A compound of formula In which - X represents oxygen, - B is thymine, - R 1 and R 2 are identical and represent a straight or branched C 2 -C 30 hydrocarbon chain, which is saturated or partially unsaturated, - R 3 is: a hydroxy group; a NR 4 R 5 R 6 group, in which R 4, R 5 and R 6, identical or different, represent a hydrogen atom, or a C 1 - C 6 straight or branched alkyl chain; a phosphate or phosphonate group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a sulfonate group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a trialkylamino group; a trialkylphoshonium group which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a silyl group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a phosphoramidite group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group, or else-O-C -(CH 2) q -C -O [(CH 2) 2 -O] r -H in which q = 2 and r is an integer from 4 to 30,and - R 7 is hydrogen, a straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group, a trifluoralkyl, a halogen or a C 1 -C 6 alkoxy group. - X represents oxygen, - B is thymine, - R 1 and R 2 are identical and represent a straight or branched C 2 -C 30 hydrocarbon chain, which is saturated or partially unsaturated, - R 3 is: a hydroxy group; a NR 4 R 5 R 6 group, in which R 4, R 5 and R 6, identical or different, represent a hydrogen atom, or a C 1 - C 6 straight or branched alkyl chain; a phosphate or phosphonate group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a sulfonate group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a trialkylamino group; a trialkylphoshonium group which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a silyl group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a phosphoramidite group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group, or else-O-C -(CH 2) q -C -O [(CH 2) 2 -O] r -H in which q = 2 and r is an integer from 4 to 30,and a hydroxy group; a NR 4 R 5 R 6 group, in which R 4, R 5 and R 6, identical or different, represent a hydrogen atom, or a C 1 - C 6 straight or branched alkyl chain; a phosphate or phosphonate group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a sulfonate group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a trialkylamino group; a trialkylphoshonium group which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a silyl group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group; a phosphoramidite group, which is unsubstituted or substituted by at least one straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group, or else-O-C -(CH 2) q -C -O [(CH 2) 2 -O] r -H in which q = 2 and r is an integer from 4 to 30,and - R 7 is hydrogen, a straight or branched, unsubstituted or substituted, C 1 -C 6 alkyl group, a trifluoralkyl, a halogen or a C 1 -C 6 alkoxy group.