hydroxyl group in its side-chain; X2, X4 and X5 are independently of each other O, S, or NH; R2, R4 and Rs are independently of each other hydrogen or an O, S or N protecting group, provided that when Xl-Xs are oxygen and R2, R4 and Rs are hydrogen, at least one of Rl and R3 is other than hydrogen or methyl; or a pharmaceutically acceptable salt, hydrate or derivative thereof.