Ring-Opening Reactions of Donor-Acceptor Cyclobutanes with Electron-Rich Arenes, Thiols, and Selenols

Efficient and versatile formation of glycosidic bonds via catalytic strain-release glycosylation with glycosyl ortho-2,2-dimethoxycarbonylcyclopropylbenzoate donors

Catalytic glycosylation is a vital transformation in synthetic carbohydrate chemistry due to its ability to expediate the large-scale oligosaccharide synthesis for glycobiology studies with the consumption of minimal amounts of promoters. Herein we introduce a facile and efficient catalytic glycosylation employing glycosyl ortho-2,2-dimethoxycarbonylcyclopropylbenzoates (CCBz) promoted by a readily accessible and non-toxic Sc(III) catalyst system. The glycosylation reaction involves a novel activation mode of glycosyl esters driven by the ring-strain release of an intramolecularly incorporated donor-acceptor cyclopropane (DAC). The versatile glycosyl CCBz donor enables highly efficient construction of O-, S-, and N-glycosidic bonds under mild conditions, as exemplified by the convenient preparation of the synthetically challenging chitooligosaccharide derivatives. Of note, a gram-scale synthesis of tetrasaccharide corresponding to Lipid IV with modifiable handles is achieved using the catalytic strain-release glycosylation. These attractive features promise this donor to be the prototype for developing next generation of catalytic glycosylation.

Diastereoselective Synthesis of Tetrahydrobenzo[b]azocines by Lu(OTf)3 Catalyzed [4 + 4] Cycloaddition of Donor–Acceptor Cyclobutanes with Anthranils

The construction of N-heterocyclic eight-membered rings remains a challenging due to unfavorable transannular strain. Herein, we report a Lu(OTf)3 catalyzed formal [4 +4] cycloaddition reaction of cyclobutane 1,1-diesters with anthranils...

Lewis acid-catalyzed [4 + 2] cycloaddition of donor–acceptor cyclobutanes with iminooxindoles: access to spiro[piperidine-3,2′-oxindoles]

A Lewis acid-catalyzed [4 + 2] cycloaddition reaction from D–A cyclobutanes and iminooxindoles, providing the corresponding spiro[piperidine-3,2′-oxindoles] under mild conditions.

Cycloadditions of Donor-Acceptor Cyclopropanes and -butanes using S=N-Containing Reagents: Access to Cyclic Sulfinamides, Sulfonamides, and Sulfinamidines

We present (3+2)‐ and (4+2)‐cycloadditions of donor–acceptor (D–A) cyclopropanes and cyclobutanes with N‐sulfinylamines and a sulfur diimide, along with a one‐pot, two‐step strategy for the formal insertion of HNSO₂ into D–A cyclopropanes. These are rare examples of cycloadditions with D–A cyclopropanes and cyclobutanes whereby the 2π component consists of two different heteroatoms, thus leading to five‐ and six‐membered rings containing adjacent heteroatoms.

Friedel-Crafts-Type Reactions with Electrochemically Generated Electrophiles from Donor-Acceptor Cyclopropanes and -Butanes

We describe a general electrochemical method to functionalize donor-acceptor (D-A) cyclopropanes and -butanes with arenes utilizing Friedel-Crafts-type reactivity. The catalyst-free strategy relies on the direct anodic oxidation of the strained carbocycles, which leads after C(sp³)-C(sp³) cleavage to radical cations that act as electrophiles for the arylation reaction. Broad reaction scopes in regard to cyclopropanes, cyclobutanes, and aromatic reaction partners are presented. Additionally, a plausible electrolysis mechanism is proposed.

Electrocatalytic Activation of Donor-Acceptor Cyclopropanes and Cyclobutanes: An Alternative C(sp3 )-C(sp3 ) Cleavage Mode

We describe the first electrochemical activation of D-A cyclopropanes and D-A cyclobutanes leading after C(sp3 )-C(sp3 ) cleavage to the formation of highly reactive radical cations. This concept is utilized to formally insert molecular oxygen after direct or DDQ-assisted anodic oxidation of the strained carbocycles, delivering β- and γ-hydroxy ketones and 1,2-dioxanes electrocatalytically. Furthermore, insights into the mechanism of the oxidative process, obtained experimentally and by additional quantum-chemical calculations are presented. The synthetic potential of the reaction products is demonstrated by diverse derivatizations.

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.

Synthetically important ring opening reactions by alkoxybenzenes and alkoxynaphthalenes

Alkoxybenzenes and alkoxynaphthalenes, as nucleophiles, have drawn great attention from organic chemists over the decades. Due to their high ring strain, those particular classes of molecules are often used in synthesis by utilizing their properties to undergo facile Friedel-Crafts alkylations. Different isomeric and low or densely substituted alkoxybenzenes are used for synthesis according to the structure of the target molecule. Isomeric methoxybenzenes, are the most commonly used molecule in this regard. This review aims to comprehensively cover the instances of different alkoxy-benzenes/naphthalenes used as nucleophiles for ring opening.

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.

Multicomponent, Tandem 1,3- and 1,4-Bisarylation of Donor-Acceptor Cyclopropanes and Cyclobutanes with Electron-Rich Arenes and Hypervalent Arylbismuth Reagents

A tandem catalytic process for 1,3- and 1,4-bisarylation of donor-acceptor (D-A) cyclopropanes and cyclobutanes is disclosed. This strategy capitalizes on the use of two distinct sources of nucleophilic and electrophilic arylating agents, affording the formation of two new C-C bonds in an orchestrated multicomponent fashion with the aid of a catalytic Lewis acid. Mechanistic investigations have revealed it to be a stereoselective process, and products could be easily elaborated into other useful compounds.

Synthesis of Functionalized Tetrahydropyridines by SnCl4 -Mediated [4+2] Cycloaddition between Donor-Acceptor Cyclobutanes and Nitriles

Cycloadditions of strained carbocycles promoted by Lewis acids are powerful methods to construct heterocyclic frameworks. In fact, the formal [3+2] cycloadditions of donor-acceptor (DA) cyclopropanes with nitriles has seen particular success in synthesis. In this work, we report on the first [4+2] cycloaddition of nitriles with DA cyclobutanes by Lewis acid activation. Tetrahydropyridine derivatives were obtained in up to 91 % yield from various aryl-activated cyclobutane diesters and aliphatic or aromatic nitriles.