Efficient functionalization of 2',5'-oligoadenylates with sulfur

To derivatize the 2'-terminus of 2',5'-oligoadenylates with a thiol group, the reaction of periodate-oxidized nucleotide and 2',5'-oligonucleotide with aminothiols was explored. Two separate synthetic approaches were employed, both of which relied upon the use of S-protected thiols. In one approach, 5'AMP was oxidized with sodium periodate to dialdehyde, which was reacted with cystamine hydrochloride. Sodium cyanoborohydride reduction of the unisolated intermediate aminal gave compound 4. The second approach involved reaction of S-(2-tetrahydropyranyl)cysteamine with the dialdehyde obtained by periodate oxidation of 5'AMP to yield, after reduction with Na(CN)BH3, the S-protected adduct 3. Intermediate 3 could be oxidized with aqueous iodine to give disulfide 4. Disulfide 4, obtained by either of the above routes, was reduced with dithiothreitol (DTT) to the thiol 5. This same reaction sequence was applied to 2-5A tetramer monophosphate, p5'A2'[p5'A2'](2)p5'A (6a), to give via 6b the 2'-terminal-modified derivative 6c. Aqueous iodine oxidation of 6c provided the disulfide 7, which reacted with DTT to give quantitative conversion to product, the free thiol 8. Both the disulfide 7 End the S-tetrahydropyranyl-protected derivative (6c) were bound effectively to the 2-5A-dependent RNase L of mouse L cells with IC50 values of 1 x 10-9 M for 7 and 8 x 10(-10) M for 6c, not significantly different from the corresponding value for the parent unmodified 2-5A (6a) itself.