2-Thio-1,3,2-dithiaphospholane, 1, and 2-thio-1,3,2-oxathiaphospholane, 2, ring systems are useful as synthetic synthons for the formation of nucleoside 5′-O-Pα-thioor -dithiophosphate analogs or oligonucleotides bearing phosphorothioate or dithioate linkages. Nucleoside 5′-O-Pα-(thio)triphosphate analogs, 2 have gained wide application in biochemistry and cell biology. These compounds have been used also for the enzymatic synthesis of DNA and RNA fragments possessing chiral phosphorothioate internucleotide bonds. The related phosphorodithioate analogs have been used as lubricants and fuel additives. During the last decade, oligodeoxynucleotide analogs containing phophorodithioate linkages (dithioate DNA) have received increasing attention for biochemical and therapeutic applications as well. This is because dithioate DNA/RNA is achiral, isoelectronic with respect to normal DNA, and nuclease resistant. Several methods for the synthesis of these analogs have been developed. Recently we have reported the synthesis and application of nucleoside-5′-O-Pα-thio/dithio-triphosphate analogs as inhibitors of nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1), for the potential treatment of CPPD disease which accompanies osteoarthritis. Importantly, we found that nucleoside 5′-O-Pα,α-dithio-triphosphate analogs are reasonably stable under acidic and basic, and under oxidizing conditions. The synthetic usefulness of ring systems 1 and 2 for the formation of a variety of research tools and therapeutic agents based on nucleoside 5′-O-Pα-thioor -dithio-or -trithiophosphate ester, has attracted our attention with a view to exploring the chemoselectivity of the ring opening of both 2-thio-1,3,2-dithiaphospholane and 2-thio-1,3,2-oxathiaphospholane rings by various nucleophiles. Establishing the chemoselectivity rules is required for learning the scope of the reaction and then for application of the rules for synthesizing useful nucleotide-conjugates towards various uses. Here we report the investigation of the reactivity and chemo-selectivity of uridine-5'-O-