STUDIES TOWARDS THE SYNTHESIS OF DIAZONAMIDE-A - UNEXPECTED FORMATIONOF A 3,4-BRIDGED INDOLE

Model studies towards the synthesis of the cytotoxic marine natural pr oduct diazonamide A are described, Three model 3-phenylbenzo[b]furan d erivatives 5, 8 and 10 were prepared using rhodium(II) catalysed decom position of the diazophenylacetate 3, Claisen rearrangement of the eth er 7, and a classical intramolecular Friedel-Crafts reaction as key st eps. Only the aromatic benzofuran system proved satisfactory in pallad ium coupling reactions; diazoacetyl(benzofuranyl)indole 18 was prepare d by Suzuki coupling of (benzofuran-7-yl)boronic acid 11 with 4-bromoi ndole 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.