NUCLEOSIDES AND NUCLEOTIDES - 174 - SYNTHESIS OF OLIGODEOXYNUCLEOTIDES CONTAINING [2-[[N-(2-AMINOETHYL)CARBAMOYL]OXY]ETHYL]THYMIDINE AND THEIR THERMAL-STABILITY AND NUCLEASE-RESISTANCE PROPERTIES

The synthesis and properties of oligodeoxynucleotides (ODNs) containin g [2-[IN-(2-aminoethyl)carbamoyl]oxy]ethyl]thymidine (3) are described . 4'alpha-(2-Hydroxyethyl)thymidine (4), which is a precursor for phos phoramidite 5, was synthesized using a newly developed intramolecular radical cyclization reaction at the 4'-position of thymidine derivativ e 7. The radical reaction of 4'beta-(phenylseleno)-3'-O- (dimethylviny lsilyl)thymidine derivative 7, which was prepared from thymidine in se veral steps, with Bu3SnH and AIBN, followed by Tamao oxidation, gave e ither 4'alpha-(2-hydroxyethyl) derivative 6 or 4'alpha-(1-hydroxyethyl ) derivative 13, respectively. With a low Bu-3-SnH concentration, the reaction gave 6, via. 6-endo-radical-cyclized product 11, as a sole pr oduct in 87% yield. The reaction of 7 in the presence of excess Bu3SnH gave 13 in 75% yield, via 5-exo-cyclized product 12, as a diastereome ric mixture. The 4'alpha-(2-hydroxyethyl) derivative 6 was then conver ted into a [2-[IN-(2-aminoethyl)carbamoyl]oxy]ethyl]thymidine derivati ve 14, which was phosphitylated to give phosphoramidite 5 in 72% yield . In this study, 3 was incorporated into a nonadecamer, d[CTGGCTCAGCTC GTCTCAT]-3', and a heptadecamer, d[CTCGTACCATTCCGCTC]-3', instead of T at various positions. ODNs containing 3 were more resistant to nucleo lytic hydrolysis by both snake venom phosphodiesterase (a 3'-exonuclea se) and DNase I (an endonuclease) than unmodified parent ODNs, althoug h ODNs containing 3 only slightly destabilized duplex formation with b oth complementary DNA and RNA strands. Furthermore, the duplex formed by an ODN containing 3 and its complementary RNA was a good substrate for Escherichia coli RNase H.