Discovery and Synthesis of C-Nucleosides as Potential New Anti-HCV Agents

Abstract

Nucleoside analogues have long been recognized as prospects for the discovery of direct acting antivirals (DAAs) to treat hepatitis C virus because they have generally exhibited cross-genotype activity and a high barrier to resistance. C-Nucleosides have the potential for improved metabolism and pharmacokinetic properties over their N-nucleoside counterparts due to the presence of a strong carbon-carbon glycosidic bond and a non-natural heterocyclic base. Three 2'CMe-C-adenosine analogues and two 2'CMe-guanosine analogues were synthesized and evaluated for their anti-HCV efficacy. The nucleotide triphosphates of four of these analogues were found to inhibit the NS5B polymerase, and adenosine analogue 1 was discovered to have excellent pharmacokinetic properties demonstrating the potential of this drug class.

Keywords: C-Nucleoside; HCV; NS5B polymerase.

Figure 1

Structures of sofosbuvir, 2′CMe-7-deaza-adenosine, and target C-nucleoside analogues.

Scheme 1. Synthesis of 1

Reagents and conditions: (a) NaH, 2,4-dichlorobenzyl chloride; (b) TFA/water 9:1; (c) Cl₃CCN, Cs₂CO₃; (d) (i) 4 Å mol. sieves, DCM, 2 h, rt, (ii) pyrrole, BF₃·OEt₂, −50 °C (60% yield, β/α 2:1) or −78 °C (80% yield, β/α 1:1), (iii) NH₃ in MeOH, −78 °C; (e) ClSO₂NCO, CH₃CN/DMF, 0 °C; (f) BF₃·OEt₂, DCM, reflux; (g) (i) NaH, THF, 0 °C, (ii) Ph₂P(O)ONH₂, THF, 0 °C; (h) formamidine acetate, DMA, 140 °C, 1–2 h; (i) H₂, Pd–C (10%), 45 °C, 18 h, NaOAc, MeOH, HOAc.

Scheme 2. Convergent Synthesis of A-Analogue 2

Reagents and conditions: (a) NBS, DMF, 86 °C, 1 h; (b)(i) nBuLi, −78 °C, THF, (ii) 17, −78 °C; (c) NH₃ in MeOH, RT to reflux; (d) BF₃·OEt₂, Et₃SiH, DCM, −78 °C to RT, 3 h; (e) H₂, Pd–C (10%), 60 °C, 18 h, NaOAc, MeOH/DCM (9:1), AcOH.

Scheme 3. Convergent Synthesis of G-Analogues 4 and 5

Reagents and conditions: (a) TEA, DMAP, Boc₂O, MeCN, RT, 46 h; (b) NBS, DCE, −10 to 0 °C, 1 h; (c)(i) nBuLi, THF −100 °C, 23 or 28 (ii) 17; (d) BF₃·OEt₂, Et₃SiH, MeCN, −78 °C to RT, 3 h; (e) H₂ (60 psi) Pd–C (10%), 60 °C, 18 h, NH₄OAc, MeOH/EtOAc (13:2) or BBr₃, −78 °C to −30 °C (30 to 5); (f) DMF-dimethylacetal, DCM/MeCN, RT to 60 °C.

Scheme 4. Improved Synthesis of 4

Reagents and conditions: (a) nBuLi, BnOH, −10 °C to RT, THF; (b) NBS, DCM, −10 °C to RT; (c)(i) nBuLi, −100 °C, 2-Me-THF, (ii) 34 containing C7 isomer, −100 °C to RT; (d) BF₃·OEt₂, Et₃SiH, DCM, −78 °C, 15 min; (e) acetamide, Pd₂(dba)₂, xantphos, Cs₂CO₃, 130 °C, 1 h; (f) H₂, MeOH, Pd–C (10%), RT to 50 °C, 70 h; (g) NaOMe, RT to 80 °C.

Scheme 5. Synthesis of Pyrazolo-Nucleoside 3

Reagents and conditions: (a) NaH, DME, 41, 0 °C to RT; (b) (CH₃)₃COCH[N(CH₃)₂]₂, DMF, 60 °C, 15 h; (c) NH₂NH₂·HCl, EtOH, 105 °C, 2 h; (d) ethyl N-cyanoformimidate, 85 °C, toluene; (e) H₂, MeOH, Pd–C (10%), 45 °C, 17 h.