This invention relates to a compound comprising a structure of formula (I): cyclic disulfide moiety-phosphorus coupling group (I). The cyclic disulfide moiety has the structure of (C-I), (C-II), or (C-III). This invention also relates to an oligonucleotide comprising one or more compounds that comprise the structure of formula (I), wherein at least one phosphorus coupling group contains a nucleoside or oligonucleotide. The invention also relates to a pharmaceutical composition comprising the oligonucleotide described herein and a method of reducing or inhibiting the expression of a target gene by administering to the subject a therapeutically effective amount of the oligonucleotide described herein.