key: cord-0942499-ql2ovi21
authors: Manchoju, Amarender; Zelli, Renaud; Wang, Gang; Eymard, Carla; Oo, Adrian; Nemer, Mona; Prévost, Michel; Kim, Baek; Guindon, Yvan
title: Nucleotide Analogues Bearing a C2′ or C3′-Stereogenic All-Carbon Quaternary Center as SARS-CoV-2 RdRp Inhibitors †
date: 2022-01-17
journal: Molecules
DOI: 10.3390/molecules27020564
sha: d0bd5713101ece2f9f5ac800c18cdcfaf576a857
doc_id: 942499
cord_uid: ql2ovi21

The design of novel nucleoside triphosphate (NTP) analogues bearing an all-carbon quaternary center at C2′ or C3′ is described. The construction of this all-carbon stereogenic center involves the use of an intramoleculer photoredox-catalyzed reaction. The nucleoside analogues (NA) hydroxyl functional group at C2′ was generated by diastereoselective epoxidation. In addition, highly enantioselective and diastereoselective Mukaiyama aldol reactions, diastereoselective N-glycosylations and regioselective triphosphorylation reactions were employed to synthesize the novel NTPs. Two of these compounds are inhibitors of the RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2, the causal virus of COVID-19.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a positive-sense RNA virus and the causal agent of coronavirus (CoV) disease 2019 . CoVs employ a multi-subunit replication/transcription machinery. The virus enters the cell by endocytosis using the ACE 2 receptors and is then uncoated. ORF1a and ORF2 of the positive strand RNA are then translated to produce non-structural protein precursors (nsp), including proteins that further cleave the precursor to form mature functional helicase and RNA-dependent RNA polymerase [1] (RdRp, or alternatively nsp 12). The latter has been recognized as an optimal target for drug design, due to its crucial role in RNA synthesis, lack of host homologues and high structural conservation between coronaviruses.

The severity of SARS-CoV-2 disease has encouraged many laboratories to evaluate potential inhibitors of RdRp from related viruses. Sofosbuvir, a potent hepatitis c virus (HCV) polymerase inhibitor, was first studied. Inhibitors of single-stranded negative RNA viruses, such as Remdesivir ( Figure 1A , Ebola) [2] and B-D-N 4 -hydroxy-cytidine NHC [3] (Molnupiravir or EIDD-1931, influenza) , along with other nucleotides, were also examined [4] .

in the Remdesivir ribose moiety and the serine 861 side chain in the nsp 12 subunit of RdRp [7] . The truncation of serine 861 to alanine or glycine renders the RdRp less sensitive or insensitive to inhibition by Remdesivir [8] . An alternative mechanism was recently proposed involving the RdRp-dependent RNA proofreading. The mechanism of NHC (Molnupiravir), presently evaluated in clinical trials and is attributed to the triple level incorporation of NHC-TP, resulting in increased mutations and, ultimately, in a process known as "lethal mutagenesis". From a drug design standpoint, the antiviral activity of nucleoside analogues (NAs) depends on their efficient transport into cells where a first phosphorylation takes place (e.g., by cytidine kinase). These two critical steps are rate-limiting in the entire cascade that leads, through subsequent phosphorylation by other kinases, to the active nucleoside triphosphate (NTP) analogues that compete with natural nucleoside triphosphates. To circumvent these limitations, phosphoramidates were developed as monophosphate lipophilic pro-drugs that facilitate intracellular transport. The pro-drug is cleaved intracellularly, releasing the NA monophosphate that is then transformed into the corresponding bioactive triphosphorylated analogue. The main objective of the present study was to evaluate a novel series of triphosphorylated nucleotide analogues to test their activity Remdesivir (RDV) was approved by the Food and Drug Administration (FDA) for the intravenous treatment of COVID-19 in hospitalized adult and pediatric patients [5] . RDV-TP (1), the active agent, is incorporated into RNA by the RdRp enzyme and then acts as a delayed chain termination of RNA synthesis at position i + 3 [6] . The Remdesivirinduced RdRp stalling is caused by a translocation barrier between the C1 -cyano group in the Remdesivir ribose moiety and the serine 861 side chain in the nsp 12 subunit of RdRp [7] . The truncation of serine 861 to alanine or glycine renders the RdRp less sensitive or insensitive to inhibition by Remdesivir [8] . An alternative mechanism was recently proposed involving the RdRp-dependent RNA proofreading. The mechanism of NHC (Molnupiravir), presently evaluated in clinical trials and is attributed to the triple level incorporation of NHC-TP, resulting in increased mutations and, ultimately, in a process known as "lethal mutagenesis".

From a drug design standpoint, the antiviral activity of nucleoside analogues (NAs) depends on their efficient transport into cells where a first phosphorylation takes place (e.g., by cytidine kinase). These two critical steps are rate-limiting in the entire cascade that leads, through subsequent phosphorylation by other kinases, to the active nucleoside triphosphate (NTP) analogues that compete with natural nucleoside triphosphates. To circumvent these limitations, phosphoramidates were developed as monophosphate lipophilic pro-drugs that facilitate intracellular transport. The pro-drug is cleaved intracellularly, releasing the NA monophosphate that is then transformed into the corresponding bioactive triphosphorylated analogue. The main objective of the present study was to evaluate a novel series of triphosphorylated nucleotide analogues to test their activity directly in vitro against RdRp [9] . If active, the corresponding phosphoramidate pro-drugs would then be installed on the parent NA for further evaluations. The nucleoside analogues reported in the literature thus far display additional substituents at C1 (cyano for Remdesivir Figure 1A ) and different C2 or C3 substituents (methyl or fluorine, Figure 1B ). This was suggestive that further modification of the C2 or C3 positions would be tolerated.

Our laboratory has a long-standing interest in the development of new methodologies to improve the synthesis of nucleoside analogues. In parallel, we have been studying carbon-centered free radicals on acyclic molecules and their reactivity in atom transfer reactions, leading to the generation of all carbon stereogenic quaternary centers. Together, these findings [10] [11] [12] [13] [14] [15] [16] led to the conceptualization and synthesis of novel nucleoside analogues bearing a quaternary all-carbon stereogenic center at C3 or C2 [16] [17] [18] .

The presence of these all-carbon quaternary centers also provides structural properties that may influence the recognition of these nucleosides or nucleotides by given targets (enzymes or a receptors). For instance, nucleosides and nucleotides are flexible molecules adopting conformations ranging between North (C3 endo, Figure 1C ) or South (C2 endo, Figure 1C ). Increasing the populations of nucleosides in their bio-active conformation may translate into a greater binding affinity to the target. We hypothesized that the presence of the quaternary center could induce a conformational bias when located at C2 or C3 , the former would favor the North conformation (RNA-like) and the latter, the South (DNA-like). These conformational changes would be induced to minimize the steric effects (gauche effects) of the quaternary centers with their proximal substituents ( Figure 1C ). The X-ray analyses of some of our analogues having stereogenic quaternary centers at C2 or C3 supports these conformational biases. In solution, these molecules will, however, still possess some plasticity, allowing for conformational realignment during binding, contrary to locked nucleosides analogues [19] . The presence of the hydroxyl on this center could also potentially act as an extended pharmacophore, providing different proximal binding. On the other hand, binding to enzymes susceptible to steric hindrance at these positions could lead to inactivity. We thus embarked on the syntheses of a small library of nucleotides having stereogenic centers at C2 or C3 bearing adenine or cytosine nucleobases ( Figure 1D ) to investigate their inhibitory profile against RdRp.

The synthesis of the all-carbon quaternary stereogenic centers was based on our findings that a carbon-centered free radical flanked by an ester and a secondary carbon bearing an electronegative substituent (hydroxyl) could stereoselectively participate in kinetically controlled atom transfer reactions. Hydrogen and allylation transfer reactions have previously been studied both experimentally and theoretically by our group [20, 21] . We recently prepared radical precursor 2 using an enantioselective Mukaiyama aldol reaction in good yield and with high diastereoselectivity in favor of the 3R isomer ( Figure 2 ) [14] . The secondary alcohol was then transformed into dimethylallysilyl ether 3. Acyclic intermediate 6, bearing the quaternary stereogenic center, was synthesized by a sequence involving an intramolecular atom transfer cyclization, an elimination reaction under photoredox catalysis and subsequent ester reduction with protection of alcohol generated.

Allylic oxidation of 6 using SeO 2 led to a modest 50% yield of the corresponding ketone 7 (Figure 2 ) [14] . The 2,4-syn diol 8 was then obtained from ketone 7 by reduction using catecholborane in the presence of cesium chloride. The key intermediate 8 was transformed to furanosides 9 and 10. The corresponding β-cytosine nucleoside analogue 11 was obtained from 9 by taking advantage of anchimeric participation by the acetate at C2 . On the other hand, α-cytosine NA 12 was accessed from 10 using Me 2 BBr [22] .

Biological evaluation of the corresponding novel nucleotides necessitated an improvement in the overall synthesis of these C3 quaternary substituted nucleosides. As reported herein (Figure 2 ), this was accomplished by introducing the hydroxyl at C2 through a stereoselective epoxidation of glycal 13 [23, 24] to the epoxide 14, followed by hydrolysis and stereoselective N-glycosylation to generate novel nucleoside analogues 16.

Molecules 2022, 27, x FOR PEER REVIEW 4 of through a stereoselective epoxidation of glycal 13 [23, 24] to the epoxide 14, followed hydrolysis and stereoselective N-glycosylation to generate novel nucleoside analogues Different pathways to prepare key intermediate 21 were explored. Cyclization of dehyde 19 in the presence of PTSA in THF/H2O led to lactols 20a,b with a 1.4:1 anome ratio (Scheme 2). Mesylation and elimination with Et3N generated glycal 21 in 51% yie Alternatively, methylfuranoside 22a,b was derived from aldehyde 19 in the presence PTSA in anhydrous methanol. Methylfuranoside 22a,b were then treated with TMSO and 2,6-lutidine to give glycal 21 in an excellent yield. through a stereoselective epoxidation of glycal 13 [23, 24] to the epoxide 14, followed by hydrolysis and stereoselective N-glycosylation to generate novel nucleoside analogues 16. Dondoni's dimethyldioxirane (DMDO) [25] , generated in situ with a catalytic amount of acetone and a stoichiometric amount of potassium peroxymonosulfate (oxone), was chosen as the oxidant. When glycal 21 was subjected to DMDO oxidation, the ribolike epoxide 23a was obtained as the major epoxide in a 7:1 ratio relative to the arabinolike epoxide 23b (Scheme 3). The stereoselectivity is rationalized by spiro-like transition states TS A and TS B (Scheme 3), where DMDO approaches the olefin from the inside face of the glycals' envelope-like conformations [26] [27] [28] . Attack from the bottom face in TS A avoids significant steric clash with the C5′ substituent in TS B (Scheme 3). Hydrolysis of the crude epoxides 23a and 23b in THF/H2O gave the ribo-like lactols 24a,b as the major compounds (63%, isolated by flash column chromatography). These lactols were then protected with acetyl groups to afford acetate ribofuranosides 25a,b in 85% yield. Scheme 3. Synthesis of diacetate ribofuranosides 25a,b. ‡ represents the "Transition State"

With ribo-like diacetate furanosides 25a,b in hand, stereoselective N-glycosylations of 25a,b were performed in the presence of a silylated base (adenine or cytidine) and TMSOTf (Scheme 4). The 1′,2′-trans ribo-like nucleosides 26-28 were obtained in excellent diastereoselectivity (>20:1 dr) and yield (75-85%) in accordance with anchimeric assistance of the C2′ acetyl group. NMR spectroscopic analysis experiments confirmed the 1,2′trans stereochemistries (2D NOESY) and, in the case of adenine coupling, N 9 isomers (HMBC). Subsequent cleavage of the C5′-OTBDPS protecting groups with 3HF‧NEt3 provided corresponding nucleoside analogues 29-31 (86-87%), which were the key precursors for the formation of the nucleoside C5′O-triphosphates. Further, cleavage of the C2′and C3′ acetyl-protecting groups with NaOMe provided 1′,2′-trans ribo-like nucleoside analogues 32-34 (80-82%). Dondoni's dimethyldioxirane (DMDO) [25] , generated in situ with a catalytic amount of acetone and a stoichiometric amount of potassium peroxymonosulfate (oxone), was chosen as the oxidant. When glycal 21 was subjected to DMDO oxidation, the ribo-like epoxide 23a was obtained as the major epoxide in a 7:1 ratio relative to the arabino-like epoxide 23b (Scheme 3). The stereoselectivity is rationalized by spiro-like transition states TS A and TS B (Scheme 3), where DMDO approaches the olefin from the inside face of the glycals' envelope-like conformations [26] [27] [28] . Attack from the bottom face in TS A avoids significant steric clash with the C5 substituent in TS B (Scheme 3). Hydrolysis of the crude epoxides 23a and 23b in THF/H 2 O gave the ribo-like lactols 24a,b as the major compounds (63%, isolated by flash column chromatography). These lactols were then protected with acetyl groups to afford acetate ribofuranosides 25a,b in 85% yield. Dondoni's dimethyldioxirane (DMDO) [25] , generated in situ with a catalytic amount of acetone and a stoichiometric amount of potassium peroxymonosulfate (oxone), was chosen as the oxidant. When glycal 21 was subjected to DMDO oxidation, the ribolike epoxide 23a was obtained as the major epoxide in a 7:1 ratio relative to the arabinolike epoxide 23b (Scheme 3). The stereoselectivity is rationalized by spiro-like transition states TS A and TS B (Scheme 3), where DMDO approaches the olefin from the inside face of the glycals' envelope-like conformations [26] [27] [28] . Attack from the bottom face in TS A avoids significant steric clash with the C5′ substituent in TS B (Scheme 3). Hydrolysis of the crude epoxides 23a and 23b in THF/H2O gave the ribo-like lactols 24a,b as the major compounds (63%, isolated by flash column chromatography). These lactols were then protected with acetyl groups to afford acetate ribofuranosides 25a,b in 85% yield. With ribo-like diacetate furanosides 25a,b in hand, stereoselective N-glycosylations of 25a,b were performed in the presence of a silylated base (adenine or cytidine) and TMSOTf (Scheme 4). The 1′,2′-trans ribo-like nucleosides 26-28 were obtained in excellent diastereoselectivity (>20:1 dr) and yield (75-85%) in accordance with anchimeric assistance of the C2′ acetyl group. NMR spectroscopic analysis experiments confirmed the 1,2′trans stereochemistries (2D NOESY) and, in the case of adenine coupling, N 9 isomers (HMBC). Subsequent cleavage of the C5′-OTBDPS protecting groups with 3HF‧NEt3 provided corresponding nucleoside analogues 29-31 (86-87%), which were the key precursors for the formation of the nucleoside C5′O-triphosphates. Further, cleavage of the C2′and C3′ acetyl-protecting groups with NaOMe provided 1′,2′-trans ribo-like nucleoside analogues 32-34 (80-82%). With ribo-like diacetate furanosides 25a,b in hand, stereoselective N-glycosylations of 25a,b were performed in the presence of a silylated base (adenine or cytidine) and TMSOTf (Scheme 4). The 1 ,2 -trans ribo-like nucleosides 26-28 were obtained in excellent diastereoselectivity (>20:1 dr) and yield (75-85%) in accordance with anchimeric assistance of the C2 acetyl group. NMR spectroscopic analysis experiments confirmed the 1,2 -trans stereochemistries (2D NOESY) and, in the case of adenine coupling, N 9 isomers (HMBC). Subsequent cleavage of the C5 -OTBDPS protecting groups with 3HF·NEt 3 provided corresponding nucleoside analogues 29-31 (86-87%), which were the key precursors for the formation of the nucleoside C5 O-triphosphates. Further, cleavage of the C2 and C3 acetyl-protecting groups with NaOMe provided 1 ,2 -trans ribo-like nucleoside analogues 32-34 (80-82%). The synthesis of the C2′ quaternary series was then explored. The five-membered ring lactone 40, bearing the quaternary center at C2′, was prepared by a route previously reported by our group [13] (Scheme 5). The Mukaiyama aldol reaction of aldehyde 35 with tetrasubstituted silylated-enolether 36 provided 37a,b. The α-bromomethylesters 37a,b were subjected to lactonization, followed by installation of a TBS at the C5 primary alcohol. Installation of vinyldimethylsilane at the C3′ secondary alcohol provided lactones 39a,b, which were subjected to an atom transfer cyclization/elimination reaction using photoredox catalysis to give lactone 40. An alternative synthetic sequence was then optimized to reduce the number of steps to reach 43a,b [13] . A TBS-protecting group was introduced on the secondary hydroxyl of 40, followed by ozonolysis resulting in aldehyde 41 in excellent yield. After the simultaneous reduction of both the lactone and aldehyde using Red-Al, benzoylation provided furanosides 43a,b. With benzoylated furanosides 43a,b in hand, stereoselective N-glycosidation of 43a,b with 2,6 dichloropurine in the presence of TMSOTf at −10 C led to the regioselective formation of N 9 nucleoside analogue 44 with high diastereoselectivity (β:α, 10:1) and good yield (73%, Scheme 6). The selective formation of the β-anomer is attributed to an anchimeric assistance from C2′ benzoate. The 1′,2′-trans stereochemistry was confirmed by 2D NOESY experiments, while the N 9 regioselectivity was verified by the key indicative three bond correlation between the H1′ of sugar and C4 of purine in 1 H/ 13 C 2D HMBC NMR The synthesis of the C2 quaternary series was then explored. The five-membered ring lactone 40, bearing the quaternary center at C2 , was prepared by a route previously reported by our group [13] (Scheme 5). The Mukaiyama aldol reaction of aldehyde 35 with tetrasubstituted silylated-enolether 36 provided 37a,b. The α-bromomethylesters 37a,b were subjected to lactonization, followed by installation of a TBS at the C5 primary alcohol. Installation of vinyldimethylsilane at the C3 secondary alcohol provided lactones 39a,b, which were subjected to an atom transfer cyclization/elimination reaction using photoredox catalysis to give lactone 40. An alternative synthetic sequence was then optimized to reduce the number of steps to reach 43a,b [13] . A TBS-protecting group was introduced on the secondary hydroxyl of 40, followed by ozonolysis resulting in aldehyde 41 in excellent yield. After the simultaneous reduction of both the lactone and aldehyde using Red-Al, benzoylation provided furanosides 43a,b. 27 The synthesis of the C2′ quaternary series was then explored. The five-membered ring lactone 40, bearing the quaternary center at C2′, was prepared by a route previously reported by our group [13] (Scheme 5). The Mukaiyama aldol reaction of aldehyde 35 with tetrasubstituted silylated-enolether 36 provided 37a,b. The α-bromomethylesters 37a,b were subjected to lactonization, followed by installation of a TBS at the C5 primary alcohol. Installation of vinyldimethylsilane at the C3′ secondary alcohol provided lactones 39a,b, which were subjected to an atom transfer cyclization/elimination reaction using photoredox catalysis to give lactone 40. An alternative synthetic sequence was then optimized to reduce the number of steps to reach 43a,b [13] . A TBS-protecting group was introduced on the secondary hydroxyl of 40, followed by ozonolysis resulting in aldehyde 41 in excellent yield. After the simultaneous reduction of both the lactone and aldehyde using Red-Al, benzoylation provided furanosides 43a,b. With benzoylated furanosides 43a,b in hand, stereoselective N-glycosidation of 43a,b with 2,6 dichloropurine in the presence of TMSOTf at −10 C led to the regioselective formation of N 9 nucleoside analogue 44 with high diastereoselectivity (β:α, 10:1) and good yield (73%, Scheme 6). The selective formation of the β-anomer is attributed to an anchimeric assistance from C2′ benzoate. The 1′,2′-trans stereochemistry was confirmed by 2D NOESY experiments, while the N 9 regioselectivity was verified by the key indicative three bond correlation between the H1′ of sugar and C4 of purine in 1 H/ 13 C 2D HMBC NMR experiments. Nucleoside 45 was formed from treatment with ammonia in methanol to give the 2-chloroadenosine and deprotection of the C2′ benzoate, followed by desilylation With benzoylated furanosides 43a,b in hand, stereoselective N-glycosidation of 43a,b with 2,6 dichloropurine in the presence of TMSOTf at −10 • C led to the regioselective formation of N 9 nucleoside analogue 44 with high diastereoselectivity (β:α, 10:1) and good yield (73%, Scheme 6). The selective formation of the β-anomer is attributed to an anchimeric assistance from C2 benzoate. The 1 ,2 -trans stereochemistry was confirmed by 2D NOESY experiments, while the N 9 regioselectivity was verified by the key indicative three bond correlation between the H1 of sugar and C4 of purine in 1 H/ 13 C 2D HMBC NMR experiments. Nucleoside 45 was formed from treatment with ammonia in methanol to give the 2-chloroadenosine and deprotection of the C2 benzoate, followed by desilylation in the presence of 3HF·NEt 3 . The 2-chloroadenosine analogue 45 was then hydrogenated to the adenosine derivative 46 in 78% yield. The cytidine analogue 47 was also prepared from the benzoylated furanoside, as reported by our group [13] . to the adenosine derivative 46 in 78% yield. The cytidine analogue 47 was also prepared from the benzoylated furanoside, as reported by our group [13] . Scheme 6. Synthesis of 1′,2′-trans ribo-like adenine nucleoside analogues.

Having established synthetic routes to access nucleoside analogues bearing an all carbon quaternary stereocenter at C2′ or C3′, we next investigated the synthesis of their C5′ triphosphate derivatives. The synthesis of NTPs represents a challenging task [29] Initial triphosphorylation attempts using Taylor's method, using trimetaphosphate and mesitylenesulfonyl chloride, were unsuccessful [30] . The one-pot synthesis approach o Huang and co-workers [31] was more appropriate for our substrates. The phosphoryla tion is accomplished under mild conditions using tributylammonium pyrophosphate in the presence of salicyl phosphorochloridite (SalPCl).

Phosphorylation of 1′,2′-trans ribo-like nucleoside analogues bearing either purine or pyrimidine nucleobases was carried out using SalPCl, (Bu2HN)2H2P2O7 and Bu3N in an hydrous DMF. Subsequent addition of protected nucleoside provided the five-cyclic tri phosphite intermediates that were then subjected to iodine oxidation and hydrolysis. The cleavage of the C2′ and C3′acetyl-protecting groups with ammonium hydroxide (NH4OH then generated the corresponding 1′,2′-trans ribo-like nucleoside 5′-triphosphate (Scheme 7). The final 1′,2′-trans ribo-like NTPs (2-4) were prepared in good yields (27-51%, Scheme 7) from the nucleoside analogues 29-31, respectively. Scheme 6. Synthesis of 1 ,2 -trans ribo-like adenine nucleoside analogues.

Having established synthetic routes to access nucleoside analogues bearing an allcarbon quaternary stereocenter at C2 or C3 , we next investigated the synthesis of their C5 triphosphate derivatives. The synthesis of NTPs represents a challenging task [29] . Initial triphosphorylation attempts using Taylor's method, using trimetaphosphate and mesitylenesulfonyl chloride, were unsuccessful [30] . The one-pot synthesis approach of Huang and co-workers [31] was more appropriate for our substrates. The phosphorylation is accomplished under mild conditions using tributylammonium pyrophosphate in the presence of salicyl phosphorochloridite (SalPCl).

Phosphorylation of 1 ,2 -trans ribo-like nucleoside analogues bearing either purine or pyrimidine nucleobases was carried out using SalPCl, (Bu 2 HN) 2 H 2 P 2 O 7 and Bu 3 N in anhydrous DMF. Subsequent addition of protected nucleoside provided the five-cyclic triphosphite intermediates that were then subjected to iodine oxidation and hydrolysis. The cleavage of the C2 and C3 acetyl-protecting groups with ammonium hydroxide (NH 4 OH) then generated the corresponding 1 ,2 -trans ribo-like nucleoside 5 -triphosphate (Scheme 7). The final 1 ,2 -trans ribo-like NTPs (2-4) were prepared in good yields (27-51%, Scheme 7) from the nucleoside analogues 29-31, respectively.

The phosphorylation of nucleoside analogues bearing a C2 quaternary stereocenter was then conducted using the same strategy. Triphosphorylation of unprotected NAs 46-47 furnished the corresponding NTPs 5-7 in low, but acceptable yields, for this challenging transformation (5-13%, Scheme 8).

hydrous DMF. Subsequent addition of protected nucleoside provided the five-cyclic phosphite intermediates that were then subjected to iodine oxidation and hydrolysis. cleavage of the C2′ and C3′acetyl-protecting groups with ammonium hydroxide (NH4O then generated the corresponding 1′,2′-trans ribo-like nucleoside 5′-triphosphate (Sch 7). The final 1′,2′-trans ribo-like NTPs (2-4) were prepared in good yields (27-51%, Sch 7) from the nucleoside analogues 29-31, respectively. The phosphorylation of nucleoside analogues bearing a C2′ quaternary stereoce was then conducted using the same strategy. Triphosphorylation of unprotected NAs 47 furnished the corresponding NTPs 5-7 in low, but acceptable yields, for this chall ing transformation (5-13%, Scheme 8).

Scheme 8. Synthesis of NTPs bearing C2′ quaternary stereocenters.

Preliminary results for SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) i bition suggested that the most active analogues were LCB-2344 (6) and LCB-2279 which both bear purine nucleobases and the quaternary stereogenic center at C2′ (Fig  3) . These NTPs (6 and 7) appear to have just slightly lower activity than the comme Remdesivir TP (1) [32] . NTP 5, bearing a cytosine nucleobase, was inactive. Nucleo analogues (2) (3) (4) , having the adenine, chloro-adenine and cytosine nucleobases at C1′ the quaternary center at C3′, showed a lower activity profile. Scheme 8. Synthesis of NTPs bearing C2 quaternary stereocenters.

Preliminary results for SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) inhibition suggested that the most active analogues were LCB-2344 (6) and LCB-2279 (7), which both bear purine nucleobases and the quaternary stereogenic center at C2 (Figure 3 ). These NTPs (6 and 7) appear to have just slightly lower activity than the commercial Remdesivir TP (1). We also prepared the REM-TP (1) using the approach described at Scheme 8, the activity against RdRp obtained with this product was similar to 6 (LCB-2344). NTP 5, bearing a cytosine nucleobase, was inactive. Nucleotide analogues (2) (3) (4) , having the adenine, chloro-adenine and cytosine nucleobases at C1 and the quaternary center at C3 , showed a lower activity profile. 

All the anhydrous solvents were purchased from Sigma Aldrich, USA. All the glassware was purchased from Pyrex USA. Reactions requiring anhydrous conditions were performed under an atmosphere of nitrogen in flame-dried glassware using standard syringe techniques. Molecular sieves (Sigma Aldrich, USA) were used to prepare the anhydrous solvents. The sieves (4 Å , 1-2 mm beads) were activated by heating at 180 °C for 48 h under vacuum prior to their addition into new bottles of solvent purged with argon. All commercially available reagents were used as received: TBSCl was purchased from TCI America, USA and TBSOTf was obtained from Oakwood Chemicals, USA. Remaining all reagents were purchased from Sigma Aldrich, USA. Flash chromatography was done on silica gel 60 (0.040-0.063 mm, Silicycle, QC, Canada) using an automated purification system. Thin-layer chromatography (TLC) was done on pre-coated (0.25 mm) Merck F-254 silica gel aluminum plates. Visualization was performed with short UV wavelengths and/or revealed with potassium permanganate solutions. See the 1 H, 13 C, and 2D NMR 

All the anhydrous solvents were purchased from Sigma Aldrich, Saint Louis, MO, USA. All the glassware was purchased from Pyrex USA. Reactions requiring anhydrous conditions were performed under an atmosphere of nitrogen in flame-dried glassware using standard syringe techniques. Molecular sieves (Sigma Aldrich, USA) were used to prepare the anhydrous solvents. The sieves (4 Å, 1-2 mm beads) were activated by heating at 180 • C for 48 h under vacuum prior to their addition into new bottles of solvent purged with argon. All commercially available reagents were used as received: TBSCl was purchased from TCI America, USA and TBSOTf was obtained from Oakwood Chemicals, USA. Remaining all reagents were purchased from Sigma Aldrich, USA. Flash chromatography was done on silica gel 60 (0.040-0.063 mm, Silicycle, QC, Canada) using an automated purification system. Thin-layer chromatography (TLC) was done on pre-coated (0.25 mm) Merck F-254 silica gel aluminum plates. Visualization was performed with short UV wave-lengths and/or revealed with potassium permanganate solutions. See the 1 H, 13 C, and 2D NMR spectra data in the Supplementary Materials. The 1 H NMR spectra were recorded at room temperature on a 700 MHz, 500 MHz and 400 MHz NMR spectrometer and the data is reported as follows: chemical shift in ppm referenced to residual solvent (CDCl 3 δ 7.26, CD 3 OD δ 3.31 and D 2 O δ 4.79 ppm), multiplicity (s = singlet, d = doublet, dd = doublet of doublets, ddd = doublet of doublets of doublets, t = triplet, td = triplet of doublets, m = multiplet, app = apparent), coupling constants (Hz), and integration. The 13 C NMR spectra were recorded at room temperature using 176 MHz and 126 MHz with the data reported as follows: chemical shift in ppm referenced to residual solvent (CDCl 3 δ 77. 16 and CD 3 OD δ 49.00 ppm). The 31 P NMR spectra were recorded at room temperature using 162 MHz. Infrared spectra were recorded on a Fourier transform infrared spectrophotometer from a thin film of purified product or with single reflection diamond attenuated total reflection module, and signals are reported in cm −1 . Mass spectra were recorded through electrospray ionization with positive ion mode. A Hybrid Quadrupole Orbitrap mass analyzer was used for high-resolution mass spectrometry (HRMS) measurements. Optical rotations were measured at room temperature from the sodium D line (589 nm) using the formula: [α] D = (100)α obs //( ·(c)), where c = (g of substrate/100 mL of solvent) and = 1 dm. The sequence from 35 to 40 in the Scheme 5, and compounds 5 and 47 [13] were prepared as reported in our previous publications [13, 14] . (17) . To a solution of secondary alcohol 5 [14] (3.90 g, 9.14 mmol, 1.00 equiv.) in anhydrous DCM (45 mL, 0.20 M), imidazole (1.56 g, 22.9 mmol, 2.50 equiv.) was added, immediately followed by triethylchlorosilane (2.30 mL, 13.7 mmol, 1.50 equiv.). The resulting mixture was stirred at room temperature for 16 h. The mixture was diluted with DCM (25 mL) and saturated NH 4 (+)-((2S,3R)-2-(((tert-Butyldiphenylsilyl)oxy)methyl)-3-methyl-2,3-dihydrofuran-3-yl)methyl acetate (21) . To a solution of methoxy lactols 22a,b (0.12 g, 0.25 mmol, 1.0 equiv.) in anhydrous DCM (1.3 mL, 0.2 M), 2,6-lutidine (0.12 mL, 1.0 mmol, 4.0 equiv.) and TMSOTf (0.09 mL, 0.5 mmol, 2 equiv.) were added at 0 • C [32] .The resulting solution was stirred for 30 min at room temperature. The mixture was then diluted in DCM (5 mL) followed by washing with water (5 mL), and the aqueous layer was extracted with DCM (3 × 5 mL). The combined organic layers were dried over MgSO 4 , filtered and concentrated under reduced pressure. Purification by silica gel flash chromatography (Hexanes/EtOAc, 90:10) provided glycal 21 (96 mg, 90%) as a colorless oil, which was confirmed to be identical to the compound formed from 20a,b (see above).

((2S,3S,4R)-2-(((tert-Butyldiphenylsilyl)oxy)methyl)-4,5-dihydroxy-3-methyltetra hydrofuran-3-yl)methyl acetate (24a,b) . To a solution of glycal 21 (118 mg, 0.278 mmol, 1.00 equiv.) in anhydrous DCM (1.3 mL, 0.22 M) at 0 • C, acetone (0.13 mL, 1.6 mmol, 6.0 equiv.) and a saturated NaHCO 3 solution (2.5 mL) were added. To the resulting biphasic mixture, a 0.37 mM solution of oxone in water (1.5 mL) was added. After sealing the flask, the resulting mixture was stirred for 30 min at 0 • C and 3 h at room temperature. After degassing the flask, the aqueous phase was extracted with DCM (3 × 5 mL). The organic layers were dried over MgSO 4 , filtered and concentrated under reduced pressure. The resulting crude epoxide 23a and 23b (dr 7:1, determined by 1 (−)-(3R,4S,5R)-4-((tert-Butyldimethylsilyl)oxy)-5-(((tert-butyldimethylsilyl)oxy)methyl)-3methyl-3-vinyldihydrofuran-2(3H)-one (S2). To a solution of secondary alcohol 40 (4.00 g, 14.0 mmol, 1.00 equiv.) in anhydrous DCM (42 mL, 0.33 M), 2,6 Lutidine (4.04 mL, 34.9 mmol, 2.50 equiv.) and TBSOTf (4.81 mL, 2.09 mmol, 1.50 equiv.) were added at 0 • C. The resulting mixture was gradually warmed to room temperature and stirring was continued for overnight. A saturated aqueous solution of NaHCO 3 (~20 mL) was added and the mixture was extracted with CH 2 Cl 2 (3 × 40 mL). The combined organic layers were washed with brine, dried over MgSO 4 , filtered and condensed under reduced pressure. (+)-(3S,4S,5R)-4-((tert-Butyldimethylsilyl)oxy)-5-(((tert-butyldimethylsilyl)oxy)methyl)-3methyl-2-oxotetrahydrofuran-3-carbaldehyde (41). The alkene S2 (761 mg, 1.90 mmol, 1.00 equiv.) was dissolved in CH 2 Cl 2 (114 mL, 0.02 M) and the mixture was cooled to −78 • C. Ozone was bubbled through the reaction mixture until a blue color appeared at which point the ozone inlet was changed for a N 2 inlet and bubbling was continued for 15 min. The solution was then purged with nitrogen to remove excess of ozone. Triethylamine (0.265 mL, 1.90 mmol, 1.00 equiv.) was added and the mixture was stirred for 30 min at −78 • C. Then, it was gradually warmed to room temperature and stirring was continued for 1 h. The reaction mixture was filtered over MgSO 4 (43a,b) . To a solution of lactol 42a,b (3.30 g, 8.11 mmol, 1.00 equiv.) in CH 2 Cl 2 (45 mL, 0.18 M), pyridine (3.94 mL, 48.7 mmol, 6.00 equiv.) and DMAP (99 mg, 0.81 mmol, 0.10 equiv.) were added. After cooling the resulting mixture to 0 • C, BzCl (4.71 mL, 40.6 mmol, 5.00 equiv.) was added dropwise. The reaction mixture was gradually warmed to room temperature and stirred for 16 h. The mixture was cooled to 0 • C and ethylenediamine (1.36 mL, 20.3 mmol, 2.50 equiv.) was added and stirring was continued for 1 h at 0 • C. The reaction mixture was then diluted with hexanes (40 mL) and passed through a celite using Et 2 O. The filtrates were concentrated under reduced pressure. The residue was purified by flash chromatography on silica gel (Hexanes/EtOAc, 4:1) to afford the 43a,b (3.8 g, 76%, dr 1. (−)((2R,3R,4S,5R)-4-((tert-Butyldimethylsilyl)oxy)-5-(((tert-butyldimethylsilyl)oxy)methyl)-2-(2,6-dichloro-9H-purin-9-yl)-3-methyltetrahydrofuran-3-yl)methyl benzoate (44). To a solution of the benzoylated furanosides 43a,b (434 mg, 0.706 mmol, 1.00 equiv.) in anhydrous MeCN (2.8 mL, 0.25 M), 2,6-dichloropurine (147 mg, 0.776 mmol, 1.10 equiv.) was added at room temperature. The resulting mixture was cooled to −10 • C and DBU (0.316 mL, 2.12 mmol, 3.00 equiv.) was added followed by dropwise addition of TMSOTf (0.520 mL, 2.82 mmol, 4.00 equiv.). The stirring was continued at −10 • C for 3 h. The mixture was warmed to room temperature, and a saturated solution of NaHCO 3 (~8 mL) was added. The aqueous layer was extracted with CH 2 Cl 2 (3 × 20 mL) and the combined organic layers were washed with brine, dried over MgSO 4 , filtered and concentrated under reduced pressure. The crude was purified by flash chromatography on silica gel (CH 2 Cl 2 /MeOH, 100:0 to 70:30) to provide the pure product 44 (375 mg, 78%, β:α = 10:1) as brown gum. R f = 0.87 (MeOH/CH 2 (−)-((2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-3-hydroxy-4-methyltetrahydrofuran-2,4-diyl) dimethanol (46). To a solution of nucleoside 45 (70 mg, 0.21 mmol, 1.0 equiv) in methanol (10 mL, 0.021 M), palladium (10 wt.%) on activated carbon (90 mg, 85 µmol, 0.40 equiv.) was added. The reaction mixture was degassed and flushed using a hydrogen-filled balloon. The resulting reaction was stirred for overnight at 40 • C. The reaction mixture was filtered through Celite ® , washed with methanol, and filtrates were concentrated under reduced pressure to provide 46 (49 mg, 78%). The crude was pure by NMR and used for the next step without purification. R f = 0.27 (CH 2 Cl 2 /CH 3 (m, 2H), 3.77 (d, J = 11.2 Hz, 1H), 0.67 (s, 3H) OH and NH signals are missing possibly due to exchange in CD 3 OD ppm; 13 31 and 45-47) , salicyl phosphorochloridite (SalPCl) and tributylammonium pyrophosphate [(Bu 3 HN) 2 H 2 P 2 O 7 ] were respectively dried under reduced pressure in 10 mL, 5 mL, 5 mL flasks for 1 h. To a solution of (Bu 3 HN) 2 H 2 P 2 O 7 (1.2-2.5 equiv.) in anhydrous DMF (0.1 M), NBu 3 (0.25 M) was added under nitrogen atmosphere and the mixture was stirred until (5 min) it became homogenious. The reaction mixture then was injected into a 5 mL flask containing SalPCl (1.2-2.5 equiv.) and the resulting mixture was stirred at room temperature for 30 min. The mixture was then transferred to a flask containing nucleoside 29-31 and 45-47 (1.0 equiv.) and the resulting mixture was stirred for 1.5 h. A solution of iodine (3% in Pyr: H 2 O 9:1 wV) was injected dropwise into the solution until a permanent brown color was persisted (~0.5 mL) and the resulting mixture was stirred for 15 min. Water (1.5 mL) was added and the solution was stirred for 1.5 h to provide the desired C5 -triphosphate which was detected by TLC (i-PrOH: NH 4 OH: H 2 O, 5:3:2). The reaction mixture was transferred into a centrifuge tube using 15 mL of EtOH. A solution of 3M NaCl was added dropwise until the reaction mixture became cloudy (~0.5 mL) and was cooled to -78 • C for 1 h. Centrifugation was conducted at 10 • C with 3200 rpm for 20 min and the resulting liquid phase was then transferred to a 50 mL Erlenmeyer flask. The resulting solid (residue) inside the centrifuge tube was air dried for 15 min. The residue was purified by reverse phase C18 flash chromatography (MeCN in 20 mM triethylammonium acetate (TEAAc) buffer, pH = 7) to provide corresponding nucleoside triphosphate triethylammonium salt, which was then lyophilized to provide pure solid nucleoside triphosphate as a white powder.

General Procedure D: If the nucleside analogues hydroxyl or amine functional groups are protected with acetyl (Ac) or benzoyl (Bz) follow the Gerenaral Procedure C until centrifiguation. Residue (solid) inside the centrifuge tube was dissolved in NH 4 OH (0.02 M) and was stirred for overnight. The mixture was concentrated under reduced pressure. The residue was purified by reverse phase C18 flash chromatography (MeCN in 20 mM triethylammonium acetate buffer, pH = 7) to provide corresponding nucleoside triphosphate triethylammonium salt, which was then lyophilized to provide pure solid nucleoside triphosphate as a white powder.

((2S,3S,4R,5R)-5-(4-Amino-2-oxopyrimidin-1(2H)-yl)-4-hydroxy-3-(hydroxymethyl)-3methyltetrahydrofuran-2-yl)methyl tetrahydrogen triphosphate 2 (LCB-2330). Following general procedure D, (Bu 3 HN) 2 H 2 P 2 O 7 (0.11 g, 0.20 mmol, 2.5 equiv.), NBu 3 ((2S,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-4-hydroxy-3-(hydroxymethyl)-3-methyltetrahydrofuran -2-yl)methyl tetrahydrogen triphosphate 3 (LCB-2332). Following general procedure D, (Bu 3 HN) 2 H 2 P 2 O 7 (0.10 g, 0.18 mmol, 2.5 equiv.), NBu 3 (0.29 mL, 0.25 M), SalPCl (37 mg,

The syntheses of novel NTP analogues bearing a quaternary all-carbon stereogenic center at C3 and C2 have been achieved. The stereogenic quaternary center at C2 or C3 were generated by photocatalyzed cyclization/elimination free-radical-based reactions through five-exo-trig cyclization in the C2 quaternary series and seven-endo trig cyclization in the C3 quaternary series. The installation of the hydroxyl at C2 was significantly improved through a stereoselective epoxidation, providing access to NA-containing quaternary carbon at C3 . A modified approach was also presented for the synthesis of NAs of C2 quaternary center series. The synthesis and purification of the corresponding nucleoside triphosphates are reported for the first time. Optimization for NTPs bearing C2 quaternary stereogenic center is under development.

Finding novel molecules that could act as antiviral agents against SARS-CoV-2, or other emerging viruses, is an important venue for the present and future treatment of these infections. We have reported herein two inhibitors, 6 (LCB-2344) and 7 (LCB-2279), against SARS-CoV-2 RdRp, which are lead molecules for further optimization. The stereogenic quaternary center at C2 will be further modified to improve the potency of the novel series of molecules. Currently the study is underway to explore the monophosphorylated pro-drugs of these molecules and their antiviral efficacity.

The final mixture was concentrated under reduced pressure. Purification by reverse phase C18 flash chromatography (flow rate of 10 mL/min., gradient run of acetonitrile from 0 to 10% in 20 mM TEAAc, pH =7) provided nucleoside triphosphate 3 (LCB-2332) triethylammonium salt (18 mg, 27%) as a white powder. Formula: C 12 H 20 N 5 O 13 P 3 ; MW: 535.24 g/mol; 1 H NMR (500 MHz, D 2 O, signals for triethylammonium denoted by *) δ 8.66 (br s, 1H)

NBu 3 (0.35 mL, 0.25 M), SalPCl (27.0 mg, 0.133 mmol, 2.20 equiv.) and nucleoside analogue 31 (25.0 mg, 0.06 mmol, 1.00 equiv.) in anhydrous DMF (0.5 mL, 0.1 M) were employed in the phosphorylation. The final mixture was concentrated under reduced pressure. Purification by reverse phase C18 flash chromatography (flow rate of 10 mL/min., gradient run of acetonitrile from 0 to 10% in 20 mM TEAAc, pH = 7) provided nucleoside triphosphate 4 (LCB-2337) triethylammonium salt (30 mg, 51%) as a white powder

24 (s, 3H) ppm; OH and NH 2 signals are missing possibly due to exchange in D 2 O; 13 C NMR (126 MHz, D 2 O, signals for triethylammonium denoted by *) δ 156.3, 153.8, 150.7 (Brs), 140.2, 117.54 (Brs)

mg, 147 µmol, 1.00 equiv.) in anhydrous DMF (1 mL, 0.1 M) were employed in the phosphorylation. The final mixture was concentrated under reduced pressure. Purification by reverse phase C18 flash chromatography (flow rate of 8 mL/min., gradient run of acetonitrile from 0 to 6% in 20 mM TEAAc

MHz, D 2 O, signals for triethylammonium denoted by *) δ 8.14 (d, J = 7.8 Hz, 1H), 6.25 (d, J = 9.5 Hz, 1H)

SalPCl (17.0 mg, 0.085 mmol, 1.20 equiv.) and nucleoside analogue 46 (21.0 mg, 0.071 mmol, 1.00 equiv.) in anhydrous DMF (0.7 mL, 0.1 M) were employed in the phosphorylation. Purification by reverse phase C18 flash chromatography (flow rate of 8 mL/min., gradient run of acetonitrile from 0 to 8% in 20 mM TEAAc, pH =7) provided nucleoside triphosphate 6 (LCB-2344) triethylammonium salt (3.4 mg, 5%) as a white powder. Formula: C 12 H 20 N 5 O 13 P 3 ; MW: 535.23 g/mol; 1 H NMR (500 MHz, D 2 O, signals for triethylammonium denoted by *) δ 8.60 (Br s, 1H), 8.28 (s, 1H)

Purification by reverse phase C18 flash chromatography (flow rate of 12 mL/min., gradient run of acetonitrile from 0 to 20% in 20 mM TEAAc, pH = 7) provided nucleoside triphosphate 7 (LCB-2279) triethylammonium salt (10 mg, 11%) as a white powder. Formula: C 12 H 19 ClN 5 O 13 P 3 ; MW: 569.68 g/mol; 1 H NMR (500 MHz, D 2 O, signals for triethylammonium denoted by *) δ 8.52 (Br s, 1H)

D 2 O) −6.42 (d, J = 21.5 Hz, 1P)

HRMS (ESI)

The RNA synthesis activity of the RdRp complex was evaluated in a reaction mixture comprising a 19-mer RNA primer, 43-mer RNA template, 25 mM TRIS-HCl (pH8), cold NTPs (50 µM ATP, CTP and UTP; 25 µM GTP), 0.1 µM [α-32P]-GTP and different concentrations of each inhibitor of interest. Nuclease-free water was added in place of the RdRp or the inhibitors for the negative control (−Pol) or no-treatment control (+Pol), respectively. After 10 min incubation at 30 • C, 5mM MnCl2 was added into each reaction mix to initiate the RdRp reaction. After another 30 min incubation at 30 • C, the RdRp reactions were terminated with formamide containing 40 mM EDTA, and were heated at 95 • C for 10 min

Structure of the RNA-dependent RNA polymerase from COVID-19 virus

Adenine C-Nucleoside (GS-5734) for the Treatment of Ebola and Emerging Viruses

Characterization of orally efficacious influenza drug with high resistance barrier in ferrets and human airway epithelia

Nucleotide Analogues as Inhibitors of SARS-CoV-2 Polymerase, a Key Drug Target for COVID-19

RNA-dependent RNA polymerase (RdRp) inhibitors: The current landscape and repurposing for the COVID-19 pandemic

Remdesivir is a direct-acting antiviral that inhibits RNA-dependent RNA polymerase from severe acute respiratory syndrome coronavirus 2 with high potency

Mechanism of SARS-CoV-2 polymerase stalling by remdesivir

Template-dependent inhibition of coronavirus RNA-dependent RNA polymerase by remdesivir reveals a second mechanism of action

Repurposing Nucleoside Analogs for Human Coronaviruses

Diastereoselective Synthesis of C2'-Fluorinated Nucleoside Analogues Using an Acyclic Approach

Dual-face nucleoside scaffold featuring a stereogenic all-carbon quaternary center

Synthesis of nucleoside analogues using acyclic diastereoselective reactions

Photoredox-Catalyzed Stereoselective Radical Reactions to Synthesize Nucleoside Analogues with a C2'-Stereogenic All-Carbon Quaternary Center

Diastereoselective Synthesis of Arabino-and Ribo-like Nucleoside Analogues Bearing a Stereogenic C3' All-Carbon Quaternary Center

Identification of a C3 '-nitrile nucleoside analogue inhibitor of pancreatic cancer cell line growth

Nucleoside Analogues and Methods of use thereof

Nucleoside and Nucleotide Analogues Bearing a Quaternary All-Carbon Stereogenic Center at the 2' Position and Methods of Use as a Cardioprotective Agent

Nucleoside and Nucleotide Analogues with Quaternary Carbon Centers and Methods of Use

The history of N-methanocarbathymidine: The investigation of a conformational concept leads to the discovery of a potent and selective nucleoside antiviral agent

Selected Diastereoselective Reactions: Free Radical Additions and Cyclizations. Chapter

Diastereoselective hydrogen-transfer reactions: An experimental and DFT study

Synthesis of 1',2'-cis-nucleoside analogues: Evidence of stereoelectronic control for SN2 reactions at the anomeric center of furanosides

Stereospecific Vorbruggen-Like Reactions of 1,2-Anhydro Sugars-An Alternative Route to the Synthesis of Nucleosides

Stereoselective Synthesis of Acortatarins A and B

Direct epoxidation of D-glucal and D-galactal derivatives with in situ generated DMDO

Epoxidation of Alkenes by Dimethyldioxirane-Evidence for a Spiro Transition-State

Nucleophilic additions to fused bicyclic five-membered ring oxocarbenium ions: Evidence for preferential attack on the inside face

Torsional control of stereoselectivities in electrophilic additions and cycloadditions to alkenes

Syntheses of nucleoside triphosphates

Synthesis of Nucleoside Triphosphates from 2'-3'-Protected Nucleosides Using Trimetaphosphate

Protection-Free One-Pot Synthesis of 2'-Deoxynucleoside 5'-Triphosphates and DNA Polymerization

2'-Fluorination of tricyclo-DNA controls furanose conformation and increases RNA affinity

A Scalable Synthesis of alpha-L-Threose Nucleic Acid Monomers

We gratefully acknowledge Baek Kim and Adrian Oo from Emory University

The data presented in this study are available in the paper and Supporting Information.