Novel ribonucleoside 3′,5′-cyclic monophosphorodithioates were synthesized by a combination of the phosphoramidite and phosphotriester methods. These include uridine 3′,5′-cyclic monophosphorodithioate (cUMPS2), cytidine 3′,5′-cyclic monophosphorodithioate (cCMPS2), and guanosine 3′,5′-cyclic monophosphorodithioate (cGMPS2). In addition, 2,6-diaminopurine ribonucleoside 3′,5′-cyclic monophosphorodithioate (cDAPMPS2) was synthesized via the O6-amination reaction of the O6-sulfonylated guanosine derivative.