Donor–Acceptor Cyclopropanes as 1,2-Dipoles in GaCl3-Mediated [4 + 2]-Annulation with Alkenes: Easy Access to the Tetralin Skeleton

Formal High-Order Cycloadditions of Donor-Acceptor Cyclopropanes with Cycloheptatrienes

The design of various cycloaddition/annulation processes is one of the most intriguing challenges in the development of donor-acceptor (D-A) cyclopropane chemistry. In this work, a new class of formal high-order [6+n]-cycloaddition and annulation processes of D-A cyclopropanes with cycloheptatriene systems has been designed and reported, to fill a significant gap in the chemistry of D-A cyclopropanes. The reactivity of methylated cycloheptatrienes from Me1 to Me5 as well as unsubstituted cycloheptatriene was studied in detail under GaCl3 activation conditions, which makes it possible to efficiently generate gallium 1,2-zwitterionic complexes or 1,3-zwitterionic intermediates starting from D-A cyclopropanes, while other Lewis acids are ineffective and non-selective. New examples of formal [6+2]-, [6+3]-, [6+4]-, [6+1]-, and [4+2]-cycloaddition and annulation reactions with cycloheptatrienes along with more complex processes were discovered. Cycloheptatriene itself can also successfully act as a hydride anion donor, which allows the ionic hydrogenation of D-A cyclopropanes to be performed under mild conditions. As a result, a number of efficient and highly diastereoselective protocols for the synthesis of seven-membered carbocycles has been developed.

Donor-Acceptor Cyclopropane Ring Opening with 6-Amino-1,3-dimethyluracil and Its Use in Pyrimido[4,5-b]azepines Synthesis

A scandium trifluoromethanesulfonate-catalyzed reaction of donor-acceptor cyclopropanes with 6-amino-1,3-dimethyluracil was found to proceed as three-membered ring opening via nucleophilic attack of the C(5) atom of an ambident nucleophile serving as an enamine equivalent. It was shown that, under basic conditions, the obtained products underwent cyclization to 6,7-dihydro-1H-pyrimido[4,5-b]azepine-2,4,8-triones, an interesting subclass of nucleobase analogues.

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.

Donor-Acceptor Cyclopropanes in the Synthesis of Carbocycles

Donor-acceptor cyclopropanes not only participate in a broad range of ring openings with nucleophiles, electrophiles, radical and red-ox agents, but also are excellent substrates for various (3+n)-cycloaddition and (3+n)-annulation processes. Moreover, under treatment with Lewis acid donor-acceptor cyclopropanes can produce new ring systems via isomerization or cyclodimerization. Authors' contribution to the synthesis of diverse carbocycles from donor-acceptor cyclopropanes is summarized in this account.

Ring-Opening 1,3-Aminochalcogenation of Donor-Acceptor Cyclopropanes: A Three-Component Approach

A 1,3-aminothiolation was realized by reacting 2-substituted cyclopropane 1,1-dicarboxylates with sulfonamides and N-(arylthio)succinimides. Under Sn(OTf)2 catalysis the transformation proceeded smoothly to the corresponding ring-opened products bearing the sulfonamide in the 1-position next to the donor and the arylthio residue in the 3-position next to the acceptor. The procedure was extended to the corresponding selenium analogues by employing N-(phenylseleno)succinimides as an electrophilic selenium source.

Expanding the Reactivity of Donor-Acceptor Cyclopropanes: Synthesis of Benzannulated Five-Membered Heterocycles via Intramolecular Attack of a Pendant Nucleophilic Group

Lewis-acid-induced domino transformations of donor-acceptor cyclopropanes, possessing a nucleophilic center embedded in a donor group, into functionalized 2,3-dihydrobenzo[ b]furans and 2,3-dihydrobenzo[ b]thiophenes are reported herein. An unusual switch of the electrophilic center in the three-membered ring, from the atom bearing a donor substituent to an unsubstituted carbon atom, was achieved by a judicious choice of Lewis acid, which induces the isomerization of a cyclopropane to an electrophilic alkene, and the length of linker, connecting a nucleophilic moiety and the small ring.

Asymmetric Sulfur-Ylide-Mediated Formal [4+1]-Annulation Reaction: Scope and Mechanism

A formal [4+1]-annulation strategy between sulfur ylides and 1,3-dienes was developed to afford functionalized cyclopentanoids. The process consists of a stereoselective cyclopropanation reaction followed, in situ, by a stereospecific MgI₂ -catalyzed vinylcyclopropane-cyclopentene rearrangement. The use of chiral sulfur ylides provided cyclopentanoids with excellent enantiocontrol. A combined experimental and computational mechanistic study showed that the stereospecificity of the rearrangement could be accounted for by a double S_N 2 reaction mechanism involving iodide.

A photocatalytic iminyl radical-mediated C–C bond cleavage/addition/cyclization cascade for the synthesis of 1,2,3,4-tetrahydrophenanthrenes

A visible light photocatalytic radical addition/cyclization cascade is developed to provide an efficient access to cyanoalkylated 1,2,3,4-tetrahydrophenanthrenes.

Donor–acceptor cyclopropanes as ortho-quinone methide equivalents in formal (4 + 2)-cycloaddition to alkenes

A new reactivity of donor–acceptor cyclopropanes as synthetic equivalents of o-quinone methides was revealed for 2-hydroxyarylcyclopropane diesters in their formal (4 + 2)-cycloaddition to alkenes yielding chromanes.

Computational study on GaCl3-mediated reactions of donor–acceptor cyclopropanes with aromatic aldehydes: mechanism and role of GaCl3 and aldehydes

The mechanism and role of GaCl₃ and aldehydes for the GaCl₃-mediated reaction of donor–acceptor (D–A) cyclopropanes are clarified through our calculations.