Catalytic hydrogenation of 5‐substituted 3‐(2‐aminobenzyl)‐1,2,4‐oxadiazoles 7 give 2‐(acylamino)indoles 10. Treatment of 2‐(benzoylamino)indoles 10a–c with acid leads to the novel 6‐phenyl‐8H‐pyrimido[1,6‐a: 4,5‐b']diindoles 19a–c. A route for the new ring formation is proposed. Catalytic hydrogenation of 19a (Raney nickel, 70°C, atmospheric pressure) saturates the 6‐phenyl ring but leaves the pentacyclic ring system intact. The structure of 19a is confirmed by an X‐ray crystallographic analysis.
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