Synthesis of a C-Nucleoside Phosphonate by Base-Promoted

Letter Cite This: Org. Lett. 2018, 20, 1203−1206

pubs.acs.org/OrgLett

Synthesis of a C‑Nucleoside Phosphonate by Base-Promoted Epimerization Peng Nie,† Elisabetta Groaz,† Steven De Jonghe,‡ Eveline Lescrinier,† and Piet Herdewijn*,† †

Medicinal Chemistry, Rega Institute for Medical Research, KU Leuven, Herestraat 49, 3000 Leuven, Belgium Laboratory of Virology and Chemotherapy, Rega Institute for Medical Research, KU Leuven, Herestraat 49 bus 1043, 3000 Leuven, Belgium

‡

S Supporting Information *

ABSTRACT: The efficient synthesis of a [2′S] C-nucleoside phosphonate and its corresponding prodrug has been realized. A phosphonomethoxy group was stereoselectively introduced at the anomeric 5′-carbon atom through glycosylation of a benzoyl protected [5′R]-acetoxy-[2′R]-9-deazaadenine. An unexpected epimerization at the 2′-position of the sugar moiety occurred upon removal of the protecting groups, but this was further exploited as a key reaction for improved synthesis of the target compound. Here, we present the first synthesis of a C-nucleoside phosphonate 1 and its prodrug 2, featuring 9-deazaadenine as a base moiety. (See Figure 1.) The focus of the proposed

C

-Nucleosides are nucleoside analogues in which the C−N bond between the sugar and base moiety is replaced by a more-stable C−C bond. Some C-nucleosides, such as formycin A and B, as well as showdomycin, have been isolated from natural sources and have been shown to possess antibiotic activity.1 Pseudouridine, which is a C-nucleoside commonly found in structural RNA,2 can reduce radiation-induced chromosome aberrations in human lymphocytes.3 Some unnatural C-nucleosides with interesting biological properties have also been synthesized, such as BCX4430 and GS-6620. While the former compound displays good activity against Ebola virus (EBOV) infections,4 GS-6620 represents a potentially useful candidate for the treatment for hepatitis C virus (HCV) infections.5 Generally, three main strategies are used for the synthesis of C-nucleoside analogues. Vorbrüggen-type reactions offer a feasible solution for the coupling between a carbohydrate and C-nucleobase and involve the use of electrophilic sugars under acidic conditions.6 Another method for the construction of Cnucleosides consists of the nucleophilic substitution between nucleobase and modified sugars; in this case, lithium−halogen exchange is normally used to convert the nucleobase to a good nucleophile.4,5,7 Lastly, it is possible to construct the nucleobase on the sugar ring via a linear approach.8 Nucleoside phosphonates have been developed as isosteric analogues of natural nucleoside phosphates. The most successful examples are tenofovir, adefovir, and cidofovir, which received marketing approval for clinical use as antiviral drugs.9 Even though a series of nucleoside phosphonates have been considered for drug development, no attention has been paid to the exploration of the synthesis of C-nucleoside phosphonates until now. © 2018 American Chemical Society

Figure 1. Structures of target C-nucleoside phosphonate 1 and bisphosphonamidate 2.

synthetic scheme is on avoiding the use of sophisticated protecting group strategies.4 The introduction of a phosphomethoxy group at the 5′-position10 of compound 9 was the key step in the synthesis, together with the design of an appropriate scheme to obtain the final compound with the correct stereochemistry. Although these types of reactions have been well-documented in the field of nucleoside chemistry,11 this is the first application in the C-nucleoside field, leading to some particular synthetic challenges. Our retrosynthetic analysis is illustrated in Scheme 1. The prodrug approach to synthesize compound 2 from 1 by using 2,2′-dithiopyridine and triphenylphosphine as activating agents is a well-established procedure.12 The stereoselective installation of the phosphonate group onto acetate 3 could be achieved through an acid-catalyzed glycosylation reaction. It has been Received: January 11, 2018 Published: February 1, 2018 1203

DOI: 10.1021/acs.orglett.8b00123 Org. Lett. 2018, 20, 1203−1206

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Organic Letters

Scheme 2. Synthesis of Compound 10, Starting from β-9deazaadenosine

Scheme 1. Retrosynthetic Analysis of Target Compounds 1 and 2

previously demonstrated that precursor 4 can be oxidatively decarboxylated to give acetate 3, which is known as the Kochi reaction. Acid 4 could arise from primary alcohol 5, which, in turn, could be prepared by a C-glycosylation reaction from commercially available 6.6a,b In case of an unsuccessful Cglycosylation of 6 into 5, an alternative approach (starting from ribonolactone 7) could be followed to prepare the protected 9deazaadenosine.8 We proposed to use the benzoyl protecting group for the nucleobase as well as the hydroxyl groups at the 2′- and 3′-position, which could be easily removed within one step. A Vorbrüggen-type glycosylation reaction between compound 6 and 9-deazaadenine under conditions of Lewis acid catalysis (SnCl4, BF3·OEt2, or TMSOTf) in different solvents, such as dichloromethane (DCM), MeCN, and 1,2-dichloroethane, did not occur in our experiments. Unfortunately, the coupling reaction between 7 and either 9bromodeazaadenine or its N6-benzoylated analogue in the presence of n-BuLi also met with failure. Therefore, 9deazaadenosine was obtained by a sequence of reactions as described by Lim and Klein.8a Selective protection of the 5′-hydroxyl group with TBDPSCl, followed by blocking of all the other active protons with BzCl, gave the fully protected compound 8 in 81% yield. The removal of the 5′-silyl protecting group is usually performed using fluoride-based reagents. However, the use of TBAF resulted in a concomitant debenzoylation reaction, while the use of TEA· 3HF led to a very slow deprotection rate. Finally, 4 N HCl in dioxane was used to remove the silyl group in the presence of methanol as a solvent.13 The primary alcohol of 5 was smoothly oxidized using (diacetoxyiodo)benzene (DAIB) and TEMPO14 at room temperature to afford acid 4. Copper acetate was employed as a catalyst for the oxidative decarboxylation reaction to provide the glycosylation precursor acetate 9 as an [R]/[S] mixture at the 5′-position. In the absence of this crucial catalyst, the decarboxylation proceeded only sluggishly, with a low conversion of the starting material.15 A survey of several Lewis acid conditions in the key step reaction for the introduction of the phosphonate group revealed that BF3·OEt2 in DCM produced 10 as a single isomer. (See Scheme 2.) However, the reaction progressed very slowly and a maximum yield of 14% could be obtained. The configuration of compound 10 was analyzed by 2D NMR (ROESY). The neighboring 4′-benzoate participation could explain the good stereoselectivity of this glycosylation reaction. An epimerization reaction at the 2′-position occurred when methanolic ammonia

was used to remove all of the benzoyl groups, affording compound 11 as an [R]/[S] mixture at the 2′-position, which could be separated by reverse-phase HPLC. The yields of the [2′R] and [2′S] isomer was 52% and 35%, respectively. Thus, such epimerization reaction hampers the obtainment of sufficient amounts of the [2′S] anomer for further investigation. A plausible mechanism of this epimerization reaction is shown in Scheme 3. A nitrogen anion is generated upon Scheme 3. Proposed Mechanism of Base-Promoted Epimerization

removal of the benzoyl group. This is followed by opening of the sugar ring, which leads to II. Subsequent reclosure of the ring through a 1,4-addition may give III, which can finally be reprotonated to form both R- and S-epimers. The epimerization occurs during the nucleophilic conjugate addition because the hydroxide can either attack the 2′ carbon from an upward or downward direction, with respect to the double bond. Potential cleavage of the phosphonate moiety was not detected during the reaction. This is presumably due to the fact that the hemiacetal at the 5′-position of II tends to form cyclic intermediate III through an intramolecular 1,4-addition, which could be faster than the fragmentation reaction. Another reason might be that the phosphonate moiety is not a good leaving group under basic conditions. Based on this experience, we designed a new synthetic approach. The two main issues encountered in the execution of the previous scheme were (i) the low yield of the glycosylation reaction at the 5′-position to introduce the phosphonomethoxy 1204

DOI: 10.1021/acs.orglett.8b00123 Org. Lett. 2018, 20, 1203−1206

Letter

Organic Letters group and (ii) the epimerization occurring during the deprotection step. It was reasoned that the use of α-9deazaadenosine as starting material might provide a decreased steric hindrance at the 5′-position during glycosylation. However, the efficiency of the epimerization reaction at the level of compound 16 could not be predicted. As shown in Scheme 4, the glycosylation precursor acetate 15 was prepared from α-9-deazaadenosine through a similar

Scheme 5. Synthesis of Target Compounds C-Nucleoside Phosphonate 1 and Bisphosphonamidate 2

Scheme 4. Synthesis of Compound 1 Starting from α-9Deazaadenosine

In summary, we have developed an effective synthetic approach for the preparation of a novel C-nucleoside phosphonate. By harnessing the intrinsic chemical properties of the key intermediates, a base-promoted epimerization was employed as a critical step in the synthesis. A five-step sequence led to key intermediate 19 in an efficient way. Unfortunately, this C-nucleoside phosphonate and its prodrug showed no activity against RNA viruses, such as RSV, Zika, Influenza A H1N1, and Polio virus.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00123. Experimental details and NMR spectra (PDF)

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synthetic route as that described earlier. The difference here is that compound 15 was obtained as a single isomer, compared to 9. This stereoselectivity could be due to the fact that all other substituents on the furanose ring are pointed downward. In addition, a much better yield was attained in the glycosylation reaction (15 to 16) by employing TMSOTf as a promoter. Fortunately, epimerization did occur during deprotection, affording a mixture of R/S anomers with the formation of the [2′S] isomer in 37% yield. The isopropyl protecting groups were cleaved by using TMSBr in the presence of 2,6-lutidine, providing target 1 in a moderate yield (60%). However, the deprotection conditions were rather harsh and involved a timeconsuming workup procedure. Therefore, the isopropyl groups were replaced by benzyl groups that could be removed by hydrogenolysis, as depicted in Scheme 5, which is the most efficient approach to obtain the target compounds. The five-step reaction sequence allowed us to prepare key intermediate 18 in large scale from α-9deazaadenosine. Compound 15 was converted to 18 by glycosylation with the phosphonate reagent using BF3·OEt2 as Lewis acid. The deprotection and epimerization reactions occurred in one step giving compound 19[2′S]. Hydrogenolysis with Pd/C as a catalyst gave compound 1 after filtration over Celite, followed by purification via high-performance liquid chromatography (HPLC). A prodrug was also prepared through coupling between target 1 and an amino acid ester by activating the phosphonate diacid using 2,2′-dithiopyridine and triphenylphosphine16 in 30% yield after HPLC purification.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Steven De Jonghe: 0000-0002-3872-6558 Piet Herdewijn: 0000-0003-3589-8503 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.N. would like to thank the China Scholarship Council (CSC) for funding. We are grateful to Prof. Jef Rozenski for recording HRMS spectra, and Luc Baudemprez for running the 2D NMR experiments (KU Leuven). KU Leuven used the service program offered by the National Institute of Health (NIH) for testing the final compounds. We wish to thank Prof. Donald Smee (Utah State University) for carrying out the antiviral screening.

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REFERENCES

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DOI: 10.1021/acs.orglett.8b00123 Org. Lett. 2018, 20, 1203−1206