Synthesis And Biological Activity of Borane Phosphonate DNA

Abstract

Borane phosphonate oligodeoxyribonucleotides are synthesized from 5′-O-[benzhydroxybis(trimethylsilyloxy)silyl]-2′-deoxyribonucleoside-3′-O-methylphosphoramidites. The exocyclic amine functions of adenine, guanine, and cytosine are protected with trimethoxytrityl, and thymine is unprotected. Using these synthons and under standard conditions via activation with S-ethylthiotetrazole, condensations on a highly crosslinked polystyrene support are in excess of 99%. Following the complete synthesis of the oligodeoxyribonucleotide phosphite triester, oxidation with THF·BH₃ yields the oligodeoxyribonucleotide borane phosphonate. Further treatment with 80% aqueous acetic acid followed by disodium 2-carbamoyl-2-cyanoethylene-1,1-dithiolate removes trimethoxytrityl from the 2′-deoxyribonucleoside bases and the methyl protecting group from the internucleotide phosphate triester, respectively. Cleavage from the support with ammonium hydroxide and purification by reverse phase HPLC affords the pure oligodeoxyribonucleotide borane phosphonate. These oligomers are taken up by cells in the absence of cationic lipids and transport biologically active interfering RNA into cells.

Keywords:

• Borane
• DNA
• phosphonate

Acknowledgments

We thank Rich Shoemaker for assistance with the NMR facility and University of Colorado Central Analytical Laboratories for the mass spectral facility. This research was supported by the University of Colorado at Boulder.

Notes

Previous research has shown that the use of 2′-deoxynucleoside 3′-H-phosphonates as synthons requires that N3 of thymine be protected preferably with N3-anisoyl. Otherwise, the combination of silylation to activate an internucleotide H-phosphonate linkage toward boranation, followed by boranation, causes reduction of the thymine base.^6,7