Dibenzofluorenones undergo reversible Friedel–Crafts acyl rearrangements in PPA at elevated temperatures. The Friedel–Crafts acyl rearrangements of 13H-dibenzo [a, i] fluoren- 13-one (DBaiF) yield both 13H-dibenzo [a, h] fluoren-13-one (DBahF) and 12H-dibenzo [b, h] fluoren-12-one (DBbhF). DBahF and DBbhF undergo reversible mutual isomerizations, and their ratio depends on the reaction conditions. The O-protonate DBahFH+ plays a ...