Oxonium/Sulfonium Ylide Rearrangements to Access Chromenes and Thiochromenes: Concerted or Stepwise Pathway?

Dirhodium(II)-catalyzed regio-divergent intramolecular rearrangement of oxonium and sulfonium ylides, generated from diazo arylidene succinimides (DAS), has been achieved at room temperature. Allyloxy-substituted DAS derivatives were strategically designed and synthesized to explore their reactivity under metal catalysis. The transformation is proposed to proceed via oxonium ylide intermediate, followed by [1,2]- and [1,4]-shifts, leading to the chromene framework. This strategy was further extended to _{thioallyl-substituted DASs} to access thiochromenes under mild reaction conditions. For the first time, intramolecular rearrangement has been explored on DAS derivatives to synthesize bioactive chromene and thiochromene scaffolds.