Diastereoselective synthesis of cyclosaligenyl-nucleosyl-phosphotriesters.

A diastereoselective synthesis of cycloSal-phosphotriesters (cycloSal=cycloSaligenyl) based on chiral auxiliaries has been developed that allows the synthesis of single diastereomers of the cycloSal-pronucleotides. In previously described synthesis routes, the cycloSal-compounds were always obtained as 1:1 diastereomeric mixtures that could be separated in only rare cases. However, it was shown that the diastereomers have different antiviral activity, toxicity, and hydrolysis stabilities. Here, first a chiral thiazoline derivative was used to prepare nonsubstituted and 5-methyl-cycloSal-phosphotriesters in 48 and ≥95% de (de=diastereomeric excess). However, this approach failed to give the important group of 3-substituted cycloSal-nucleotides. Therefore, two other chiral groups were discovered that allowed the synthesis of (R(P))- and (S(P))-3-methyl-cycloSal-phosphotriesters as well. The antiviral activity was found to be five- to 20-fold different between the two individual diastereomers, which proved the importance of this approach.