiroannulation/(2 + 2) cycloreversion sequence. Even the formation of seven-membered S-heterocycles was realized by (4 + 3)-cycloaddition processes. In 2016, we demonstrated the synthesis of benzo-fused dithiepines from in situ generated ortho-bisthioquinones, whereas the utilization of thia-Michael systems as a hetero-4π-component delivered tetrahydrothiepine derivatives containing just one sulfur atom embedded in the ring system.

Brønsted Acid Catalyzed (4 + 2) Cyclocondensation of 3-Substituted Indoles with Donor–Acceptor Cyclopropanes

Acylcyclopropanes are employed as useful donor–acceptor cyclopropanes that undergo formal (4 + 2) cyclocondensation with N-unprotected 3-substituted indoles in the presence of a Brønsted acid catalyst. The reaction involves the simultaneous alkylation of both the N and C-2 positions of the indole and provides access to the 8,9-dihydropyrido[1,2-a]indole scaffold that is the central core of several biologically relevant indole alkaloids in excellent yields and good selectivities.

Synthesis of disubstituted γ-butyrolactones and spirocyclopropanes via a multicomponent reaction of aldehydes, Meldrum's acid and sulfoxonium ylides

An efficient synthesis of disubstituted γ-butyrolactones and spirocyclopropanes via multicomponent reaction of aldehydes, Meldrum's acid and sulfoxonium ylides has been developed.

Chiral 3-Acylglutaric Acid Derivatives from Strain-Induced Nucleophilic Retro-Claisen Ring-Opening Reactions

A nucleophilic retro-Claisen ring-opening of donor-acceptor cyclobutenes, formed with high stereocontrol by [3 + 1]-cycloaddition of TIPS-protected enoldiazoacetates with α-acyl sulfur ylides, has been developed. Removal of the TIPS group to form the isolable β-keto ester precedes the strain-induced ring-opening. Various amines, alcohols, thiols, and amino acid derivatives are effective nucleophiles, and their products are formed in very high yields via stoichiometric reactions. The chirality of the reactant donor-acceptor cyclobutenes is fully retained in the ring-opening reactions. The 3-acylglutaric acid products are converted to various valuable structures, including amido-diols, γ-aminobutyric acid (GABA) derivatives, and heterocycles.

Desymmetrization of 1-Symmetrical Donor-Acceptor (D-A) Cyclopropanes via Reactions with 1,3-Cyclodiones

A new type of 1-unsymmetrical D-A cyclopropanes containing a cyclic enone motif was obtained by the desymmetrization of 1-symmetrical D-A cyclopropanes via first the Lewis acid-catalyzed O-nucleophilic ring-opening reaction with 1,3-cyclodiones followed by an organobase-promoted unexpected multistep intramolecular transformation.

Uncovering the Neglected Similarities of Arynes and Donor-Acceptor Cyclopropanes

Arynes and donor-acceptor (D-A) cyclopropanes are two classes of strained systems having the potential for numerous applications in organic synthesis. The last two decades have witnessed a renaissance of interest in the chemistry of these species primarily because of the mild and robust methods for their generation or activation. Commonly, arynes as easily polarizable systems result in 1,2-disubstitution, whereas D-A cyclopropanes as polarized systems lead to 1,3-bisfunctionalization thereby showing striking similarities. Transformations with 1,2- and 1,3-dipoles afford cyclic structures. With arynes, emerging four-membered rings as intermediates might react further, whereas the analogous five-membered rings obtained from D-A cyclopropanes are most often the final products. However, there are a few cases where these intermediates behave surprisingly differently. This Minireview highlights the parallels in reactivity between arynes and D-A cyclopropanes thereby shedding light on the neglected similarities of these two reactive species.

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.

DBU-promoted ring-opening reactions of multi-substituted donor–acceptor cyclopropanes: access to functionalized chalcones with a quaternary carbon group

A strategy to synthesize highly stereoselective chalcones with alkylcyanoacetate subunits via DBU-promoted ring-opening reactions of multi-substituted D–A cyclopropanes has been developed without the requirement of a transition metal catalyst and extra solvent.

A thermal decarboxylative Cloke–Wilson rearrangement of dispirocyclopropanes derived from para-quinone methides and bromo-Meldrum's acids: an approach to synthesize spirobutyrolactone para-dienones

An unprecedented approach to synthesize spirobutyrolactone para-dienones from para-quinone methides and bromo-Meldrum's acids has been developed.

Stereoselective synthesis of trifluoromethyl-substituted 2H-furan-amines from enaminones

A straightforward strategy for synthesis of highly functionalized trifluoromethyl 2H-furans is described.

Enantioselective cyanation via radical-mediated C-C single bond cleavage for synthesis of chiral dinitriles

Ring-opening reaction via selective cleavage of C–C bond is known as a powerful strategy for construction of complex molecules. Complementary to the ionic process focusing on mostly small ring systems, radical-mediated C–C bond cleavage offers a solution for further diverse enantioselective functionalization benefited from its mild conditions, whereas such asymmetric transformations are still limited to three-membered rings so far. Herein, we describe radical-mediated ring-opening and enantioselective cyanation of four- and five-membered cycloketone oxime esters to access chiral 1,5- and 1,6-dinitriles. Employment of dual photoredox/copper catalysis is essential for the asymmetric ring-opening cyanation of cyclopentanone oxime esters. Both reactions proceed under mild conditions giving chiral dinitriles in high yields and enantioselectivity with low catalyst loading and broad substrate scope. The products dinitriles can be converted to valuable optically active diamides and diamines. Mechanistic studies indicate that the benzylic radical generated via C–C single bond cleavage is involved in the catalytic cycle.

Ring-opening reaction via selective cleavage of C–C bond is a powerful strategy to increase molecular complexity. Here the authors present the facile synthesis of chiral 1,5- and 1,6-dinitriles via the radical-mediated ring-opening and enantioselective cyanation of cycloketone oxime esters.

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.