Synthesis of the Core Tetracycle of the Spirobiflavonoid Daphnodorin C

The first synthesis of the spirobi[benzofuran]dione core structure of the spirobiflavonoid daphnodorin C is described. The key step is the oxidative ring contraction of electron‐rich benzofuro[3,2‐ b ]chromenones by treatment with dimethyldioxirane, which afforded novel 2,3′‐spirobi[benzofuran]diones, rather than the isomeric spiroketals. The conversion probably proceeds via epoxidation, followed by Meinwald‐type rearrangement with acyl migration. The starting materials were obtained by the condensation of α ‐brominated o ‐fluoroacetophenone and methyl salicylate derivatives. The structures of several of the novel spiro compounds were confirmed by X‐ray analysis.