c) (i) reacting a compound of formula (II) with a compound of formula (V): wherein R2 is a suitable protecting group, for example benzyl or benzyloxycarbonyl, to provide a compound of formula (VI): (ii) removing protecting group R2 to provide a compound of formula (IV); and (iii) reacting the compound of formula (IV) with a compound of formula (II) to provide a compound of formula (I), wherein X,