A benzonaphthalene compound of the formula ##STR11## wherein R.sub.1 represents (i) ##STR12## or (ii) --CH.sub.2 OH, R.sub.6 represents ##STR13## wherein r' and r" represent hydrogen, lower alkyl, mono or polyhydroxyalkyl, aryl or a residue of an amino sugar selected from the group consisting of glucosamine, galactosamine and mannosamine,
- R.sub.2 represents hydrogen, branched or straight chain alkyl having 1-15 carbon atoms, alkoxy having 1-4 carbon atoms or a cycloaliphatic radical,
- R.sub.3 represents hydrogen, hydroxy, branched or straight chain alkyl having 1-4 carbon atoms, alkoxy having 1-10 carbon atoms, a cycloaliphatic radical selected from the group consisting of 1-methyl cyclohexyl and 1-adamantyl, a thiocycloaliphatic radical, or --O--Si(CH.sub.3).sub.2 --R.sub.8 wherein R.sub.8 represents linear or branched lower alkyl, and
- R.sub.4 and R.sub.5 each independently represent hydrogen, lower alkyl, hydroxy or lower acyloxy,
- or a salt thereof.