Nitration of Indoles. III. Polynitration of 2-Alky lindoles

Nitration in concentrated nitric acid of 2-methylindole and 1,2-dimethylindole gives the corresponding 3,6-dinitroindoles 5 and 16; 5 was also obtained from 2-methylindole-3-carboxaldehyde and 2-methyl-6-nitroindole (3). 2-Methyl-3-nitroindole (1) and 2-methyl-3H-indol-3-one oxime (2) gives both 5 and 3,4-dinitro-2-methylindole (6); 6 was also obtained from 2-methyl-4-nitroindole (4). Further nitration of 5 and 16 or their precursors gives the 3,4,6-trinitroindoles 7 and 17; 3-acetyl-2-methyl-4-nitroindole (9) and 4 also gave 7. Nitration in concentrated nitric acid of 2-methyl-5-nitroindole (20) and l,2-dimethyl-5-nitroindole (27) gives the corresponding 3,5-dinitroindoles 22 and 28. Further nitration of 22 and 28 (or 27) gives the 3,5,6-trinitroindoles 24 and 31; 24 was also obtained from the 1-acetyl derivative (21) or 20. Dimethylation of 22 gave 3,5-dinitro-2-ethyl-1-methylindole (36), as proved by preparation from 2-ethylindole via the 5-nitro (34) and 3,5-dinitro (35) derivatives and methylation to 36. Nitration in concentrated sulfuric acid of 1,2,3-trimethylmdole and 2,3,3-trimethyl-3H-indole gives the corresponding 5-nitro derivatives 41 and 43. The differing mechanisms of nitration of indoles in sulfuric acid and in nitric or acetic acids are discussed, and a set of orientation rules for nitration of 2-alkylindoles is presented.