Syntheses of tetracyclic and pentacyclic condensed thiophene systems

In 1951-1953 Rabindran and Tilak51,52 introduced a new dibenzo-thiophene synthesis and extended it to tetracyclic and pentacyclic systems. This method involves a two-step process of condensing a thioarenol with an a-halo ketone or a-haloacetal to form an a-(arylthio) ketone or a-(arylthio)-acetal intermediate, which is subsequently cyclized intramolecularly. The three principal variations introduced by Tilak and co-workers are presented in Schemes 3, 5, and 6 and were summarized in I960.528... 

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