Bemerkungen zur Synthese von 3‐Aminotoluol‐5‐sulfonsäure und 2‐Aminotoluol‐3‐sulfonsäure

Some comments on the synthesis of 3‐aminotoluene‐5‐sulfonic acid and 2‐aminotoluene‐3‐sulfonic acid. Sulfonation of 3‐nitrotoluene ( 5 ) yields predominantly the unsymetrical isomer 5‐nitrotoluene‐2‐sulfonic acid ( 7 ), and lesser amounts of 5‐nitrotoluene‐3‐sulfonic acid ( 6 ), previously reported as the major product. The desired 5‐aminotoluene‐3‐sulfonic acid ( 3 ) was synthesized in preparative amounts from 6‐aminotoluene‐3‐sulfonic acid (4) via the following sequence of reactions: diazotation and Sandmeyer replacement of 6‐chlorotoluene‐3‐sulfonic acid ( 13 ), nitration of the sulfonyl chloride 14 under suitable conditions to give isomer free 6‐chloro‐5‐nitrotoluene‐3‐sulfonyl chloride ( 15 ), hydrolysis to the sulfonic acid 16 and finally, simultaneous hydrogenolysis and reduction to 3 . The isomeric 7 was unequivocally prepared from 2‐amino‐5‐nitrotoluene ( 9 ) via two routes: (1) diazotation, Sandmeyer thiocyanatation to 5‐nitro‐2‐thiocyanatotoluene ( 10 ), Na 2 S reduction to the di(2‐methyl‐4‐nitro‐phenyl)‐disulfide ( 11 ), treatment with nitric acid and chlorine to give 5‐nitrotoluene‐2‐sulfonyl chloride ( 12 ) and finally alkaline hydrolysis to 7 ; (2) Meerwein's SO 2 treatment of the diazonium salt derived from 9 leads directly to 12 and thence to 7 . 2‐Aminotoluene‐3‐sulfonic acid ( 1 ) was prepared from the key intermediate 3‐amino‐2‐nitrotoluene ( 18 ) via the same two routes used to prepare 7 from 9 . Both reaction sequences provided 2‐nitrotoluene‐3‐sulfonly chloride, the hydrolysis product of which was reduced to 1 . Intermediate 18 was prepared in the following four steps from m ‐toluic acid ( 19 ): nitration to the 2‐nitroderivative ( 20 ), whose acid chloride ( 21 ) was converted to 2‐nitro‐ m ‐toluamide ( 22 ), and Hoffmann rearrangement to 18 .