Synthesis of 2,5-Disubstituted 3,6-Diamino-1,4-benzoquinones

Article abstract of DOI:10.1021/jo00134a018

A general synthetic approach to a wide variety of 2,5-disubstituted 3,6-diamino-1,4-benzoquinones was developed.Bromanil was diaminated with ammonia, and adjacent NH2 and OH groups were protected as benzoxazoles by treatment with a carboxylic acid chloride followed by a polyphosphate ester cyclization-dehydration.The resulting 2,5-dibromobenzobis(oxazoles) were monolithiated by halogen-metal exchange with n-butyllithium and then reacted with a variety of electrophiles.The remaining bromide was replaced in a similar fashion.Alternatively the second bromide was replaced by reaction with ? allylnickel halide complexes.The benzoxazole protecting group could be hydrolyzed with zinc(II) chloride/HCl-aqueous ethanol under an inert atmostphere.Air oxidation of the resulting hydroquinone under neutral conditions gave the desired 2,5-disubstituted 3,6-diamino-1,4-benzoquinone in good to excellent overall yield.This method was used to synthesize precursors to the basic ring system of the mitomycin antineoplastic antibiotics.Acid hydrolysis of the benzoxazole protecting group under oxidizing conditions resulted in the production of 2,5-disubstituted 3,6-dihydroxy-1,4-benzoquinone.Methylation followed by reaction with ammonia gave the desired diaminoquinone.