Nucleoside phosphorothiolates, synthesis

The synthesis of pyrimidine 3 -phosphorothioamidites has been carried out to prepare ODNs containing 3 -5 -phosphorothiolate linkages for mechanistic studies.The stereospecific synthesis of 3 -deuterated pyrimidine nucleosides for NMR studies of ODNs has also been reported.The incorporation into DNA of a thymidine analogue bearing a 3 -methyl group was shown to be destabilising with both complementary DNA and RNA, though less so with RNA. ° ... 

Cosstick and co-workers have reported an improved procedure for the synthesis of phosphorothiolate-containing DNA using Arbuzov chemistry in which the nucleoside disulfides are generated in situ from the corresponding thioesters. Several nucleoside disulfides have been prepared and in particular the regioselectivity of reaction of these with a phosphite has been examined. The disulfide (59) allows the preparation of the desired diribonucleoside phosphoro-thiolate (60) but the reaction is accompanied by some (2-10%) formation of the pyridyl phosphorothiolate. 

The use of the Michaelis-Arbuzov reaction has been made in the synthesis of dinucleoside-3 -5-phosphorothiolates using disulphides (73) and nucleoside 5 -dialkylphosphite derivatives. Particularly efficient reactions were achieved using a 5 -Wj(trimethylsilyl)phosphite (74) which afforded the phosphorothiolate directly after aqueous work up. Derivative (74) was obtained by treating the corresponding nucleoside H-phosphonate with chlorotrimethylsilane in pyridine in the presence of triethylamine. 

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