"a) reaction of the compound of formula (III), or a protected form thereof, with a 4-thio sugar compound of formula (IV). ##STR5## wherein Z3 and Z5 are as defined above and L is a leaving group, for example halogen, e.g. chloro, acyloxy (e.g. C1-6 alkanoyloxy such as acetoxy), or S-benzyl." . . . .