Abstract: A chain-extended oligonucleotide is synthesized by condensing a mononucleotide, oligonucleotide, mononucleoside succinate or oligonucleotide succinate in which only the terminal 5'-hydroxyl group is not blocked and the other functional groups are blocked by a plurality of low-molecular-weight protecting groups and one polysaccharide derivative protecting group, with a mononucleotide or oligonucleotide in which the functional groups other than the terminal phosphoric acid group ar blocked by low-molecular-weight protecting groups in the homogeneous system in a common solvent.A compound represented by the following formula is preferred as the compound forming the polysaccharide derivative protecting group: ##STR1## wherein C.sub.6 H.sub.7 O.sub.2 stands for an anhydrous glucose residue, R stands for ##STR2## n is a number of 10 to 2,000, x is a number of 0.4 to 0.8, and y is a number of 1.0 to 2.0.

Abstract: 2'-Modified kanamycins, including 2'-deoxykanamycin A, 2'-epikanamycin A and 2'-epikanamycin B, as new compounds are produced starting from kanamycin A by consecutive reaction steps. These new compounds are useful as antibacterial agent.