Studies towards the synthesis of diazonamide A. Unexpected formation of a 3,4-bridged indole

Abstract

Model studies towards the synthesis of the cytotoxic marine natural product diazonamide A are described. Three model 3-phenylbenzo[b]furan derivatives 5, 8 and 10 were prepared using rhodium(II) catalysed decomposition of the diazophenylacetate 3, Claisen rearrangement of the ether 7, and a classical intramolecular Friedel–Crafts reaction as key steps. Only the aromatic benzofuran system proved satisfactory in palladium coupling reactions; diazoacetyl(benzofuranyl)indole 18 was prepared by Suzuki coupling of (benzofuran-7-yl)boronic acid 11 with 4-bromoindole 14 to give 17, followed by diazo-transfer. Rhodium(II) catalysed decomposition of 18 in acetonitrile resulted in the formation of the 3,4-bridged indole 19 rather than the desired oxazole 20.