Stereospecific synthesis of phosphotriesters by using new condensing reagents

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Deoxyadenylyl-(3'-5')-phosphoro-o-chlorophenyl deoxyadenosine having the Sp configuration was synthesized stereoselectively by condensing N,5'-bis-dimethoxytrityldeoxyadenosine 3'-(o-chlorophenyl)phosphate with N,3'-bis-dimethoxytrityldeoxyadenosine using 1-2,4,6-triisopropylbenzene-5-(pyridin-2-yl)tetrazole as the condensing reagent followed by dedimethoxytritylation. The absolute configuration of this diastereomer was determined spectroscopically by comparison of two corresponding diastereomers which were prepared by using a conventional condensing reagent and isolated by chromatography on silica gel. Their nuclear Overhauser effect in proton magnetic resonances and the circular dichroism of these two diastereoisomers could only be explained if the product obtained by the stereoselective synthesis of this type exist in solution as an equilibrium of folded and extended forms.