³¹P NMR Study of the Mechanism of Activation and Coupling Reactions in the Synthesis of Oligodeoxyribonucleotides by the Phosphotriester Method

The phosphotriester method provides a rapid and convenient procedure for synthesizing oligonucleotides. The mechanism has been revealed and intermediates have been identified by ³¹P NMR methodology. It was found that reaction of a 5^'-protected nucleoside 3^'-(p-chlorophenyl phosphate) with mesitylenesulfonyl chloride (MSC1) or 1-(mesitylyl-2-sulfonyl)-3-nitro-1,2,4-triazole (MSNT) in anhydrous pyridine yields only two products within 5 min, the sulfonic acid-phosphate mixed anhydride 2 and the (3^'-3^') symmetrical pyrophosphate tetraester 3 which can be isolated as a mixture. Reaction of 2 and 3 with 3^'-O-acetylthymidine yields the phosphotriester dimer [(MeO)₂Tr]NpTOAc. The reaction rate and yield of dimers are closely dependent on the presence of catalysts. The reaction finished within minutes when tetrazole or 3-nitro-1,2,4-triazole was used. On the contrary, the reaction completed in hours when imidazole or 1,2,4-triazole was used as catalysts. The possible mechanisms are explored and discussed in detail.