A compound of the formula: ##STR1## wherein B is a purin-9-yl group or a heterocyclic isostere of a purin-9-yl group; or a pyrimidin-1-yl group or a heterocyclic isostere of a pyrimidin-1-yl group; A is --CH-- or A--G taken together is --C(.dbd.O)--, --C(.dbd.CH.sub.2)--, --C(OH)(CH.sub.2 OH)-- or ##STR2## and G and D are functional groups; or a pharmaceutically acceptable salt or ester thereof.