Synthesis of 4‐Aminopyrimidines from 1,2,4‐Oxadiazoles, V[1]. Reductive Formation of 2‐(Benzoylamino)indole from 3‐(2‐Aminobenzyl)‐5‐phenyl‐1,2,4‐oxadiazole and Its Transformation to 6‐Phenyl‐8H‐pyrimido‐[1,6‐a: 4,5‐b']diindole, a New Heterocyclic Ring System