AbstractThis paper demonstrates that some azolium triflates, such as N‐phenylimidazolium triflate, benzimidazolium triflate and N‐methylbenzimidazolium triflate, are more useful than 1H‐tetrazole as promoters for the stereoselective condensation of a 5′‐O‐free nucleoside and a stereochemically pure 5′‐O‐(p,p′‐dimethoxytrityl)‐3′‐O‐{(4R)‐1H,3H‐pyrrolo[1,2‐c]‐1,3,2‐oxazaphospholidin}‐2‐yl 2′‐deoxyribonucleoside (Rc‐1) or 5′‐O‐(p,p′‐dimethoxytrityl)‐3′‐O‐{(4S)‐1H,3H‐pyrrolo[1,2‐c]‐1,3,2‐oxazaphospholidin}‐2‐yl 2′‐deoxyribonucleoside (Sc‐2) (Agrawal strategy). The azolium triflates allowed the stereoselective formation of an internucleotide phosphorothioate bond via the above‐described condensation using a stereochemically pure phosphoramidite, followed by sulfurization using bis[3‐triethoxysilylpropyl]tetrasulfide. The highest diastereoexcess values of the products in the synthesis of dideoxyribonucleoside phosphorothioates using a suitable azolium triflate such as benzimidazolium triflate, N‐methylbenzimidazolium triflate or N‐phenylimidazolium triflate were 90–96 % in solution phase or 80–88 % in solid phase; these values were higher than those obtained in the synthesis using 1H‐tetrazole as a promoter for the condensation of a nucleoside phosphoramidite and a nucleoside. This paper also describes that studies on the absolute configurations of stereogenic phosphorus atoms in the phosphoramidites Rc‐1 and Sc‐2 by meansof two different existing methods, i. e., the Beaucage method, gave contrary conclusions, and thus the configurations should be determined by an absolutely reliable method, such as X‐ray analysis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)