Novel regioselective synthesis of decahydrobenzocarbazoles using rhodium generated carbonyl ylides with indoles

Intermolecular cycloadditions of five-membered cyclic carbonyl ylides 3 with indole, N-methylindole and N-benzylindole afforded decahydrobenzo[c]carbazole 4a–f or decahydrocyclopenta[c]carbazole 4g–h derivatives with high regioselectivity. With N-benzoylindole and N-sulphonylindole, decahydrobenzo[c]carbazoles 4i,j and the regioisomer decahydrobenzo[a]carbazoles 5i,j are isolated. The electron withdrawing substituent reduces both regioselectivity and reactivity of the cycloadditions. This methodology generated oxa-bridged (unnatural) decahydrobenzocarbazole derivatives with complete control of four stereocenters.