Concise approach to γ-(het)aryl- and γ-alkenyl-γ-aminobutyric acids. Synthesis of vigabatrin

γ-Aminobutyric acid (GABA) and GABA derivatives have attracted increased attention over the years in the fields of medicinal chemistry and chemical biology due to their interesting biological properties and synthetic relevance. Here, we report a short synthetic route to γ-(het)aryl- and γ-alkenyl-γ-aminobutyric acids, including the antiepileptic drug vigabatrin, from readily available donor-acceptor cyclopropanes and ammonia or methylamine. This protocol includes a facile synthesis of 2-oxopyrrolidine-3-carboxamides and their acid hydrolysis to γ-aryl- or γ-alkenyl-substituted GABAs, which can serve as perspective building blocks for the synthesis of various GABA-based N-heterocycles and bioactive compounds.

Synthesis of 2-[2-(Ethoxymethoxy)phenyl]spiro[cyclopropane-1,2′-indene]-1′,3′-dione

An 1,3-indanedione-derived donor–acceptor cyclopropane, bearing the ethoxymethyl-protected phenolic group at the ortho-position of the donor aryl substituent, has been synthesized using a reaction sequence involving the Knoevenagel condensation of 1,3-indanedione with the corresponding protected salicylaldehyde followed by the Corey–Chaykovsky cyclopropanation of the obtained adduct with dimethylsulfoxonium methylide. The structure of the synthesized cyclopropane was unambiguously proved by single-crystal X-ray diffraction data.

Extended Corey–Chaykovsky reactions: transformation of 2-hydroxychalcones to benzannulated 2,8-dioxabicyclo[3.2.1]octanes and 2,3-dihydrobenzofurans

We report the divergent synthesis of benzannulated 2,8-dioxabicyclo[3.2.1]octanes and 2,3-dihydrobenzofurans using the concept of extended Corey–Chaykovsky reactions.

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.

Protic Ionic Liquid as Reagent, Catalyst, and Solvent: 1-Methylimidazolium Thiocyanate

We propose a new concept of the triple role of protic ionic liquids with nucleophilic anions: a) a regenerable solvent, b) a Brønsted acid inducing diverse transformations via general acid catalysis, and c) a source of a nucleophile. The efficiency of this strategy was demonstrated using thiocyanate-based protic ionic liquids for the ring-opening of donor-acceptor cyclopropanes. A wide variety of activated cyclopropanes were found to react with 1-methylimidazolium thiocyanate under mild metal-free conditions via unusual nitrogen attack of the ambident thiocyanate ion on the electrophilic center of the three-membered ring affording pyrrolidine-2-thiones bearing donor and acceptor substituents at the C(5) and C(3) atoms, respectively, in a single time-efficient step. The ability of 1-methylimidazolium thiocyanate to serve as a triplex reagent was exemplarily illustrated by (4+2)-annulation with 1-acyl-2-(2-hydroxyphenyl)cyclopropane, epoxide ring-opening and other organic transformations.

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.

DBU-promoted ring-opening reactions of multi-substituted donor–acceptor cyclopropanes: access to functionalized chalcones with a quaternary carbon group

A strategy to synthesize highly stereoselective chalcones with alkylcyanoacetate subunits via DBU-promoted ring-opening reactions of multi-substituted D–A cyclopropanes has been developed without the requirement of a transition metal catalyst and extra solvent.

Chameleon-Like Activating Nature of the Spirooxindole Group in Donor-Acceptor Cyclopropanes

The concept of a chameleon activating group is considered in the context of donor-acceptor cyclopropane chemistry. When spiro-conjugated with cyclopropane, oxindole can act as an acceptor or a donor depending on the electronic nature of vicinal substituents. This dichotomy is reflected in the alteration of chemoselectivity of spiro[oxindole-1,3'-cyclopropane] ring opening with nucleophiles.

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

Donor-acceptor (D-A) cyclobutanes with two geminal ester groups as acceptors are reacted with electron-rich arenes as nucleophiles to afford ring-opened products. AlCl₃ mediates this Friedel-Crafts-type reaction. A variety of donors and electron-rich arenes are used. Nucleophilic thiols and selenols also trigger this ring-opening reaction. Furthermore, a comparison of various physical parameters has been carried out for several D-A cyclobutanes.

Convenient Synthesis of Functionalized Cyclopropa[c]coumarin-1a-carboxylates

A simple method has been developed for the synthesis of cyclopropa[c]coumarins, which belong to the donor-acceptor cyclopropane family and, therefore, are promising substrates for the preparation of chromene-based fine chemicals. The method, based on the acetic acid-induced intramolecular transesterification of 2-arylcyclopropane-1,1-dicarboxylates, was found to be efficient for substrates containing hydroxy group directly attached to the aromatic ring.

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.