Concise syntheses of the 1,7-dihydropyrano[2,3-g]indole ring system of the stephacidins, aspergamides and norgeamides

Three approaches towards the synthesis of the 1,7-dihydropyrano[2,3-g]indole ring system of the stephacidins, paraherquamides and norgeamides have been investigated. The first involves a tandem nitrene insertion/aromatic Claisen rearrangement. The second consists of a more conventional approach from commercially available 6-benzyloxyindole. The third approach is a revised synthesis of the 2-prenylated pyrano indole necessary for a biomimetic Diels–Alder approach towards the stephacidins, aspergamides and the norgeamides.