Compounds useful in the treatment of patients suffering from senile dementia, Alzheimer's disease or memory defects, of the formula ##STR1## in which R represents hydrogen or a linear, branched or cyclic alkyl, alkenyl, or alkynyl, containing up to 8 carbon atoms, or the radical ##STR2## in which R.sub.1 represents a linear or branched, alkyl, alkenyl or alkynyl, containing up to 8 carbon atoms, a linear or branched, saturated or unsaturated, alkoxy containing up to 8 carbon atoms, or aryl containing up to 14 carbon atoms, or the radical ##STR3## in which R.sub.2 represents a linear or branched alkyl, alkenyl or alkynyl, containing up to 8 carbon atoms, aryl containing up to 14 carbon atoms, possibly substituted, or aralkyl containing up to 18 carbon atoms, possibly substituted, and R' represents a linear, branched or cyclic alkyl, alkenyl or alkynyl, containing up to 8 carbon atoms, as well as their addition salts with organic or mineral acids.