4-Acyl-5-alkyl-3-aminophenols

Sulfur Bake. The yellow, orange, and brown sulfur dyes belong to this group. The dyes are usually made from aromatic amines, diamines, and their acyl and nuclear alkyl derivatives. These may be used in admixture with nitroanilines and nitrophenols or aminophenols to give the desired shade. The color formed is said to be the result of the formation of the thiazole chromophore, evident in dye structure (1). 

The aminophenols are chemically reactive, undergoing reactions involving both the aromatic amino group and the phenolic hydroxyl moiety, as well as substitution on the benzene ring. Oxidation leads to the formation of highly colored polymeric quinoid structures. 2-Aminophenol undergoes a variety of cyclization reactions. Important reactions include alkylation, acylation, diazonium salt formation, cyclization reactions, condensation reactions, and reactions of the benzene ring. 

Due to the attractivity of this method several groups have developed onium salt supported versions of classical reactions. For example, starting from hydroxyl derived imidazolium salts, formation of supported acrylates with acryloyl chloride followed by reaction with diene in refluxing toluene afforded Diels Alder adduct in good yields (>65%). After saponification, products are isolated without further purification [127], Alternatively, starting from carboxylic acid derived imidazolium salts, acyl chloride formation with thionyl chloride in acetonitrile, followed by reaction with 4-aminophenol led to supported N-arylamide. Williamson alkylation using NaOH as a base and subsequent cleavage from the onium salt support under acidic condition (HCI/I I2()/ AcOH) allowed for isolation of various alkoxy substituted anilines with >98% purity... 

The benzimidazole was prepared from 8. Acylation of the amino group and nitration of the resulting trifluoroacetamide (9) gave a mixture of regioi-somers 10 and 11. The nitro group of the 10 was reduced and the resulting o-aminophenol was converted to benzimidazole 12 upon reaction with acetyl chloride. N-Alkylation of the amide provided 13, which ultimately played the role of 6 in the synthetic plan. 

---