A new synthetic route to the tetracyclic framework of Strychnos alkaloids via intramolecular aldol reaction

By treatment of 2,3,4,9-tetrahydrospiro[1H-carbazole-1,2'-[1,3]dithiolan]-4(9H)- one (1) with ethanolamine, followed by reduction of the corresponding imine (6) with NaBH4 to the amine (7) and benzoylation, N-benzoyl-N-(2-hydroxyethyl)-{2,3,4,9-tetrahydrospiro-[1H-carbazole-1,2'-(1,3)dithiolan]-4-yl}amine (8) is formed, which can be deprotected to 4-[Benzoyl-(2-hydroxyethyl)amino]-2,3,4,9-tetrahydro-1H-carbazol-1-one (9). Oxidation of the primary hydroxyl group yields [benzoyl-(1-oxo-2,3,4,9-tetrahydrocarbazol-1-one-4-yl)amino]acetaldehyde (10), a key-intermediate for the cyclization to 2-benzoyl-4-hydroxy-1,2,3,4,5,7-hexahydro-1, 5c-methanoazocino[4,3-b]indol-6-one (11), which represents the tetracyclic skeleton of Strychnos-type alkaloids.