In order to mimic the transition state of the cap RNA 2′-O-methylation, the design of the SAM mimetics relies on the coupling of the adenosine moiety of the SAM cofactor to another adenosine modified at 2′-O-position with an ethyl group to form the link. Thus, we first synthesized the dinucleoside 1 with a 2′-O-ethyl amino link between both adenosines (Scheme 2 ). The major advantage of this N-containing linker is the possibility to functionalize the secondary amine with a large variety of groups which may lead to additional binding with specific sites of RNA 2′-O-MTases. According to the schematic representation of the transition state of the 2′-O-methylation (Scheme 1), an accurate mimic was represented by compound 2 with the α-amino acid chain of the SAM branched to the nitrogen atom [16]. We further modified this side chain under α-amino-ester form or α-amino-amide form to result in compounds 3 and 4, respectively. Then instead of polar modifications, we introduced hydrophobic substituents on the secondary amine in dinucleosides 5, 6 and 7. Finally we chose to obtain compound 9 with a nitrobenzenesulfonamide moiety as a structural particular element of the dinucleoside. Indeed, as a global observation in medicinal chemistry the N-arylsulfonamide motif is regularly found in antitumor agents as in some antiviral inhibitors [19,20]. Further, we explored the combination of the nitro group with another substituent (MeO, CF3, Cl) at diverse positions in the phenyl ring resulting in the compounds 10–13 and we also removed the nitro group in 14 (Scheme 3 ). In addition, we chose to replace the sulfone by a methylene group in 15–16 (Scheme 4 ) to assess the sulfone contribution to the inhibitory activity obtained with dinucleosides containing the N-arylsulfonamide motif.
Scheme 2 Synthesis of dinucleosides 1–9. (a) K2CO3, KI, DMF, 50 °C, 24 h, 74%; (b) PhSH, K2CO3, DMF, 25 °C, 2 h, 76%; (c) TFA/H2O 8/2, 25 °C, 3 h, 76% for 1 and 34% for 9; (d) (i) 21, AcOH, CH2Cl2, 25 °C, 2 h, (ii) NaBH(OAc)3, 25 °C, 2 h, 93%; (e) (i) TFA/H2O 8/2, 25 °C, 3 h, (ii) 2 M aqueous solution LiOH, 25 °C, 0.5 h, 32%; (f) TFA/H2O 8/2, 25 °C, 3 h, 35%; (g) (i) TFA/H2O 8/2, 25 °C, 3 h, (ii) 7 M NH3/MeOH, 30 °C, 24 h, 38%; (h) 1-bromobutane, DIEA, NMP, microwave 110 °C, 4 h, 47% for 23; 1-bromo-3-phenylpropane, DIEA, NMP, microwave 110 °C, 4 h, 53% for 24, methyl 4-bromobutyrate, DIEA, NMP, microwave 110 °C, 3.5 h, 58% for 25; (i) (i) TFA/H2O 8/2, 25 °C, 6 h, 72% for 5; 3 h, 28% for 6; 5.5 h, 60% for 7; 5.5 h for 8. (ii) 2 M aqueous solution LiOH, 0 °C, 0.5 h, 36% for 8.
Scheme 3 Synthesis of dinucleosides 10–13. (a) Ns-Cl, NEt3, DMF, 25 °C, 3.5 h, 40–72%; (b) 17, KI, K2CO3, DMF, 50 °C, 24 h, 43–52%; (c) TFA/H2O 8/2, 25 °C, 3 h–5 h, 13–20%.
Scheme 4 Synthesis of dinucleosides 14–16. (a) 4-chlorobenzenesulfonyl chloride, NEt3, CH2Cl2, 0 °C, 3 h, 90%. (b) (i) 4-chloro-3-nitrobenzaldehyde, AcOH, DCE, 40 °C, 20 min; (ii) NaBH(OAc)3, 40 °C, 16 h, 87%. (c) (i) 4-chlorobenzaldehyde, AcOH, DCE, 40 °C, 20 min. (ii) NaBH(OAc)3, 40 °C, 16 h, 72%. (d) TFA/H2O 8/2, 25 °C, 3 h, 48% for 14; 6 h, 25% for 15; 6 h, 37% for 16.