Coupling of two diazotized 3-aminothieno[3,4-c]coumarins with aromatic amines

The coupling reactions of two diazotized 3-aminothieno[3,4-c]coumarins were investigated. Compounds 1a,b both react with sodium nitrite in concentrated sulphuric acid at 0-5°C to give the diazotized intermediates 2 and 3, the latter resulting from the acid-catalyzed hydrolysis of the lactonic ring of 2. The in situ formed diazonium salts react with aromatic amines (4) to afford a series of arylazothiophenes dyes in the form of their ammonium sulfate salts. With diazotized aniline, besides the normally expected phenylazothiophene 10 from the reaction with compound 1a, the corresponding product of acid hydrolysis 11 was also isolated. In at least one of the cases, the thienyl diazonium salt 2 undergoes a Gomberg-Bachmann arylation reaction with p-nitroaniline to give the 2-arylthiophene 9. The direct hydrolysis of compounds 1a,b by concentrated sulphuric acid and subsequent oxidative dimerization of the primary product of acid hydrolysis led to compound 12.