Figure 3.

(A) Separation of PS dinucleotide diastereomers (III^Rp* and III^Sp** of scheme 1) by thin layer chromatography (TLC). The R_P and S_P diastereomers of the dimers 5′-asa, 5′-Afsa, 5′-usu 5′-asUf, 5′-usUf, and 5′-Ufsu except 5′-gsa were well separated on TLC and were isolated by flash column chromatography. The R_P and S_P isomers of 5′-gsa were separated by HPLC. Lowercase u, a, and g indicate 2′-OMe nucleoside, lowercase s indicates PS linkage, and Uf and Af indicate 2′-F U and A nucleosides, respectively. (B) The fully protected PS chirally pure Rp and Sp diastereomer phosphoramidites synthesized and isolated for incorporation into oligonucleotide under solid phase synthesis conditions to obtain chirally pure PS backbone containing siRNAs. From left to right and top to bottom, the compounds are 5′-usu (5a ^Rp, 5b ^Sp), 5′-usUf (11a ^Rp, 11b ^Sp), 5′-Ufsu (17a ^Rp, 17b ^Sp), 5′-asUf (23a ^Rp, 23b ^Sp), 5′-asa (29a ^Rp, 29b ^Sp), 5′-Afsa (35a ^Rp, 35b ^Sp) and 5′-gsa (41a ^Sp, 41 ^Rp).