Ring-Opening Cyclization of Cyclohexane-1,3-dione-2-spirocyclopropanes with Amines: Rapid Access to 2-Substituted 4-Hydroxyindole

Gold-Catalyzed "Back-to-Front" Synthesis of 4-Silyloxyindoles

A series of pyrrol-yn-glycol derivatives were easily prepared from simple pyrroles through a three-step sequence involving hydroxyalkylation-alkynylation-O-silylation. Their reaction with IPrAuNTf2 triggers a C2-pyrrole attack onto the activated alkyne and subsequent highly selective 1,2-migration of the oxyalkyl group in the intermediate spirocycle. This approach enables the efficient synthesis of a wide selection of regioselectively functionalized 4-hydroxyindoles, which represent important yet challenging indole scaffolds, in high yields.

Ring expansion of spirocyclopropanes with stabilized sulfonium ylides: highly diastereoselective synthesis of cyclobutanes

A novel method was devised for regioselective ring expansion of Meldrum's acid-derived spirocyclopropanes to spirocyclobutanes with stabilized sulfonium ylides, affording 1,2-trans-disubstituted 6,8-dioxaspiro[3.5]nonane-5,9-diones in up to 87% yields without the formation of any isomers. The aforementioned reaction was also applied to the barbituric acid-derived spirocyclopropane, resulting in the formation of the corresponding cyclobutanes.

Synthesis of 1,5-Substituted Pyrrolidin-2-ones from Donor-Acceptor Cyclopropanes and Anilines/Benzylamines

We developed a straightforward synthetic route to pharmacologically important 1,5-substituted pyrrolidin-2-ones from donor-acceptor cyclopropanes bearing an ester group as one of the acceptor substituents. This method includes a Lewis acid-catalyzed opening of the donor-acceptor cyclopropane with primary amines (anilines, benzylamines, etc.) to γ-amino esters, followed by in situ lactamization and dealkoxycarbonylation. The reaction has a broad scope of applicability; a variety of substituted anilines, benzylamines, and other primary amines as well as a wide range of donor-acceptor cyclopropanes bearing (hetero)aromatic or alkenyl donor groups and various acceptor substituents can be involved in this transformation. In this process, donor-acceptor cyclopropanes react as 1,4-C,C-dielectrophiles, and amines react as 1,1-dinucleophiles. The resulting di- and trisubstituted pyrrolidin-2-ones can be also used in subsequent chemistry to obtain various nitrogen-containing polycyclic compounds of interest to medicinal chemistry and pharmacology, such as benz[g]indolizidine derivatives.

Ambident reactivity of 5-aminopyrazoles towards donor-acceptor cyclopropanes

Lewis acid-catalysed reactions of donor-acceptor cyclopropanes with 1,3-disubstituted 5-aminopyrazoles were investigated. Under catalysis with gallium(III) chloride, products of the three-membered ring opening via a nucleophilic attack of the exocyclic amino group were obtained in a chemoselective manner. Oppositely, in the presence of scandium(III) triflate, products of either N-alkylation or C(4)-alkylation, or a mixture of both were formed. The products of the C(4) alkylation were transformed in one step into tetrahydropyrazolo[3,4-b]azepines that are attractive for medicinal chemistry and pharmacology.

An iron-catalyzed domino reaction of donor-acceptor cyclopropanes: a diastereoselective approach towards diversely functionalized pyrrolo-quinazolines

An expeditious synthetic route to access functionalized pyrrolo[2,1-b]quinazoline scaffolds has been achieved via domino ring opening cyclization (DROC) reactions of donor-acceptor (D-A) cyclopropanes and 2-amino(methyl)aniline derivatives. This novel iron catalyzed transformation is amenable to a wide range of substrates. Three new C-N bonds and two rings were sequentially constructed in this divergent one-pot process. The advantages of simple operation, high yields and general applicability make this procedure highly attractive and practical too.

Synthesis of 3C-alkylated active methylene substituted 2H-indazole derivatives via sequential ring opening of donor–acceptor cyclopropanes and reductive cyclization reaction

A facile synthesis of 3C-alkylated active methylene substituted 2H-indazoles from o-nitrocyclopropanes and primary arylamines catalyzed by Ni(ClO4)2.6H2O/SnCl2.2H2O is reported.

Synthesis of substituted 1,2-dihydroisoquinolines via Ni(ii) and Cu(i)/Ag(i) catalyzed double nucleophilic addition of arylamines to ortho-alkynyl donor-acceptor cyclopropanes (o-ADACs)

A concise approach for the synthesis of substituted 1,2-dihydroisoquinolines via double nucleophilic addition of primary arylamines to ortho-alkynyl donor-acceptor cyclopropanes (o-ADACs) in the presence of a catalytic Ni(ClO₄)₂·6H₂O and CuI/AgOTf system has been developed. Further applying this protocol, some of the derived malonates were converted into the corresponding monoesters under Krapcho decarboxylation reaction conditions. Thereafter, these esters were transformed into the respective acids and alcohols. In addition, multifunctionalized 4-(2,2,2-trifluoroacetyl) 1,2-dihydroisoquinolines were also obtained with excess TFAA.

Lewis acid catalysed asymmetric cascade reaction of cyclopropyl ketones: concise synthesis of pyrrolobenzothiazoles

A concise method to synthesize pyrrolobenzothiazoles via a cascade reaction of cyclopropyl ketones was developed by using a chiral N,N′-dioxide-scandium(iii) complex catalyst.

Blue LED Irradiation of Iodonium Ylides Gives Diradical Intermediates for Efficient Metal-free Cyclopropanation with Alkenes

A facile and highly chemoselective synthesis of doubly activated cyclopropanes is reported where mixtures of alkenes and β-dicarbonyl-derived iodonium ylides are irradiated with light from blue LEDs. This metal-free synthesis gives cyclopropanes in yields up to 96 %, is operative with cyclic and acyclic ylides, and proceeds with a variety of electronically-diverse alkenes. Computational analysis explains the high selectivity observed, which derives from exclusive HOMO to LUMO excitation, instead of free carbene generation. The procedure is operationally simple, uses no photocatalyst, and provides access in one step to important building blocks for complex molecule synthesis.

Asymmetric Synthesis of Enantioenriched 2-Aryl-2,3-Dihydrobenzofurans by a Lewis Acid Catalyzed Cyclopropanation/Intramolecular Rearrangement Sequence

A cyclopropanation/intramolecular rearrangement initiated by the Michael addition of in situ generated ortho-quinone methides (o-QMs) has been developed for the enantioselective synthesis of 2-aryl-2,3-dihydrobenzofurans containing two consecutive stereogenic centers, including a quaternary carbon atom. In the presence of a chiral oxazaborolidinium ion catalyst, the reaction proceeded in excellent yields (up to 95 %) with excellent stereoselectivity (up to >99 ee, up to >20:1 d.r.).

Ring-opening cyclization of spirocyclopropanes with stabilized sulfonium ylides for the construction of a chromane skeleton

Regioselective ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes with stabilized sulfonium ylides provided 2,3-trans-disubstituted 2,3,4,6,7,8-hexahydro-5H-1-benzopyran-5-ones in high yields without the formation of any isomers. The obtained product was readily converted into highly substituted chromane.

An unexpected thermal-ring-rearrangement of benzochromenes to inden-3-yl-naphthols with pTsOH

Described here is the first report of an unexpected thermal-ring rearrangement (TRR) of benzochromenes to indene derivatives promoted by pTsOH, which proceeds through the protonation of benzochromenes by an acid catalysts followed by ring-opening and ring-closure by an intramolecular Friedel–Crafts cyclization.

Asymmetric ring-opening of cyclopropyl ketones with β-naphthols catalyzed by a chiral N,N′-dioxide–scandium(iii) complex

An efficient asymmetric ring-opening reaction of cyclopropyl ketones with 2-naphthols has been realized using a chiral N,N′-dioxide–Sc^III complex catalyst. A variety of chiral β-naphthol derivatives were obtained in excellent outcomes (up to 99% yield, 97% ee).

Diastereoselective Synthesis of Spirocyclopropanes under Mild Conditions via Formal [2 + 1] Cycloadditions Using 2,3-Dioxo-4-benzylidene-pyrrolidines

A highly diastereoselective cyclopropanation of cyclic enones with sulfur ylides was developed under catalyst-free conditions, producing multifunctional spirocyclopropanes in generally excellent yields (up to 99% yield and >99:1 d.r.). The asymmetric version of this method was realized by using an easily available chiral sulfur ylide, affording products with moderate to good stereoselectivity.

PIFA-Mediated oxidative cyclization of 1-aroyl-N-arylcyclopropane-1-carboxamides and their application in the synthesis of pyrrolo[3,2-c]quinolinones

A PIFA-promoted ring-closing reaction for the synthesis of spirocyclopropane quinolinediones, and their application toward pyrrolo[3,2-c]quinolinones.

An Efficient Method for the Synthesis of 2',3'-Nonsubstituted Cycloalkane-1,3-dione-2-spirocyclopropanes Using (2-Bromoethyl)diphenylsulfonium Trifluoromethanesulfonate

An efficient and practical synthesis of 2',3'-nonsubstituted cyclohexane-1,3-dione-2-spirocyclopropanes using a sulfonium salt was achieved. The reaction of 1,3-cyclohexanediones and (2-bromoethyl)diphenylsulfonium trifluoromethanesulfonate with powdered K₂CO₃ in EtOAc at room temperature (r.t.) provided the corresponding spirocyclopropanes in high yields. The synthetic method was also applied to 1,3-cyclopentanedione, 1,3-cycloheptanedione, 1,3-indanedione, acyclic 1,3-diones, ethyl acetoacetate, and Meldrum's acid.

Lewis Acid and (Hypo)iodite Relay Catalysis Allows a Strategy for the Synthesis of Polysubstituted Azetidines and Tetrahydroquinolines

A catalytic [3 + 1]-annulation reaction between cyclopropane 1,1-diester and aromatic amine is developed based on the relay catalysis strategy. Lewis acid-catalyzed nucleophilic ring opening of cyclopropane 1,1-diester with aromatic amine and (hypo)iodite-catalyzed C-N bond formation are combined successfully in one reaction. Using this reaction, biologically important azetidines and tetrahydroquinolines can be prepared directly.

Catalytic asymmetric [3 + 3] annulation of cyclopropanes with mercaptoacetaldehyde

A highly diastereo- and enantioselective [3 + 3] annulation of donor–acceptor cyclopropanes with mercaptoacetaldehyde has been developed.