P-stereocontrolled synthesis of oligo(nucleoside N3'→O5' phosphoramidothioate)s - opportunities and limitations

Synthesis and hybridizing properties of P-stereodefined chimeric [PS]-{DNA:RNA} and [PS]-{DNA:(2′-OMe)-RNA} oligomers

Oxathiaphospholane derivatives of 2′-OMe-ribonucleosides and 2′-O-TBDMS-ribonucleosides (^MN-OTP and ^TN-OTP, respectively; nucleobase protected) were synthesized and separated into pure P-diastereomers. X-ray analysis showed the R_P absolute configuration of the phosphorus atom in the fast-eluting diastereomer of ^TA-OTP. The fast- and slow-eluting P-diastereomers of ^MN-OTP and ^TN-OTP were used in the solid-phase synthesis of phosphorothioate dinucleotides (^MN_PST and N_PST, respectively), which were subsequently hydrolyzed with R_P-selective phosphodiesterase svPDE and S_P-selective nuclease P1 to determine the absolute configuration of the phosphorus atoms. P-Stereodefined phosphorothioate ([PS]) 10-mer chimeric oligomers [PS]-{DNA:(2′-OMe)-RNA} and isosequential [PS]-{DNA:RNA} containing two ^MN_PS or N_PS units were synthesized. Melting experiments performed for their complexes with Watson–Crick paired DNA matrix showed that ^MN_PS or N_PS units decrease the thermal stability of the duplexes (ΔT_m = −0.5 ÷ −5.5 °C per modification) regardless of the absolute configuration of the P-atoms. When the (2′-OMe)-RNA matrix was used an increase in T_m was noted in all cases (ΔT_m = +1 ÷ +7 °C per modification). The changes in thermal stability of the duplexes formed by [PS]-chimeras with DNA and (2′-OMe)-RNA matrices do not correlate with the absolute configuration of the phosphorus atoms.

P-Diastereomers 1 were used in the stereoretentive synthesis of chimeric [PS]-{DNA:(2′-OMe)-RNA/RNA} oligomers and the stability of their duplexes did not correlate with the P-stereochemistry.