Nitration of Indoles. II. The Mononitration of Methylindoles

Nitration of 2-methylindole and 1,2-dimethylindole in concentrated sulfuric acid gives the corresponding 2-methyl-5-nitroindoles (Ia, 84%, and Ib, 82%). Methylation of Ia with sodamide and dimethyl sulfate in liquid ammonia gave Ib (47%). Acetylation of Ia with sodium acetate and acetic anhydride gave the 1-acetyl (V, 64%) and 3-acetyl (VI, 2%) derivatives. Chromic acid oxidation of the 1-acetyl derivative gave N-acetyl-5-nitroanthranilic acid (III, 46%), thus proving the 5-position as the position of mononitration in Ia and Ib, analogous to the corresponding nitration of 2,3-dimethylindole. Chromic acid oxidation of Ia and Ib gave the oxidative dimers IVa (19%) and IVb (31%). Catalytic hydrogenation of Ia and Ib gave the corresponding 5-amino-2-methylindoles (IIa, 61%, and IIb, 37%). Condensation of Ia with formaldehyde and benzaldehyde gave the corresponding 3,3′-methylenediindoles (XIIa, 100%, and XIIb, 58%), while the Mannich reaction of Ia with formaldehyde and dimethylamine gave 2-methyl-5-nitrogramine (XIII, 18%, and XIIa, 51%). The mechanism of formation of the various products is discussed.