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

Abstract

We have developed an efficient method for the synthesis of the pentaleno[2,1-<i>b</i>:5,4-<i>b</i>′]diindole and pentaleno[2,1-<i>b</i>:5,6-<i>b</i>′]diindole skeletons via twofold Fischer indole cyclization. In this new procedure, <i>cis</i>-bicyclo[3.3.0]octane-3,7-dione on treatment with 1-methyl-1-phenylhydrazine in the presence of a low-melting mixture of <sc>l</sc>-(+)-tartaric acid and <i>N</i>,<i>N</i>′-dimethylurea at 70 °C undergoes a twofold Fischer indole cyclization to deliver the unusual <i>C_s </i>-symmetrical product pentaleno[2,1-<i>b</i>:5,6-<i>b</i>′]diindole, along with the expected <i>C₂ </i>-symmetrical pentaleno[2,1-<i>b</i>:5,4-<i>b</i>′]diindole product.