Compounds of the formula ##STR1## or a non-toxic acid addition salt thereof wherein R.sub.1 is alkyl of from 1 to 20 carbon atoms; R.sub.2 is alkyl of from 12 to 20 carbon atoms; R.sub.3 is selected from the group consisting of hydrogen and hydroxyalkyl of from 2 to 8 carbon atoms; and R.sub.4 is selected from the group consisting of hydrogen, alkyl of from 1 to 8 carbon atoms and hydroxyalkyl of from 2 to 8 carbon atoms, said compounds are useful for combating viral infections in vertebrate animals.