A compound of formula (I) where B1 is a radical of a nucleoside base, R1 is hydrogen or a hydroxy-protecting group, R2 is hydrogen, hydroxy or a 2' nucleoside-modifying atom or group, R3 is an unsubstituted or substituted C¿1? to C10 alkyl, C2 to C10 alkenyl, C4 to C10 cycloalkylalkyl, C6 to C10 aryl or C7 to C13 aralkyl group, and Z is halogen or a group of formula (II) where R?4 and R5¿ are each independently an unsubstituted or substituted C¿1? to C10 alkyl, C2 to C10 alkenyl, C4 to C10 cycloalkylalkyl, C6 to C10 aryl or C7 to C13 aralkyl group, or R?4¿ is said group and R5 is hydrogen, or R?4 and R5¿ together with the nitrogen atom to which they are attached denote a five- to thirteen-membered heterocyclic ring, or Z is a group of formula (III): Nuc-¿O?-, where Nuc is the residue of a natural or synthetic nucleoside or oligonucleotide after removal of a 5'-hydroxyl group therefrom attached through a 5' methylene thereof to the indicated oxygen atom.