A new activating mode of donor–acceptor cyclopropane: the tug-of-war between strain and aromaticity, transient generation of quinone methides and their reactions with C-nucleophiles

Here, we present a new approach for activation of donor-acceptor cyclopropane systems in ring-opening reactions, which does not require the use of Lewis or Brønsted acid as a catalyst. Under treatment with a base, donor-acceptor cyclopropanes containing a phenolic group as the donor undergo deprotonation and fast isomerisation to the corresponding quinone methides. This innovative strategy was applied to realise [4+1] annulation of 2-(2-hydroxyaryl)-substituted DA cyclopropanes with sulfur ylides, affording functionalised dihydrobenzofurans. A plausible mechanism of this process has been proposed based on theoretical calculations. Additionally, the generated ortho- and para-quinone methides as well as their aza-analogues can be trapped by deprotonated CH-acids as nucleophiles providing a simple path toward acyclic polyfunctional compounds as promising building blocks for the synthesis of more complexed and biologically interesting cyclic structures.