Four-Membered Cycle Formation Challenge: GaCl3 -Promoted Formal [2+2]-Cycloaddition of Donor-Acceptor Cyclopropanes to Bicyclobutylidene

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.

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.

Photoredox-catalyzed hydroxymethylation of β-ketoesters: application to the synthesis of [3.3.3] propellane lactones

Photoredox-catalysed hydroxymethylation of β-ketoesters substituted by an allyl subunit on the α-position afforded directly the corresponding bicyclic lactones possessing both a hydroxy group and an unsaturation. A subsequent regioselective iodoetherification led to the formation of original [3.3.3] propellane structures. Substitution of the iodine atom by various nucleophiles afforded highly functionnalized structures including triazolomethyl derivatives.

GaCl3-Mediated Cascade [2 + 4]-Cycloaddition/[4 + 2]-Annulation of Donor-Acceptor Cyclopropanes with Conjugated Dienes: Strategy for the Construction of Benzobicyclo[3.3.1]nonane Skeleton

Structurally important benzobicyclo[3.3.1]nonane derivatives were synthesized by a gallium trichloride mediated reaction of readily available donor-acceptor cyclopropanes (DACs) with 1,3-dienes as a one-pot cascade ionic [2 + 4]-cycloaddition/Friedel-Crafts-type cyclization process. At the first stage, DACs act as sources of formal gallium 1,2-zwitterionic complexes to give 6-benzylcyclohex-3-ene-1,1-dicarboxylates that are converted under certain conditions in the presence of GaCl₃ to give benzobicyclo[3.3.1]nonanes in 30-74% yields. The process is tolerant of varying substituents at different positions of the main framework. Further, potentially useful modifications of benzobicyclo[3.3.1]nonane derivatives are demonstrated. 2-Cyclopropylbutadiene reacts with DAC at higher temperature more deeply with cleavage of three-membered rings in both cyclopropane substrates, and twofold alkylation of the Ph-substituent at ortho- and meta-positions, that leads to a 1,2,7,8,9,10-hexahydro-6H-7,10a-methanocycloocta[cd]indene skeleton. Cyclopropane-1,1-diesters containing a bulky substituent in the ester group react with isoprene in a different way to give bicyclic lactones containing a 2-oxabicyclo[2.2.2]octan-3-one moiety.

Recent advances in ring-opening of donor acceptor cyclopropanes using C-nucleophiles

Ring-opening transformations of donor-acceptor cyclopropanes (DAC) with carbon-centered nucleophiles is a simple, straight-forward approach to 1,3-bifunctional compounds that has witnessed remarkable progress over the past several years. To date, different reactivity patterns of DACs have been successfully exploited in racemic/stereoselective syntheses of various acyclic compounds or carbocycles with an impressive structural diversity. The thriving strategies have been successfully utilized in multistep synthesis of complex target molecules. Herein, the recent advances (2015-present) in the ring-opening of DAC involving electron rich arenes and indoles, active methylene compounds, various dipolarophiles, organoborates/boronates, vinyl ethers etc. following Friedel-Crafts alkylation, annulation/formal cycloaddition reaction, organocatalytic reaction, Nazarov cyclisation etc. are presented.

"Cyclopropanation of Cyclopropanes": GaCl3-Mediated Ionic Cyclopropanation of Donor-Acceptor Cyclopropanes with Diazo Esters as a Route to Tetrasubstituted Activated Cyclopropanes

A new ionic cyclopropanation process involving the addition of diazo esters to donor-acceptor cyclopropanes (DAC) activated by GaCl₃ has been developed. The reactions occur via 1,2-zwitterionic gallium complexes with elimination of nitrogen in all cases to give 1,1,2,3-tetrasubstituted cyclopropanes as the main products. Also, a number of related processes with the formation of various polysubstituted cyclopropanes, alkenes, and cyclobutanes, including products of multiple diazo ester addition, have been developed. Obtained by the developed method tetrasubstituted cyclopropanes are activated cyclopropanes such as DAC and can be used for further synthesis in this capacity. Their new reaction with benzaldehyde promoted by TiCl₄ and involving one of the additional functional groups has been demonstrated, which leads to five-membered lactones. The mechanisms of the occurring processes, as well as the structures and stereochemistry of a rich range of products formed, are discussed in detail.

Reactions of Styrylmalonates with Aromatic Aldehydes: Detailed Synthetic and Mechanistic Studies

Reactions of styrylmalonates with aromatic aldehydes in the presence of Lewis and Brønsted acids and their regularities have been studied in detail. Approaches to the synthesis of various polysubstituted 5,6-dihydropyran-2-ones, indenes, aryl-containing dienes and trienes, cyclopentadienes, and polycyclic lactones have been developed with chemo-, regio-, stereo-, and diastereoselectivity control. The mechanisms of these reactions were studied in depth using multinuclear NMR experiments, monitoring the reactions in the NMR tube, and ¹⁸O isotope labeling.

Synthesis of (Het)aryl 2-(2-hydroxyaryl)cyclopropyl Ketones

A simple general method for the synthesis of 1-acyl-2-(ortho-hydroxyaryl)cyclopropanes, which belong to the donor–acceptor cyclopropane family, has been developed. This method, based on the Corey–Chaykovsky cyclopropanation of 2-hydroxychalcones, allows for the preparation of a large diversity of hydroxy-substituted cyclopropanes, which can serve as promising building blocks for the synthesis of various bioactive compounds.

Donor-Acceptor Bicyclopropyls as 1,6-Zwitterionic Intermediates: Synthesis and Reactions with 4-Phenyl-1,2,4-triazoline-3,5-dione and Terminal Acetylenes

The bicyclopropyl system activated by incorporation of donor and acceptor groups in the presence of Lewis acids was used as a synthetic equivalent of 1,6-zwitterions. Opening of both cyclopropane rings in 2'-aryl-1,1'-bicyclopropyl-2,2-dicarboxylates (D-A bicyclopropyl, ABCDs) in the presence of GaI₃ + Bu₄N⁺GaI₄^- results in 5-iodo-5-arylpent-2-enylmalonates as products of HI formal 1,6-addition to the bicyclopropyl system. The use of GaCl₃ or GaBr₃ as a Lewis acid and terminal aryl or alkyl acetylenes as 1,6-zwitterion interceptors allows the alkyl substituent to be grown to give the corresponding acyclic 7-chloro(bromo)-hepta-2,6-dienylmalonates. The reaction of ABCDs with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) catalyzed by Yb(OTf)₃ also results in the opening of both cyclopropane rings. The reaction products are tetrahydropyridazine derivatives - (7,9-dioxo-1,6,8-triazabicyclo[4.3.0]non-3-en-2-ylmethyl)malonates - containing one more PTAD moiety in the malonyl group.