SYNTHESIS OF DEOXYDINUCLEOSIDE PHOSPHORODITHIOATES

The synthesis of a new class of DNA analogues called phosphorodithioate DNA is described. This analogue, which has a deoxynucleoside-OPS2O-deoxynucleoside internucleotide linkage, is isosteric and isopolar with the normal phosphodiester, inert toward nucleases, and potentially useful for a large number of biochemical and biological applications. Two methods are described for synthesizing this derivative. One route begins by condensing a deoxynucleoside phosphorodiamidite with a second appropriately protected deoxynucleoside to yield a deoxydinucleoside phosphoramidite. Sulfhydrolysis with H2S generates the H-phosphonothioate, which upon oxidation with sulfur yields the deoxydinucleoside phosphorodithioate. Alternatively, sequential treatment of the deoxydinucleoside phosphoramidite with a mercaptan and sulfur yields the deoxydinucleoside phosphorodithioate triester. These deoxydinucleotides in protected form can then be used to introduce the dithioate internucleotide linkage into DNA. The second route for generating dithioate DNA uses deoxynucleoside phosphorothioamidites. Two derivatives, the deoxynucleoside 3'-N,N-dimethyl- or 3'-(N,N-tetramethylenephosphorothioamidite), were found to be especially attractive synthons as they could be prepared in stable form via a one-flask synthesis procedure and used to for the deoxydinucleoside thiophosphite rapidly (1-2 min with tetrazole as activator) in high yield. Subsequent oxidation with sulfur generates the completely protected phosphorodithioate linkage.