Synthesis of dihydropyrano[3,2-e]indoles as rotationally restricted phenolic analogs of 5-hydroxyindole—thermal Claisen approach versus gold catalysis

The Claisen rearrangement/cyclization of 5-propargyloxyindoles (2) to afford dihydropyrano[3,2-e]indoles (3) as direct precursors to tetrahydropyrano[3,2-e]indoles (1, a rotationally restricted phenolic analog of 5-hydroxyindole) was examined using either refluxing bromobenzene (156 °C) or Au+1 catalysis in refluxing dioxane (101 °C). This transformation was best effected using Au+1 catalysis (i.e., tris[triphenylphosphinegold(I)] oxonium tetrafluoroborate) because this method required a lower reaction temperature and gave better yields when compared to the simple thermal reaction conditions (156 °C).