A compound having the formula 4, wherein R 1 and R 2, which are the same or different, are independently selected from hydroxy-protecting groups of the following Groups (i) to (iii): (i) R 1 and R 2, independent of each other, are selected from alkyl, halo-alkyl, benzyl, t-BuPh 2 Si, (i-Pr) 3 Si or Et 3 Si; or (ii) R 1 and R 2, independent of each other, are selected from t-BuMe 2 Si, t-BuPh 2 Si, benzoyl, tetrahydropyran-2-yl, benzoyl in which the phenyl ring bears substituent(s), and biphenyl-4-formyl, provided that R 1 and R 2 are not both t-BuMe 2 Si; or (iii) R 1 and R 2 together with the five-member carbocycle to which they are attached form one of the following fused ring systems: wherein R 3 is hydrogen atom, C 1-6 alkyl, phenyl or substituted phenyl in which the substituent on said phenyl is preferably selected from methoxy, ethoxy, halo, phenyl and nitro; R 4 and R 5, which are the same or different, are independently selected from C 1-6 alkyl or aryl, preferably tert-butyl or phenyl; wherein the symbol * represents the attachment point through which said fused ring is attached to the remaining moiety of the molecule, with the proviso that the compound has not the following formula: in which Ph represents the phenyl group. R 1 and R 2, which are the same or different, are independently selected from hydroxy-protecting groups of the following Groups (i) to (iii): (i) R 1 and R 2, independent of each other, are selected from alkyl, halo-alkyl, benzyl, t-BuPh 2 Si, (i-Pr) 3 Si or Et 3 Si; or (ii) R 1 and R 2, independent of each other, are selected from t-BuMe 2 Si, t-BuPh 2 Si, benzoyl, tetrahydropyran-2-yl, benzoyl in which the phenyl ring bears substituent(s), and biphenyl-4-formyl, provided that R 1 and R 2 are not both t-BuMe 2 Si; or (iii) R 1 and R 2 together with the five-member carbocycle to which they are attached form one of the following fused ring systems: wherein R 3 is hydrogen atom, C 1-6 alkyl, phenyl or substituted phenyl in which the substituent on said phenyl is preferably selected from methoxy, ethoxy, halo, phenyl and nitro; R 4 and R 5, which are the same or different, are independently selected from C 1-6 alkyl or aryl, preferably tert-butyl or phenyl; wherein the symbol * represents the attachment point through which said fused ring is attached to the remaining moiety of the molecule, with the proviso that the compound has not the following formula: in which Ph represents the phenyl group. (i) R 1 and R 2, independent of each other, are selected from alkyl, halo-alkyl, benzyl, t-BuPh 2 Si, (i-Pr) 3 Si or Et 3 Si; or (ii) R 1 and R 2, independent of each other, are selected from t-BuMe 2 Si, t-BuPh 2 Si, benzoyl, tetrahydropyran-2-yl, benzoyl in which the phenyl ring bears substituent(s), and biphenyl-4-formyl, provided that R 1 and R 2 are not both t-BuMe 2 Si; or (iii) R 1 and R 2 together with the five-member carbocycle to which they are attached form one of the following fused ring systems: wherein R 3 is hydrogen atom, C 1-6 alkyl, phenyl or substituted phenyl in which the substituent on said phenyl is preferably selected from methoxy, ethoxy, halo, phenyl and nitro; R 4 and R 5, which are the same or different, are independently selected from C 1-6 alkyl or aryl, preferably tert-butyl or phenyl; wherein the symbol * represents the attachment point through which said fused ring is attached to the remaining moiety of the molecule, with the proviso that the compound has not the following formula: in which Ph represents the phenyl group.