Synthesis of the indeno[1,2-]indole core of janthitrem B

The tetracyclic core structure of the majority of indole diterpenoids features a trans -hydrindane moiety that is fused to an indole unit. We report here a novel synthetic route that includes a photo-Nazarov cyclization of a 3-acylindole precursor initially providing the thermodynamically preferred cis -hydrindanone. After reduction and conversion to the cyclopentadiene, dihydroxylation and hydrogenation provided the indoline. The key step generated the trans -system by stereospecific hydride shift on a dioxaphospholane under Grainger's conditions, for the first time applied to an N-heterocycle. When starting from the corresponding indole, we observed the formation of hitherto unknown methanocyclohepta[ b ]indolones. Deoxygenation of the trans -hydrindanone was achieved after conversion to the 1,3-dithiolane, followed by RANEY® Ni reduction. A novel synthetic route to a trans -fused indeno[1,2- b ]indole proceeds via photo-Nazarov cyclization of a 3-acylindole precursor and stereospecific hydride shift at an in situ -formed dioxaphospholane.