In the Hoesch reaction, an alkyl cyanide reacts with a polyhydric benzene ring, and forms an imine salt by attacking the activated benzene ring. Most acid derivatives (and even alkyl cyanide itself with phenol) results in attack at the alcohol's oxygen, which is more nucleophilic than the benzene ring. In this case however, the major product doesn't form by attack on the alcohol.
Where lies the difference? If it were in the fact that benzene is more activated because of the many alcohol groups in the polyhydric compound, wouldn't it mean that even another acid derivative, like acid chloride, would react at the benzene instead and not at the O (thus carrying out a Friedel-Crafts acylation)?