Metathesis reactions for the synthesis of ring-fused carbazoles

The metathesis reaction is used as a key step for the synthesis of the indolo[2,3-α]carbazole core of rebeccamycin 13 and the sulfur analog of furostifoline 21. Using the same methodology for the attempted synthesis of furostifoline, we unexpectedly formed tert-butyl-2a-methyl-1,2,2a,10c- tetrahydro-6H-cyclobuta[c]furo[3,2-α]carbazole-6-carboxylate 26 from the unstable diene, tert-butyl 2-(2-isopropenyl-3-furyl)-3-vinyl-1H-indole-1- carboxylate 25, presumably via a spontaneous π8 electrocyclization reaction. © 2005 American Chemical Society.