GaCl₃‑Mediated “Inverted” Formal [3 + 2]-Cycloaddition of Donor–Acceptor Cyclopropanes to Allylic Systems

A new process of “inverted” formal [3 + 2]-cycloaddition of donor–acceptor cyclopropanes (DACs) to allylic systems to give polyfunctionalized cyclopentanes has been developed. Unlike the classical version of formal [3 + 2]-cycloaddition, a DAC acts in this process as a two-carbon synthon (1,2-zwitterionic reactivity type), while an alkene acts as a three-carbon synthon. Tetramethylethylene, allylbenzenes, homoallylbenzene, terminal and internal aliphatic alkenes, and methylenecyclobutane were successfully used as alkenes. Furthermore, in order to suppress annulation at the aromatic ring in 2-arylcyclopropane-1,1-dicarboxylates and enhance the selectivity of the reaction with the allylic system of alkenes, we suggested a new approach for managing the reactivity of DACs involving substitution at both <i>ortho</i> positions of the aromatic ring. Multinuclear NMR spectroscopy was used to study the structure of the 1,2-zwitterionic gallium complexes generated and their properties and to examine the mechanisms of the occurring reactions

Cascade Cleavage of Three-Membered Rings in the Reaction of D–A Cyclopropanes with 4,5-Diazaspiro[2.4]hept-4-enes: A Route to Highly Functionalized Pyrazolines

A new cascade process for reactions of donor–acceptor cyclopropanes (DACs) with spiro­[cyclo­propane­pyrazolines] in the presence of EtAlCl₂ or Ga halides is reported. The action of a Lewis acid results in DAC activation and addition of the carbocationic intermediate to the azocyclopropane system of the pyrazoline with opening of the second three-membered ring and addition of a halide anion from the Lewis acid. A specific feature of this process is that one activated cyclopropane ring activates another one, and depending on the component ratio, the process can involve two DAC molecules and one pyrazoline molecule or one DAC molecule and two pyrazoline molecules. The process is tolerant to various functional groups and occurs with a wide range of substrates to give polyfunctionalized structures based on a 2-pyrazoline moiety