US Patent 6919437 Synthetic methods and intermediates for triester oligonucleotides

Methods for the preparation of oligonucleotides having bioreversible phosphate blocking groups are disclosed. In one aspect, the present invention provides compounds comprising a sequence of nucleotide units that includes a first segment having at least one internucleoside linkage of formula:3′-O—P(X)(O−)—O-5′and a second segment having at least one internucleoside linkage of formula:3′-O—P(X)(Y1—(CH2)q—Y2—Y3—Z)—O-5′wherein each of X, Y1, and Y2 is, independently, O or S; q is 2 to about 4; Y3 is C(═O) or S; and Z is aryl having 6 to about 14 carbon atoms or alkyl having from one to about six carbon atoms.