Boron Trifluoride-Promoted Indium(III) Triflate-Catalyzed Sequential One-Pot Synthesis of (1,2-Diaryl-2-oxoethyl)malonates fromtrans-2-Aryl-3-nitrocyclopropane-1,1-dicarboxylates and Activated Arenes

Nitrocyclopropanes as Valuable Building Blocks in Organic Synthesis and Biology: Exploring the Origin of the Nitro Group

Cyclopropane derivatives serve as important building blocks in organic synthesis, due to their three-membered stretched ring. Nitro-substituted cyclopropanes (NCPs) represent a special class of donor-acceptor cyclopropanes (DACs) that contain a strong electron-withdrawing nitro group as substituent on the cyclopropyl moiety. Due to the versatile nature of the nitro group, which can be converted into other functional azo groups or heterocyclic scaffolds, NCPs are considered important building blocks in the design of complex organic compounds. Herein, the present review is organized in two main sections focused on the synthesis and application of NCPs. The part containing the synthetic methodologies of NCPs has been further divided into distinct sections based mainly on the nature of the starting materials, such as: a) from nitroalkenes, b) from nitro-bearing diazo compounds, c) from nitroalkanes in combination with alkenes, d) from aminocyclopropanes under oxidative conditions, and e) from various nitro compounds, through miscellaneous processes. In the part concerning the applications and use of NCPs, the categorization includes three main subcategories, concerning their reduction to 1-aminocyclopropane-1-carboxylic acids (ACC), the ring opening of NCPs leading to valuable open-chain heterocycles, and the ring expansion to larger cyclic and heterocyclic compounds.

Synthesis of β-enamino malonates through caesium carbonate-promoted reaction of nitro-substituted donor-acceptor cyclopropanes

A caesium carbonate-promoted reaction of nitro-substituted donor-acceptor cyclopropanes (DACs) with primary aromatic amines in water provides a convenient access to β-enamino malonates under mild reaction conditions. The transformation takes place through the formation of allene intermediates from the DACs followed by the conjugate addition of various primary aromatic amines to the intermediates. The reaction proceeds more efficiently in water as compared with organic solvents and the products were isolated in good yields by filtering through a silica gel column without any prior extraction procedure.

First synthesis of acylated nitrocyclopropanes

Although nitrocyclopropanedicarboxylic acid esters are widely used in organic syntheses, nitrocyclopropanes with an acyl group have not yet been synthesized. When adducts of β-nitrostyrene and 1,3-dicarbonyl compounds are treated with (diacetoxyiodo)benzene and tetrabutylammonium iodide, iodination occurs at the α-position of the nitro group, and the subsequent O-attack of the enol moiety leads to 2,3-dihydrofuran. Cyclopropane was successfully synthesized through C-attack as the acyl group became bulkier. The obtained nitrocyclopropane was transformed into furan upon treatment with tin(II) chloride via a ring-opening/ring-closure process.

Cascade Ring-Opening/Cyclization Reaction of Spiro(nitrocyclopropane)oxindoles with Huisgen Zwitterions and Synthesis of Pyrazolo[3,4-b]indoles

Herein, we report a ring-opening/cyclization cascade reaction of spiro(nitrocyclopropane)oxindoles with in situ generated Huisgen zwitterions (HZs) from PPh₃ and azodicarboxylates. This reaction provides an array of polyfunctionalized pyrazolo[3,4-b]indole derivatives in moderate-to-excellent yields and generally high stereoselectivities with a broad substrate scope. The annulation products obtained from di-tert-butyl azodicarboxylates can be readily transformed into aromatic-substituted pyrazolo[3,4-b]indoles in moderate yields upon treatment with trifluoroacetic acid, thus providing a new entry to this fused heterocycle skeleton. In terms of nitro-substituted donor-acceptor cyclopropane, this work significantly broadens the substrate scope for the annulation reaction of nitrocyclopropanes and HZs. The dual roles of the oxindole moiety in the ring opening of cyclopropane and a plausible mechanism for the cascade reaction are also discussed.

The Cloke-Wilson rearrangement of aroyl-substituted donor-acceptor cylopropanes containing arylethyl donors

The chemistry of donor-acceptor (D-A) cyclopropanes containing alkyl donors has been scantily investigated. In the present work, we have synthesized new D-A cyclopropanes containing arylethyl donors and explored their reactivity in the presence of Lewis acids. Upon treatment with SnCl₄, these cyclopropanes underwent the Cloke-Wilson rearrangement to yield 3,4,5-trisubstituted γ-butyrolactones in good yields with high diastereoselectivity.

In situ generation of highly reactive allenes from nitrocyclopropanes: controllable synthesis of enynes and enesters

A new and efficient strategy for ring-opening reactions of nitrocyclopropanes is developed for the first time for the divergent synthesis of enynes and enesters via in situ generated highly reactive electron-deficient intermediate allenes. Controllable approaches resulted in enynes and enesters with up to 89% and 90% yields, respectively. The reaction features easy operation, involves green solvents and simple inorganic bases, and is transition-metal free.

The Regioselective Functionalization Reaction of Unprotected Carbazoles with Donor-Acceptor Cyclopropanes

The regioselective functionalization reaction of unprotected carbazoles with donor-acceptor (D-A) cyclopropanes has been demonstrated for the first time. With Sc(OTf)₃ as Lewis acid catalyst, the N-H functionalization of carbazoles takes place through a highly selective nitrogen-initiated nucleophilic ring opening reaction. Significantly, by engaging TfOH as Brønsted acid catalyst, a straightforward C-H functionalization at the 3-position of the unprotected carbazole proceeds via Friedel-Crafts-type addition. This strategy facilitates the diversity-oriented synthesis of carbazole-containing heterocycles and expands the novel application of D-A cyclopropanes.