Deoxyxylothymidine 3′-O-phosphorothioates: Synthesis, stereochemistry and stereocontrolled incorporation into oligothymidylates

New reagent, 5′-O-DMT-xylothymidine 3′-O-(2-thio-4,4-spiropentamethylene-1,3,2-oxathiaphospholane) and its separated diastereomers can be used for stereocontrolled incorporation of xylothymidine 3′-O-phosphorothioate of predetermined sense of P-chirality into desired oligonucleotide constructs.