Synthesis and biological evaluation of C-nucleoside analogues

C-Nucleosides are an important family of compounds with promising biological activities. In C-nucleosides, the labile C-N glycosidic linkage of classical N-nucleosides is replaced by a more stable C-C bond, which makes C-nucleosides resistant against hydrolysis. Some naturally occurring C-nucleosides such as formycin and showdomycin are potent antibiotics, while pseudouridine, an isomer of uridine, is able to stabilize RNA duplexes by replacing uridine. A number of synthetic C-nucleosides display potent antiviral activity. The most significant examples are BCX4430 (galidesivir) and GS-5734 (remdesivir), which are both currently being developed as drugs for the treatment of Ebola virus (EBOV) infections. Nucleoside phosphonates are isosteres of nucleoside monophosphates, in which the labile P-O bond is substituted by a stable P-C bond. Hence, nucleoside phosphonates possess an improved metabolic stability against phosphatase-catalyzed hydrolysis. In addition, the presence of a phosphonate moiety allows to skip the first phosphorylation step that is required for nucleoside activation. The most successful examples of nucleoside phosphonates are acyclic nucleoside phosphonates, such as adefovir, tenofovir, and cidofovir, which received marketing approval as antiviral drugs. The main goal of this Ph.D. thesis was to investigate the chemical and biological properties of novel C-nucleosides as well as C-nucleoside phosphonates. To this aim, 9-deazaadenine and pyrrolo[2,1-f][1,2,4]triazin-4-amine were selected as nucleobases, based on their promising biological profile. In the first part of this thesis (Chapter 2), the synthesis and antiviral evaluation of a 5'-phosphonate-C-nucleoside with 9-deazaadenine as nucleobase is described. The initial introduction of a phosphonate moiety onto a benzoyl protected 5'-acetoxy-[2'S]-9-deazaadenosine derivative was unsatisfactory, since an unexpected epimerization at the 2'-position occurred upon removal of the benzoyl groups under basic conditions. However, an efficient synthetic route was established through the stereoselective installation of the phosphonomethoxy group at the anomeric 5'-position of [5'R]-acetoxy-[2'R]-9-deazaadenosine, followed by a base-promoted epimerization reaction to provide the desired [2'S] compound. Such C-nucleoside phosphonate and its prodrug were submitted for antiviral evaluation against a panel of RNA viruses such as RSV, Zika virus, Influenza A H1N1, Polio virus, and MERS coronavirus. Unfortunately, no significant antiviral activity or cytotoxicity was observed. To further study the properties of 5'-phosphonate-C-nucleosides, a 3'-deoxy derivative and its corresponding phosphonoamidate prodrug were synthesized, as discussed in Chapter 3. The 2'-deoxy-9-deazaadenosine derivative was prepared from a 2'-bromo intermediate via reduction under Barton-McCombie deoxygenation conditions. The target 5'-phosphonate-3'-deoxy-C-nucleoside was then obtained by a base-promoted epimerization reaction as established in Chapter 2. The assessment of the antiviral activity of this 5'-phosphonate-3'-deoxy-C-nucleoside and its prodrug against a number of DNA viruses (including HSV, CMV, and VZV) revealed that they completely lacked antiviral activity and cytotoxicity. In Chapter 4, the synthesis and antiproliferative evaluation of xylo-C-nucleosides containing pyrrolo[2,1-f][1,2,4]triazin-4-amine as nucleobase are described. The synthesis started with the coupling between a benzyl protected d-xylono-1,4-lactone and 4-(methylthio)pyrrolo[2,1-f][1,2,4]triazine under basic conditions. A survey of several reaction conditions for the deprotection of the benzyl groups revealed that a palladium-catalyzed hydrogen transfer reaction allowed to isolate the target compound in good yield. An in vitro antiproliferative assay showed that this xylo-C-nucleoside was active against a panel of tumor cell lines such as the human leukemia HL-60 (IC50 = 1.9 µM) and lung cancer NCI-H460 (IC50 = 2.0 µM) cells. To extend the scope of our study in the field of C-nucleoside phosphonates, the synthesis of a l-threose C-nucleoside with a phosphonomethoxy moiety at the 3'-position and pyrrolo[2,1-f][1,2,4]triazin-4-amine as nucleobase is described in Chapter 5. This C-nucleoside was obtained by coupling a benzyl protected l-threono-1,4-lactone with 4-(methylthio)pyrrolo[2,1-f][1,2,4]triazine. A tert-butyldiphenylsilyl group was selected for the regioselective protection of the 3'-hydroxyl group and then cleaved to give the key intermediate with a free hydroxyl group at the 3'-position. A phosphonate unit was successfully introduced under basic conditions to give the desired compound. Unfortunately, antiviral evaluation against RNA viruses and DNA viruses revealed that the synthesized threose C-nucleosides exhibited no significant antiviral activity or cytotoxicity. Further screening against HBV is currently undergoing.

Publication year:2019

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