General scheme of the phosphotriester condensation in the oligodeoxyribonucleotide synthesis with arylsulfonyl chlorides and arylsulfonyl azolides.

Phosphotriester condensation (RO)(R'O)PO-2 (PDE) + R"OH (RO)(R'O)(R"O)PO (PTE) in the presence of arylsulfonyl chloride (ArSO2Cl) as well as arylsulfonyl azolides proceeds in two steps as revealed by 31P NMR spectroscopy. Pyrophosphotetraester (PPTE) accumulates in over 80% yield in the first step and converts to PTE in the second one. Nucleophilic catalysts of pyridine type (Nu1) are necessary in the first step. The second step is catalyzed by Nu1 as well as by catalysts of the tetrazole type (Nu2H). Base catalysis operates in the latter case. With Nu1 catalysts (pyridine, 4-N,N-dimethylaminopyridine, N-methylimidazole) the general scheme may be presented as follows: ArSO2Cl + Nu1 in equilibrium ArSO2Nu+1 + Cl-; ArSO2Nu+1 + PDE----(RO)(R'O)P(O)OSO2Ar (I); I + Nu+1----(RO)(R'O)P(O)Nu+1 (II); II + PDE in equilibrium [(RO)(R'O)PO]20; II + R"OH----(RO)(R'O)(R"O)PO. Catalysts of Nu2H type don't accelerate PPTE formation. In the second step they participate most probably in the process PPTE + Nu2H in equilibrium (RO)(R'O)P(O)Nu2 (III) + PDE; III + R"OH----(RO)(R'O)(R"O)PO + H+. The latter step is subjected to strong base catalysis.