Neighbouring group participation in the unblocking of phosphotriesters of nucleic acids

Two examples of neighbouring gròup participation during the removal of protecting groups from phosphotriesters of partially or fully protected intermediates of nucleic acids are presented. The first example shows that ammonolysis of aryl groups from phosphotriesters of partially protected - 5′ hydroxy free - nucleic acids (e.g., 4b; Ar=2C1C₆ H₄) gives rise to the formation of unnatural nucleic acids (e.g., 7 and 8). The second one illustrates that fluoride ion promoted hydrolysis of 2,2,2-trichloroethyl groups from phosphotriesters of fully protected nucleic acids (e.g., 18a), having t-butyldimethylsilyl groups at the 2′-positions, leads to the formation of a considerable amount of side-products (e.g., 20 and 21).