Anomalous Behaviour of cis-Bicyclo[3.3.0]octane-3,7-dione and Its Derivatives During Twofold Fischer Indole Cyclization Using Low-Melting Mixtures

We have developed an efficient method for the synthesis of the pentaleno[2,1-b: 5,4-b'] diindole and pentaleno[2,1-b: 5,6-b'] diindole skeletons via twofold Fischer indole cyclization. In this new procedure, cis-bicyclo[3.3.0] octane-3,7-dione on treatment with 1-methyl-1-phenylhydrazine in the presence of a low-melting mixture of L-(+)-tartaric acid and N, N'-dimethylurea at 70 degrees C undergoes a twofold Fischer indole cyclization to deliver the unusual C-s-symmetrical product pentaleno[2,1-b: 5,6-b'] diindole, along with the expected C-2-symmetrical pentaleno[2,1-b: 5,4-b'] diindole product