| abstract |
1. A compound of formula (I) <CHEM> or a salt thereof, wherein R1 and R2 are each independently hydrogen, acyl or phosphate, provided that when one of R1 or R2 is phosphate, the other is hydrogen; or R1 and R2 are joined together to form a cyclic acetal group, a cyclic carbonate group or a cyclic phosphate group. Processes for preparing these compounds and their use in therapy is also described. |