A strategy for the stereoselective preparation of thymidine phosphorothioates with the (R) or the (S) configuration at the stereogenic phosphorus atom and their application to the synthesis of oligodeoxyribonucleotides with stereochemically pure phosphate/phosphorothioate chimeric backbones

This paper describes a new method for the highly stereoselective preparation of dithymidine phosphorothioates (TpsT) with the (R) or the (S) configuration at the stereogenic phosphorus atom [(Rp)- or (Sp)-TpsT, respectively], together with the synthesis of oligodeoxyribonucleotides with stereochemically pure phosphate/phosphorothioate mixed backbones through the use of (Rp)- or (Sp)-TpsT as building blocks. Stereochemically pure (Rp)- or (Sp)-TpsT was produced through a five-step process: 1) 1 H-tetrazole-promoted thermodynamic equilibration of a diastereomeric mixture of allyl (Rp)- and (Sp)-thymidine 3,5'-cyclic phosphate to give the (Sp) isomer as the major component, 2) stereospecific sulfurization of the allyl (Sp)-thymidine 3',5'-cyclic phosphate to afford an allyl (Rp)-thymidine T,5'-cyclic phosphorothioate, 3) regioselective and stereoselective methanolysis of the allyl (Rp)-thymidine 3',5'-cyclic phosphorothioate to provide an allyl methyl (Rp)-thymidine T-phosphorothioate as the main product, 4) stereospecific removal of the methyl group from the phosphorothioate moiety to give an allyl (Rp)-5'-O-dimethoxytritylthymidine X-phosphorothioate diester, or stereospecific removal of the allyl group from the phosphate moiety to form a methyl (Sp)-5'-O-dimethoxytritylthyniidine 3'-phosphorothioate diester, and 5) stereospecific condensation of these (Rp)- and (Sp)-diesters with a 5'-O-free thymidine in a Mitsunobu reaction to produce the allyl (3'-5')-linked (Sp)dithymidine phosphorothioate and the methyl (3'-5')-linked (Rp)-dithymidine phosphorothioate, respectively. The resulting (Rp)- and (Sp)-dithymidine phosphorothioates were converted into their 3'-phosphoramidites. Oligodeoxyribonucleotides with stereochemically pure phosphate/phosphorothioate-mixed backbones were then synthesized by use of these phosphoramidites as building blocks. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006.