Compounds of formula (I) ##STR1## (wherein A represents a ##STR2## grouping; R.sup.1 represents a hydrogen or halogen (e.g. chlorine or bromine) atom or a hydroxy or amino group, R.sup.2 represents a hydrogen atom or a C.sub.1-4 alkyl (e.g. methyl) group, X represents an oxygen or sulphur group and m and n are each 0 or 1 provided that m+n=1) and physiologically acceptable salts and esters thereof, have been found to have potent antiviral activity particularly against herpes infections.