the following description sets forth a process for producing rhein , diacerhein and other diacyl derivatives thereof , which comprises the following steps : treatment of a diphenylketone ## str1 ## in which r 1 is -- or &# 39 ;, -- nr &# 39 ; r &# 34 ;, -- sr &# 39 ;, where r &# 39 ; and r &# 34 ; are h , an alkyl or aromatic group ; r 2 is h or a protective group , r 3 is -- oh or c 1 - c 4 alkyl , with an acid or superacid to give a 1 - aminoanthraquinone derivative , diazotisation , replacement of the -- nh 2 group by -- oh , optional removal of the protective group , and acylation .