Methyl-substituted cycloSal-pronucleotides of d4TMP were synthesized with high diastereoselectivities in satisfying chemical yields. The individual diastereomers were tested against HIV-1 and HIV-2 infected wild-type CEM/0 and HIV-2 infected thymidine kinase deficient CEM cells. All diastereomers tested showed significant antiviral activity in CEM/0 and strong activity in CEM/TK(-) cell cultures. The antiviral activities were strongly dependent on the chirality at the phosphate group and the position of the methyl-group(s) in the cycloSal moiety. In CEM/TK(-) cell cultures the difference in antiviral potency was found to be 7- to 20-fold. The stability of each diastereomer was studied in aqueous phosphate buffer and in CEM/0 cell extracts. Large differences in the half-lives were found. A comparison of the relative lipophilicity of the methyl-substituted cycloSal triesters was performed based on the retention times obtained by reversed phase HPLC. The results obtained clearly confirm the importance of a diastereoselective synthesis of cycloSal-pronucleotides.

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 (RP)‐ and (SP)‐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.

CycloSal‐nucleosyl‐phosphate triesters are a known class of highly effective nucleotide prodrugs (pronucleotides) of antivirally active nucleoside analogues. Until recently, the synthesis of these compounds always gave diastereoisomeric mixtures. Then, a convergent route for the stereospecific synthesis of cycloSal‐triesters was described to give isomerically pure cycloSal‐prodrugs for the treatment of viral diseases. Here, the development of a stereoselective synthesis of these pronucleotides using various chiral auxiliaries is described. In contrast to pyrrolidine‐ or pyrrolidinone derivatives it was found that a thiazolidine derived from valinol fulfilled all three requirements to act as a suitable chiral moiety, allowing: (i) strong chirality transfer, (ii) the formation of separable diastereoisomeric intermediates, and (iii) a suitable leaving group that allows the introduction of the nucleoside analogue (e.g., d4T) in the final step under mild reaction conditions. The title compounds were obtained with very high diastereoisomeric excesses of more than 95 %.

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