Solution phase synthesis phosphorothioates

The commercially available 3-amino-1,2,4-dithiazole-5-one, (3), has been attached to a hydroxyl resin via a succinic acid linker, and has been used as an efficient sulfur-transfer reagent for the solution-phase synthesis of phos-phorothioates. DNA synthesis scale-up to lOOg of a 20-mer phosphorothioate has been examined " key to this was the purification of the product, which was carried out using a high efficiency polymeric anion exchange chromatographic media. 

However, conversion of the dinucleoside //-phosphonate diester 10 to the phosphorothioate analogue 11 effected by a 0.02 M solution of 2 in 2% aqueous pyridine is relatively slow it reportedly takes 3 h for a complete reaction. While a solution of 2 in 2% aqueous acetonitrile containing triethylamine can completely transform 10 into 11 within 30 s, the use of this sulfurization mixture is incompatible with automated solid-phase oligonucleotide synthesis given the rapid formation of a yellow precipitate caused by triethylamine. In the absence of triethylamine, 10 is not sulfurized under these conditions. Nonetheless, 10 is completely converted to 11 within 20 min when a 0.02 M solution of 1 in 2% aqueous pyridine is used for the sulfurization reaction. Thus, 3i/-l,2-benzodithiol-3-one in aqueous pyridine is compatible with automated solid-phase synthesis of both DNA and RNA oligonucleoside phosphorodithioates or phosphorothioates from appropriate //-phosphonate derivatives. ... 

As a final addendum to the commentary in Section 3., it is worthwhile to discuss briefly two subjects of continual interest in the area of potential pharmaceutical applications of oligonucleoside phosphorothioates scale-up vis-a-vis solid-phase or solution synthesis and stereochemistry. ... 

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