3 is selected from the group consisting of hydrogen c 1 6 alkyl and c 1 6 acyl groups r 4 and r 5 are independently selected from the group consisting of hydrogen c 1 6 alkyl bromine chlorine fluorine and iodine r 6 is selected from the group of hydrogen halogen cyano carboxy c 1 6 alkyl c 1 6 alkoxycarbonyl c 1 6 acyl c 1 6 acyloxy carbamoyl and thiocarbamoyl and x and y are independently selected from the group of hydrogen bromine chlorine fluorine iodine amino and hydroxyl groups in a particularly preferred embodiment r 2 is cytosine or an analogue or derivative thereof most preferably r 2 is cytosine n acetylcytosine or n acetyl 5 fluorocytosine in preferred embodiments r 3 is h in another preferred embodiment r 3 is c 1 4 acyl such as acetyl in preferred embodiments r 4 and r 5 are independently selected from hydrogen c 1 4 alkyl such as methyl or ethyl and halogen such as f cl i or br in particularly preferred embodiments r 4 and r 5 are hydrogen in another particularly preferred embodiment r 4 and r 5 are f in preferred embodiments r 6 is selected from hydrogen halogen carboxy and c 1 4 alkyl in particularly preferred embodiments r 6 is h f or cl and most preferably h in preferred embodiments x and y are independently selected from the group of.