Synthesis of 4′-Thionucleoside Analogues Bearing a C2′ Stereogenic All-Carbon Quaternary Center

The design of novel 4′-thionucleoside analogues bearing a C2′ stereogenic all-carbon quaternary center is described. The synthesis involves a highly diastereoselective Mukaiyama aldol reaction, and a diastereoselective radical-based vinyl group transfer to generate the all-carbon stereogenic C2′ center, along with different approaches to control the selectivity of the N-glycosidic bond. Intramolecular SN2-like cyclization of a mixture of acyclic thioaminals provided analogues with a pyrimidine nucleobase. A kinetic bias favoring cyclization of the 1′,2′-anti thioaminal furnished the desired β-D-4′-thionucleoside analogue in a 7:1 ratio. DFT calculations suggest that this kinetic resolution originates from additional steric clash in the SN2-like transition state for 1′,4′-trans isomers, causing a significant decrease in their reaction rate relative to 1′,4′-cis counterparts. N-glycosylation of cyclic glycosyl donors with a purine nucleobase enabled the formation of novel 2-chloroadenine 4′-thionucleoside analogues. These proprietary molecules and other derivatives are currently being evaluated both in vitro and in vivo to establish their biological profiles.


Introduction
Natural nucleosides and nucleotides are involved in a plethora of biological processes including metabolism, cell signalling, and replication, all of which are often disrupted in various pathological conditions.Consequently, significant efforts have been dedicated to the development, synthesis, and investigation of nucleoside analogues with the aim of restoring normal cellular or organ homeostasis.These analogues act by competing with natural nucleosides with improved binding to targeted enzymes or receptors with some level of selectivity.This approach has led to the discovery of clinically important antiviral and anticancer agents [1,2].Modifications of the furanose ring have been extensively studied, and new synthetic approaches are continuously being developed.Substituting the endocyclic oxygen with a sulfur provides 4 ′ -thionucleoside analogues.The enhanced metabolic stability of 4 ′ -thionucleosides or nucleotides towards phosphorylases, phosphatases, and hydrolysis justifies these structural alterations [3].The presence of a larger and less electronegative heteroatom in the ring can lead to subtle changes in the anomeric effect and the conformation of the ring structure [4].Consequently, these modifications may influence the biological behavior of a nucleoside compared to its 4 ′ -thionucleoside counterpart.
The synthesis of 4 ′ -thionucleosides still requires improvement and the development of novel approaches, an objective pursued herein.The approaches for the synthesis of 4 ′ -thionucleosides have been divided into three main categories (Scheme 1) [3,5].The first Over the last decade, inspired by the acyclic strategy pioneered by Liotta's synthesis of AZT [8], we have dedicated substantial efforts to developing a novel and complementary acyclic approach (Scheme 1c) for synthesizing nucleosides and 4 ′ -thionucleoside analogues.From a conceptual standpoint, our approach takes advantage of a cyclization of an acyclic precursor already containing the nucleobase and a thioether at C1 ′ that may serve as a leaving group to give the corresponding nucleoside following an O4 ′ -to-C1 ′ cyclization (Scheme 2a).Alternatively, when a leaving group is installed at C4 ′ , the C1 ′ thioether may serve as a nucleophile resulting in 4 ′ -thionucleosides through a S1 ′ -to-C4 ′ cyclization.We demonstrated that both intramolecular cyclizations involve S N 2-like nucleophilic displacements.In the O4 ′ -to-C1 ′ displacement, the stereochemistry of the thioether at C1 ′ is inverted, whereas in the S1 ′ -to-C4 ′ cyclization, the stereochemistry of the thioether at C1 ′ remains unaltered.Both cyclizations are very robust considering that a change in the C2 ′ and C3 ′ stereochemistry results in high levels of stereoselectivity and yield, regardless of the steric tension generated in the newly formed furanoside ring [9].Scheme 2. (a) Acyclic approach for the intramolecular cyclization of 1′,2′-syn thioaminals leading to either 1′,2′-trans furanosides through an O4′-to-C1′ cyclization or 1′,2′-cis 4′-thioanalogues through a S1′-C4′ cyclization [9,10].(b) Transition state for the stereoselective addition of the silylated nucleobase [11].(c) Acyclic approach for the synthesis of C2′ F 4′-thiofuranosides in addition to those bearing an all-carbon quaternary center at C3′ (d) [10,12].
In the early stages of developing our acyclic strategy for nucleoside synthesis, both the 1′,2′-syn and anti thioaminals were prepared as a mixture from the corresponding dithioacetals.In presence of a C2 oxygen, activation at low temperatures with Me2S(SMe)BF4 or I2 resulted in a significant increase in 1′,2′-syn product formation [9,11].DFT transition state calculations indicated that this selectivity could be attributed to the preferred addition opposite to the R group and the counteranion of thiocarbenium intermediates having the C2-alkoxy group gauche to the thioether moiety (Scheme 2b) [11].With this approach, 4′-thionucleosides bearing a C2′-alkoxy or fluoride have been successfully synthesized in both the L-and D-series (Scheme 2a,c) [9,10,12].
Our interest in investigating acyclic carbon-centered free radicals and their reactivity in atom transfer reactions has enabled the generation of all-carbon stereogenic quaternary centers.These centers have been successfully incorporated at the C3′ or C2′ positions of furanoside scaffolds, leading to the development of novel families of nucleoside or nucleotide analogues (an example of which is depicted in Scheme 2d).The presence of an allcarbon quaternary center is expected to induce a conformational bias favoring a south conformation (DNA-like) when located at C3′ and a north conformation (RNA-like) when at C2′.The presence of the hydroxymethyl group on the quaternary center could act as an extended pharmacophore providing additional binding to proximal entities.Alternatively, nucleoside analogues bearing a C2′ or C3′ all-carbon quaternary center will not be recognized by enzymes or receptors susceptible to steric hinderance at these positions.These novel nucleosides bearing a quaternary center at C3′ have shown activities against gemcitabine-resistant KRAS mutated pancreatic cell lines [13,14].C2′ analogues showed inhibition of SARS-CoV-2 RNA dependant RNA polymerase (RdRp), the causal virus of , while others have shown great promise as cardioprotective agents for the treatment of heart failure [16].
In the early stages of developing our acyclic strategy for nucleoside synthesis, both the 1 ′ ,2 ′ -syn and anti thioaminals were prepared as a mixture from the corresponding dithioacetals.In presence of a C2 oxygen, activation at low temperatures with Me 2 S(SMe)BF 4 or I 2 resulted in a significant increase in 1 ′ ,2 ′ -syn product formation [9,11].DFT transition state calculations indicated that this selectivity could be attributed to the preferred addition opposite to the R group and the counteranion of thiocarbenium intermediates having the C2-alkoxy group gauche to the thioether moiety (Scheme 2b) [11].With this approach, 4 ′ -thionucleosides bearing a C2 ′ -alkoxy or fluoride have been successfully synthesized in both the L-and D-series (Scheme 2a,c) [9,10,12].
Our interest in investigating acyclic carbon-centered free radicals and their reactivity in atom transfer reactions has enabled the generation of all-carbon stereogenic quaternary centers.These centers have been successfully incorporated at the C3 ′ or C2 ′ positions of furanoside scaffolds, leading to the development of novel families of nucleoside or nucleotide analogues (an example of which is depicted in Scheme 2d).The presence of an all-carbon quaternary center is expected to induce a conformational bias favoring a south conformation (DNA-like) when located at C3 ′ and a north conformation (RNA-like) when at C2 ′ .The presence of the hydroxymethyl group on the quaternary center could act as an extended pharmacophore providing additional binding to proximal entities.Alternatively, nucleoside analogues bearing a C2 ′ or C3 ′ all-carbon quaternary center will not be recognized by enzymes or receptors susceptible to steric hinderance at these positions.These novel nucleosides bearing a quaternary center at C3 ′ have shown activities against gemcitabine-resistant KRAS mutated pancreatic cell lines [13,14].C2 ′ analogues showed inhibition of SARS-CoV-2 RNA dependant RNA polymerase (RdRp), the causal virus of , while others have shown great promise as cardioprotective agents for the treatment of heart failure [16].
Herein, we report the synthesis of 4 ′ -thioanalogues bearing a C2 ′ all-carbon stereogenic center (Scheme 3a).From the onset, intrinsic challenges were recognized using our acyclic approach; namely, the C2 ′ quaternary center could hinder the desired cyclization.Our efforts towards the synthesis of the targeted 4 ′ -thionucleosides using the acyclic approach with a pyrimidine nucleobase will be presented, in addition to a complementary cyclic approach, to access purine-bearing 4 ′ -thioanalogues (Scheme 3a).The formation of the C2′ all-carbon stereogenic center resulting from a vinyl ato transfer provides a single isomer 25.This key intermediate can efficiently provide acce to both the dithioacetals 28 or 30 with opposite stereochemistries at the C2′ quaternar center by derivatizing either the alkene or the ester moiety towards the required oxidatio state for the C1′ anomeric center (Scheme 3b).

Acyclic Approach for the Synthesis of 4′-Thionucleoside Analogues
The synthesis of the targeted novel 4′-thioanalogues required the construction of th key dithioacetal bearing the C2 all-carbon quaternary center.Following the literature pr cedures, aldehyde 32 was prepared in five steps from L-serine (Scheme 4) [ [17][18][19].Aimin to generate a 3,4-syn diol, aldehyde 32 was engaged in a Cram-Chelate controlled Muka yama aldol reaction in the presence of a mixture of tetrasubstituted enoxysilanes 33 [20 and MgBr2•OEt2, a bidentate Lewis acid.The desired 3,4-syn products 34a,b were forme in a >20:1 ratio with a 1:1 mixture of C2 bromides.The relative 3,4 stereochemistry wa confirmed after removal of the silyl ether and lactonization (see Supplementary Info mation for further details).No efforts were invested in controlling the C2-selectivity as th generated tertiary bromides 34a,b lead to a common radical species in the subsequen radical-based reaction.The formation of the C2 ′ all-carbon stereogenic center resulting from a vinyl atom transfer provides a single isomer 25.This key intermediate can efficiently provide access to both the dithioacetals 28 or 30 with opposite stereochemistries at the C2 ′ quaternary center by derivatizing either the alkene or the ester moiety towards the required oxidation state for the C1 ′ anomeric center (Scheme 3b).

Acyclic Approach for the Synthesis of 4 ′ -Thionucleoside Analogues
The synthesis of the targeted novel 4 ′ -thioanalogues required the construction of the key dithioacetal bearing the C2 all-carbon quaternary center.Following the literature procedures, aldehyde 32 was prepared in five steps from L-serine (Scheme 4) [ [17][18][19].Aiming to generate a 3,4-syn diol, aldehyde 32 was engaged in a Cram-Chelate controlled Mukaiyama aldol reaction in the presence of a mixture of tetrasubstituted enoxysilanes 33 [20], and MgBr 2 •OEt 2 , a bidentate Lewis acid.The desired 3,4-syn products 34a,b were formed in a >20:1 ratio with a 1:1 mixture of C2 bromides.The relative 3,4 stereochemistry was confirmed after removal of the silyl ether and lactonization (see Supplementary Information for further details).No efforts were invested in controlling the C2-selectivity as the generated tertiary bromides 34a,b lead to a common radical species in the subsequent radical-based reaction.The formation of the C2′ all-carbon stereogenic center resulting from a vinyl atom transfer provides a single isomer 25.This key intermediate can efficiently provide access to both the dithioacetals 28 or 30 with opposite stereochemistries at the C2′ quaternary center by derivatizing either the alkene or the ester moiety towards the required oxidation state for the C1′ anomeric center (Scheme 3b).

Acyclic Approach for the Synthesis of 4′-Thionucleoside Analogues
The synthesis of the targeted novel 4′-thioanalogues required the construction of the key dithioacetal bearing the C2 all-carbon quaternary center.Following the literature procedures, aldehyde 32 was prepared in five steps from L-serine (Scheme 4) [ [17][18][19].Aiming to generate a 3,4-syn diol, aldehyde 32 was engaged in a Cram-Chelate controlled Mukaiyama aldol reaction in the presence of a mixture of tetrasubstituted enoxysilanes 33 [20], and MgBr2•OEt2, a bidentate Lewis acid.The desired 3,4-syn products 34a,b were formed in a >20:1 ratio with a 1:1 mixture of C2 bromides.The relative 3,4 stereochemistry was confirmed after removal of the silyl ether and lactonization (see Supplementary Information for further details).No efforts were invested in controlling the C2-selectivity as the generated tertiary bromides 34a,b lead to a common radical species in the subsequent radical-based reaction.The installation of vinyldimethylsilane on secondary alcohols 34a,b provided a mixture of the corresponding silyl ethers 35a,b (Scheme 5).This mixture was then subjected to a free-radical-based atom transfer reaction, using triethylborane as the initiator.Cyclization through the preferred 5-exo-trig diastereoselective transition state with carbon-carbon bond formation from the bottom face of the radical intermediate resulted in intermediate A, which was subsequently treated with AcOH for exclusive formation (>20:1) of methylester 36 [21].DIBAL-H reduction of the methylester, benzoylation of the two alcohols, and ozonolysis of the vinyl moiety, provided aldehyde 38 in excellent yield.The installation of vinyldimethylsilane on secondary alcohols 34a,b provided a mixture of the corresponding silyl ethers 35a,b (Scheme 5).This mixture was then subjected to a free-radical-based atom transfer reaction, using triethylborane as the initiator.Cyclization through the preferred 5-exo-trig diastereoselective transition state with carboncarbon bond formation from the bottom face of the radical intermediate resulted in intermediate A, which was subsequently treated with AcOH for exclusive formation (>20:1) of methylester 36 [21].DIBAL-H reduction of the methylester, benzoylation of the two alcohols, and ozonolysis of the vinyl moiety, provided aldehyde 38 in excellent yield.Scheme 5. All-carbon quaternary center formation from a free-radical-based atom transfer reaction.
The formation of the requisite dithioacetal was first attempted with t-butylthiol, but this only resulted in cyclized products 41a,b (Table 1).Despite varying the equivalents of Lewis acid used (entries 1 and 2), debenzylation and cyclization was favored over the formation of dithioacetal 39, presumably due to steric congestion.Using the less hindered benzyl mercaptan, dithioacetal 40 was formed in excellent yield (entry 3).Subsequent removal of the C4 benzyl ether from dithioacetal 40 proved to be difficult using boron-based Lewis acids, including Me2BBr [22], providing a mere 15% yield when using BCl3 (see Section 3).Reversing the order of these reactions was therefore considered.Hydrogenolysis of the C4-benzyl ether moiety of aldehyde 38 provided lactols 42a,b in excellent yield (Table 2).Thioacetylation with BF3•OEt2 gave a mixture of cyclic thioacetals 44a,b (entry 1), while SnCl4 pushed the equilibrium to provide 44% of the targeted dithioacetal 43 (entry 2).The use of TiCl4 then gave an excellent 80% yield (entry 3).Scheme 5. All-carbon quaternary center formation from a free-radical-based atom transfer reaction.
‡ : The symbol is used to denote transition states.
The formation of the requisite dithioacetal was first attempted with t-butylthiol, but this only resulted in cyclized products 41a,b (Table 1).Despite varying the equivalents of Lewis acid used (entries 1 and 2), debenzylation and cyclization was favored over the formation of dithioacetal 39, presumably due to steric congestion.Using the less hindered benzyl mercaptan, dithioacetal 40 was formed in excellent yield (entry 3).The installation of vinyldimethylsilane on secondary alcohols 34a,b provided a mixture of the corresponding silyl ethers 35a,b (Scheme 5).This mixture was then subjected to a free-radical-based atom transfer reaction, using triethylborane as the initiator.Cyclization through the preferred 5-exo-trig diastereoselective transition state with carboncarbon bond formation from the bottom face of the radical intermediate resulted in intermediate A, which was subsequently treated with AcOH for exclusive formation (>20:1) of methylester 36 [21].DIBAL-H reduction of the methylester, benzoylation of the two alcohols, and ozonolysis of the vinyl moiety, provided aldehyde 38 in excellent yield.Scheme 5. All-carbon quaternary center formation from a free-radical-based atom transfer reaction.
The formation of the requisite dithioacetal was first attempted with t-butylthiol, but this only resulted in cyclized products 41a,b (Table 1).Despite varying the equivalents of Lewis acid used (entries 1 and 2), debenzylation and cyclization was favored over the formation of dithioacetal 39, presumably due to steric congestion.Using the less hindered benzyl mercaptan, dithioacetal 40 was formed in excellent yield (entry 3).Subsequent removal of the C4 benzyl ether from dithioacetal 40 proved to be difficult using boron-based Lewis acids, including Me2BBr [22], providing a mere 15% yield when using BCl3 (see Section 3).Reversing the order of these reactions was therefore considered.Hydrogenolysis of the C4-benzyl ether moiety of aldehyde 38 provided lactols 42a,b in excellent yield (Table 2).Thioacetylation with BF3•OEt2 gave a mixture of cyclic thioacetals 44a,b (entry 1), while SnCl4 pushed the equilibrium to provide 44% of the targeted dithioacetal 43 (entry 2).The use of TiCl4 then gave an excellent 80% yield (entry 3).Subsequent removal of the C4 benzyl ether from dithioacetal 40 proved to be difficult using boron-based Lewis acids, including Me 2 BBr [22], providing a mere 15% yield when using BCl 3 (see Section 3).Reversing the order of these reactions was therefore considered.Hydrogenolysis of the C4-benzyl ether moiety of aldehyde 38 provided lactols 42a,b in excellent yield (Table 2).Thioacetylation with BF 3 •OEt 2 gave a mixture of cyclic thioacetals 44a,b (entry 1), while SnCl 4 pushed the equilibrium to provide 44% of the targeted dithioacetal 43 (entry 2).The use of TiCl 4 then gave an excellent 80% yield (entry 3).The addition of silylated thymine to C4-mesylated dithioacetal 45 in the presence of iodine resulted in a 1:1 mixture of thioaminals 46a,b at room temperature or 50 °C (Table 3, entries 3 and 4), while lower temperatures allowed for a modest increase in formation of the 1′,2′-anti thioaminal 46a (entries 1 and 2).The marginal stereoselectivities observed contrasted with the high 1′,2′-syn induction for the introduction of a nucleobase at C1 in the presence of an electron-withdrawing group adjacent to the dithioacetal.Nonetheless, the formation of these thioaminals provided the opportunity to examine the following cyclization step while exploring strategies to improve selectivity.The two thioaminals were cyclized separately using NaI in the presence of 2,6lutidine at reflux (Table 4).1′,2′-anti thioaminal 46a reacted accordingly to give the β-Danomer 47a in excellent yield (entry 1).The cyclization results were strikingly different for the 1′,2′-syn isomer 46b, which yielded a low amount of α-D-anomer 47b with recovery of starting material and a secondary product (48) isolated in 25% yield (entry 2).To confirm this difference in reactivity under identical conditions, a 1:1 mixture of thioaminals 46a,b was submitted to the cyclization conditions (entry 3).A 7:1 ratio in favor of β-anomer 47a was obtained, confirming the faster cyclization of the 1′,2′-anti isomer, and indicating the potential for developing a kinetic resolution strategy to address the absence of induction in nucleobase coupling to dithioacetals not bearing an electron-withdrawing group at the C2 position.The addition of silylated thymine to C4-mesylated dithioacetal 45 in the presence of iodine resulted in a 1:1 mixture of thioaminals 46a,b at room temperature or 50 • C (Table 3, entries 3 and 4), while lower temperatures allowed for a modest increase in formation of the 1 ′ ,2 ′ -anti thioaminal 46a (entries 1 and 2).The marginal stereoselectivities observed contrasted with the high 1 ′ ,2 ′ -syn induction for the introduction of a nucleobase at C1 in the presence of an electron-withdrawing group adjacent to the dithioacetal.Nonetheless, the formation of these thioaminals provided the opportunity to examine the following cyclization step while exploring strategies to improve selectivity.The addition of silylated thymine to C4-mesylated dithioacetal 45 in the presence of iodine resulted in a 1:1 mixture of thioaminals 46a,b at room temperature or 50 °C (Table 3, entries 3 and 4), while lower temperatures allowed for a modest increase in formation of the 1′,2′-anti thioaminal 46a (entries 1 and 2).The marginal stereoselectivities observed contrasted with the high 1′,2′-syn induction for the introduction of a nucleobase at C1 in the presence of an electron-withdrawing group adjacent to the dithioacetal.Nonetheless, the formation of these thioaminals provided the opportunity to examine the following cyclization step while exploring strategies to improve selectivity.The two thioaminals were cyclized separately using NaI in the presence of 2,6lutidine at reflux (Table 4).1′,2′-anti thioaminal 46a reacted accordingly to give the β-Danomer 47a in excellent yield (entry 1).The cyclization results were strikingly different for the 1′,2′-syn isomer 46b, which yielded a low amount of α-D-anomer 47b with recovery of starting material and a secondary product (48) isolated in 25% yield (entry 2).To confirm this difference in reactivity under identical conditions, a 1:1 mixture of thioaminals 46a,b was submitted to the cyclization conditions (entry 3).A 7:1 ratio in favor of β-anomer 47a was obtained, confirming the faster cyclization of the 1′,2′-anti isomer, and indicating the potential for developing a kinetic resolution strategy to address the absence of induction in nucleobase coupling to dithioacetals not bearing an electron-withdrawing group at the C2 position.The two thioaminals were cyclized separately using NaI in the presence of 2,6-lutidine at reflux (Table 4). 1 ′ ,2 ′ -anti thioaminal 46a reacted accordingly to give the β-D-anomer 47a in excellent yield (entry 1).The cyclization results were strikingly different for the 1 ′ ,2 ′ -syn isomer 46b, which yielded a low amount of α-D-anomer 47b with recovery of starting material and a secondary product (48) isolated in 25% yield (entry 2).To confirm this difference in reactivity under identical conditions, a 1:1 mixture of thioaminals 46a,b was submitted to the cyclization conditions (entry 3).A 7:1 ratio in favor of β-anomer 47a was obtained, confirming the faster cyclization of the 1 ′ ,2 ′ -anti isomer, and indicating the potential for developing a kinetic resolution strategy to address the absence of induction in nucleobase coupling to dithioacetals not bearing an electron-withdrawing group at the C2 position.

Entry
The secondary product 48 seemingly originated from displacement of the C4 ′ -mesylate in 1 ′ ,2 ′ -syn thioaminal 46b with traces of water.To determine its structure, the C4 ′ hydroxyl of 48 was mesylated and treated with NaI in 2,6-lutidine (Scheme 6).Interestingly, the cyclization was very efficient, with the β-L-anomer 50 being the only isolated product in 77% yield.The secondary product 48 seemingly originated from displacement of the C4′-mesylate in 1′,2′-syn thioaminal 46b with traces of water.To determine its structure, the C4′ hydroxyl of 48 was mesylated and treated with NaI in 2,6-lutidine (Scheme 6).Interestingly, the cyclization was very efficient, with the β-L-anomer 50 being the only isolated product in 77% yield.Scheme 6. Cyclization of thioaminal 49.

DFT Computational Study
With the aim of identifying the principle steric and electronic factors influencing the rates of cyclizations in thioaminals having different relative stereochemistries, we examined model compounds 51, 54 and 58 through DFT calculations (Schemes 7 and 8).The calculated energy landscape was first explored with 51 and was consistent with rate-limiting intramolecular displacement of the C4′-mesylate, generating sulfonium intermediate 52a (TS A1, 32.8 kcal/mol) with the -SBn chain in the bottom position, trans to the C5′ center.Dealkylation in the presence of iodide (i.e., TS B, Scheme 7) would then furnish product 53 through TS B (29.1 kcal/mol).TS A2, with the -SBn occupying the upper position, exhibited significantly higher energy (41.7 kcal/mol) due to a severe steric clash between the -SBn chain and both the C5′ and base substituents.A noteworthy observation is that the conformation of the examined 4′-thiofuranoside shows lower energy with the C2′endo envelop having the C3′-OAc oriented in the pseudo-axial conformation, perpendicular to the axis of the SN2-like bond breaking and bond forming.This could be rationalized by favorable stabilization achieved through the orientation of the best C3′-H3′ and C3′-C2′ sigma donors towards the C4′ center, as confirmed by NBO analysis.This orientation Molecules 2024, 29, x FOR PEER REVIEW 7 of 27 The secondary product 48 seemingly originated from displacement of the C4′-mesylate in 1′,2′-syn thioaminal 46b with traces of water.To determine its structure, the C4′ hydroxyl of 48 was mesylated and treated with NaI in 2,6-lutidine (Scheme 6).Interestingly, the cyclization was very efficient, with the β-L-anomer 50 being the only isolated product in 77% yield.Scheme 6. Cyclization of thioaminal 49.

DFT Computational Study
With the aim of identifying the principle steric and electronic factors influencing the rates of cyclizations in thioaminals having different relative stereochemistries, we examined model compounds 51, 54 and 58 through DFT calculations (Schemes 7 and 8).The calculated energy landscape was first explored with 51 and was consistent with rate-limiting intramolecular displacement of the C4′-mesylate, generating sulfonium intermediate 52a (TS A1, 32.8 kcal/mol) with the -SBn chain in the bottom position, trans to the C5′ center.Dealkylation in the presence of iodide (i.e., TS B, Scheme 7) would then furnish product 53 through TS B (29.1 kcal/mol).TS A2, with the -SBn occupying the upper position, exhibited significantly higher energy (41.7 kcal/mol) due to a severe steric clash between the -SBn chain and both the C5′ and base substituents.A noteworthy observation is that the conformation of the examined 4′-thiofuranoside shows lower energy with the C2′endo envelop having the C3′-OAc oriented in the pseudo-axial conformation, perpendicular to the axis of the SN2-like bond breaking and bond forming.This could be rationalized by favorable stabilization achieved through the orientation of the best C3′-H3′ and C3′-C2′ sigma donors towards the C4′ center, as confirmed by NBO analysis.This orientation  The secondary product 48 seemingly originated from displacement of the C4′-mesylate in 1′,2′-syn thioaminal 46b with traces of water.To determine its structure, the C4′ hydroxyl of 48 was mesylated and treated with NaI in 2,6-lutidine (Scheme 6).Interestingly, the cyclization was very efficient, with the β-L-anomer 50 being the only isolated product in 77% yield.Scheme 6. Cyclization of thioaminal 49.

DFT Computational Study
With the aim of identifying the principle steric and electronic factors influencing the rates of cyclizations in thioaminals having different relative stereochemistries, we examined model compounds 51, 54 and 58 through DFT calculations (Schemes 7 and 8).The calculated energy landscape was first explored with 51 and was consistent with rate-limiting intramolecular displacement of the C4′-mesylate, generating sulfonium intermediate 52a (TS A1, 32.8 kcal/mol) with the -SBn chain in the bottom position, trans to the C5′ center.Dealkylation in the presence of iodide (i.e., TS B, Scheme 7) would then furnish product 53 through TS B (29.1 kcal/mol).TS A2, with the -SBn occupying the upper position, exhibited significantly higher energy (41.7 kcal/mol) due to a severe steric clash between the -SBn chain and both the C5′ and base substituents.A noteworthy observation is that the conformation of the examined 4′-thiofuranoside shows lower energy with the C2′endo envelop having the C3′-OAc oriented in the pseudo-axial conformation, perpendicular to the axis of the SN2-like bond breaking and bond forming.This could be rationalized by favorable stabilization achieved through the orientation of the best C3′-H3′ and C3′-C2′ sigma donors towards the C4′ center, as confirmed by NBO analysis.This orientation Scheme 6. Cyclization of thioaminal 49.

DFT Computational Study
With the aim of identifying the principle steric and electronic factors influencing the rates of cyclizations in thioaminals having different relative stereochemistries, we examined model compounds 51, 54 and 58 through DFT calculations (Schemes 7 and 8).The calculated energy landscape was first explored with 51 and was consistent with rate-limiting intramolecular displacement of the C4 ′ -mesylate, generating sulfonium intermediate 52a (TS A1, 32.8 kcal/mol) with the -SBn chain in the bottom position, trans to the C5 ′ center.Dealkylation in the presence of iodide (i.e., TS B, Scheme 7) would then furnish product 53 through TS B (29.1 kcal/mol).TS A2, with the -SBn occupying the upper position, exhibited significantly higher energy (41.7 kcal/mol) due to a severe steric clash between the -SBn chain and both the C5 ′ and base substituents.A noteworthy observation is that the conformation of the examined 4 ′ -thiofuranoside shows lower energy with the C2 ′ -endo envelop having the C3 ′ -OAc oriented in the pseudo-axial conformation, perpendicular to the axis of the S N 2-like bond breaking and bond forming.This could be rationalized by favorable stabilization achieved through the orientation of the best C3 ′ -H3 ′ and C3 ′ -C2 ′ sigma donors towards the C4 ′ center, as confirmed by NBO analysis.This orientation could also relieve electrostatic repulsion by distancing the leaving group (-OMs) and the C3 ′ acetate group.Even in TS A2, which experiences severe strain on the upper face with the C2 ′ -Me axial, the C2 ′ -endo conformation at 41.7 kcal/mol is preferable to the corresponding C2 ′ -exo TS, where the top face C2 ′ -Me is in a pseudo-equatorial position, with an energy of 44.9 kcal/mol (see Supplementary Information).
could also relieve electrostatic repulsion by distancing the leaving group (-OMs) and th C3′ acetate group.Even in TS A2, which experiences severe strain on the upper face wi the C2′-Me axial, the C2′-endo conformation at 41.7 kcal/mol is preferable to the corr sponding C2′-exo TS, where the top face C2′-Me is in a pseudo-equatorial position, wi an energy of 44.9 kcal/mol (see Supplementary Information).Scheme 7. Relative Gibbs free energy (kcal/mol) calculated in Gaussian 16 [23] at the M06-2x/6-31 [24,25] level of theory in 2,6-lutidine using the continuum solvation model (PCM) [26].Cyclizati of 1′,2′-anti thioaminal 51 as a model of 46a.U = Uracil, ∆Gact corresponds to the TS free energy min the lowest acyclic thioaminal 51 free energy.Otherwise, all free energies are arbitrarily referenc to the lowest energy acyclic conformer of compound 51 at 423.2 K. CYLview structures for TS a shown [27].could also relieve electrostatic repulsion by distancing the leaving group (-OMs) and C3′ acetate group.Even in TS A2, which experiences severe strain on the upper face w the C2′-Me axial, the C2′-endo conformation at 41.7 kcal/mol is preferable to the cor sponding C2′-exo TS, where the top face C2′-Me is in a pseudo-equatorial position, w an energy of 44.9 kcal/mol (see Supplementary Information).Scheme 7. Relative Gibbs free energy (kcal/mol) calculated in Gaussian 16 [23] at the M06-2x/6-31 [24,25] level of theory in 2,6-lutidine using the continuum solvation model (PCM) [26].Cyclizat of 1′,2′-anti thioaminal 51 as a model of 46a.U = Uracil, ∆Gact corresponds to the TS free energy min the lowest acyclic thioaminal 51 free energy.Otherwise, all free energies are arbitrarily referenc to the lowest energy acyclic conformer of compound 51 at 423.2 K. CYLview structures for TS shown [27].Consistent with the observed slower cyclization for the 1 ′ ,2 ′ -syn thioaminal 46b (model compound 54, Scheme 8a), the calculated TS energies for the cyclization through TS C1 (36.8 kcal/mol) with the benzyl group up or TS C2 (37.2 kcal/mol) with the benzyl group down are significantly higher in energy than the lowest TS A1 for the 1 ′ ,2 ′ -anti thioaminal (32.8 kcal/mol, Scheme 7).Both TS C1 and TS C2 suffer, respectively, from either additional syn pentane interactions (SBn and C2 ′ -Me moiety) or from an additional gauche interaction (Uracil and SBn).In TS C2, the nucleobase is also forced to occupy a less favorable pseudoaxial position.The acyclic precursor 54 minima was found to be slightly higher than for 51, leading to a predicted activation energy of 36.6 kcal/mol and therefore slower kinetics for 1 ′ ,4 ′ -trans thiofuranoside formation.Interestingly, previous cyclizations of thioaminals with 1 ′ ,4 ′ -cis and 1 ′ ,4 ′ -trans stereochemistries not bearing the C2 ′ quaternary group displayed similar rates of reactivity [9].The syn-pentane steric clashes, therefore, seem to impact more severely the reactivity of the isomers leading to 1 ′ ,4 ′ -trans thionucleosides.This was further confirmed in the formation of L-1 ′ ,4 ′ -cis thiofuranoside 60.This cyclization was observed experimentally to progress readily (Schemes 6 and 8b), in accordance with a TS energy (TS D, 32.5 kcal/mol) comparable to TS A1.The starting thioaminal 58 was 0.26 kcal/mol higher than 51, therefore leading to a calculated ∆G act of 32.2 kcal/mol.
These studies shed light on why the S1 ′ -to-C4 ′ cyclization can lead to a kinetic resolution favoring formation of the biologically relevant 1 ′ ,4 ′ -cis thionucleoside.In the context of generating the targeted analogues presented here, deprotection of the primary silyl group of a mixture of 4 ′ -thionucleosides 47a,b using 3HF•NEt 3 provided an inseparable mixture of anomers 61a,b.Following benzoate removal with NaOMe, the final molecules 23a and 23b were isolated in 42% and 21%, respectively (Scheme 9).
difference with the lowest corresponding acyclic thioaminal free energy 54 and 58.Otherwise, all free energies are arbitrarily referenced to the lowest energy acyclic conformer of compound 51 at 423.2 K (refer to Scheme 7).CYLview structures for TS are shown [27].
Consistent with the observed slower cyclization for the 1′,2′-syn thioaminal 46b (model compound 54, Scheme 8a), the calculated TS energies for the cyclization through TS C1 (36.8 kcal/mol) with the benzyl group up or TS C2 (37.2 kcal/mol) with the benzyl group down are significantly higher in energy than the lowest TS A1 for the 1′,2′-anti thioaminal (32.8 kcal/mol, Scheme 7).Both TS C1 and TS C2 suffer, respectively, from either additional syn pentane interactions (SBn and C2′-Me moiety) or from an additional gauche interaction (Uracil and SBn).In TS C2, the nucleobase is also forced to occupy a less favorable pseudo-axial position.The acyclic precursor 54 minima was found to be slightly higher than for 51, leading to a predicted activation energy of 36.6 kcal/mol and therefore slower kinetics for 1′,4′-trans thiofuranoside formation.Interestingly, previous cyclizations of thioaminals with 1′,4′-cis and 1′,4′-trans stereochemistries not bearing the C2′ quaternary group displayed similar rates of reactivity [9].The syn-pentane steric clashes, therefore, seem to impact more severely the reactivity of the isomers leading to 1′,4′-trans thionucleosides.This was further confirmed in the formation of L-1′,4′-cis thiofuranoside 60.This cyclization was observed experimentally to progress readily (Schemes 6 and 8b), in accordance with a TS energy (TS D, 32.5 kcal/mol) comparable to TS A1.The starting thioaminal 58 was 0.26 kcal/mol higher than 51, therefore leading to a calculated ∆Gact of 32.2 kcal/mol.
These studies shed light on why the S1′-to-C4′ cyclization can lead to a kinetic resolution favoring formation of the biologically relevant 1′,4′-cis thionucleoside.In the context of generating the targeted analogues presented here, deprotection of the primary silyl group of a mixture of 4′-thionucleosides 47a,b using 3HF•NEt3 provided an inseparable mixture of anomers 61a,b.Following benzoate removal with NaOMe, the final molecules 23a and 23b were isolated in 42% and 21%, respectively (Scheme 9).Scheme 9. Synthesis of final 4′-thionucleoside analogues bearing a thymine nucleobase.

Cyclic Approach for the Synthesis of 4′-Thionucleoside Analogues
As discussed in Scheme 3, a second approach was considered to access such 4′-thioanalogues, in which the thiofuranoside was formed prior to addition of the nucleobase.After mesylation of dithioacetal 43, treatment with TBAI in the presence of a base provided thiobenzylfuranoside 62 (Scheme 10) [28].Glycosylation using silylated thymine in the presence of DMTSF provided a 1.2:1 ratio of thioanalogues 47a,b.Scheme 10.Synthesis of thymine-bearing 4′-thionucleoside analogues from glycosylation of cyclic thiofuranoside 62. Scheme 9. Synthesis of final 4 ′ -thionucleoside analogues bearing a thymine nucleobase.

Cyclic Approach for the Synthesis of 4 ′ -Thionucleoside Analogues
As discussed in Scheme 3, a second approach was considered to access such 4 ′thioanalogues, in which the thiofuranoside was formed prior to addition of the nucleobase.After mesylation of dithioacetal 43, treatment with TBAI in the presence of a base provided thiobenzylfuranoside 62 (Scheme 10) [28].Glycosylation using silylated thymine in the presence of DMTSF provided a 1.2:1 ratio of thioanalogues 47a,b.
free energies are arbitrarily referenced to the lowest energy acyclic conformer of compound 51 at 423.2 K (refer to Scheme 7).CYLview structures for TS are shown [27].
Consistent with the observed slower cyclization for the 1′,2′-syn thioaminal 46b (model compound 54, Scheme 8a), the calculated TS energies for the cyclization through TS C1 (36.8 kcal/mol) with the benzyl group up or TS C2 (37.2 kcal/mol) with the benzyl group down are significantly higher in energy than the lowest TS A1 for the 1′,2′-anti thioaminal (32.8 kcal/mol, Scheme 7).Both TS C1 and TS C2 suffer, respectively, from either additional syn pentane interactions (SBn and C2′-Me moiety) or from an additional gauche interaction (Uracil and SBn).In TS C2, the nucleobase is also forced to occupy a less favorable pseudo-axial position.The acyclic precursor 54 minima was found to be slightly higher than for 51, leading to a predicted activation energy of 36.6 kcal/mol and therefore slower kinetics for 1′,4′-trans thiofuranoside formation.Interestingly, previous cyclizations of thioaminals with 1′,4′-cis and 1′,4′-trans stereochemistries not bearing the C2′ quaternary group displayed similar rates of reactivity [9].The syn-pentane steric clashes, therefore, seem to impact more severely the reactivity of the isomers leading to 1′,4′-trans thionucleosides.This was further confirmed in the formation of L-1′,4′-cis thiofuranoside 60.This cyclization was observed experimentally to progress readily (Schemes 6 and 8b), in accordance with a TS energy (TS D, 32.5 kcal/mol) comparable to TS A1.The starting thioaminal 58 was 0.26 kcal/mol higher than 51, therefore leading to a calculated ∆Gact of 32.2 kcal/mol.
These studies shed light on why the S1′-to-C4′ cyclization can lead to a kinetic resolution favoring formation of the biologically relevant 1′,4′-cis thionucleoside.In the context of generating the targeted analogues presented here, deprotection of the primary silyl group of a mixture of 4′-thionucleosides 47a,b using 3HF•NEt3 provided an inseparable mixture of anomers 61a,b.Following benzoate removal with NaOMe, the final molecules 23a and 23b were isolated in 42% and 21%, respectively (Scheme 9).Scheme 9. Synthesis of final 4′-thionucleoside analogues bearing a thymine nucleobase.

Cyclic Approach for the Synthesis of 4′-Thionucleoside Analogues
As discussed in Scheme 3, a second approach was considered to access such 4′-thioanalogues, in which the thiofuranoside was formed prior to addition of the nucleobase.After mesylation of dithioacetal 43, treatment with TBAI in the presence of a base provided thiobenzylfuranoside 62 (Scheme 10) [28].Glycosylation using silylated thymine in the presence of DMTSF provided a 1.2:1 ratio of thioanalogues 47a,b.Scheme 10.Synthesis of thymine-bearing 4′-thionucleoside analogues from glycosylation of cyclic thiofuranoside 62.A similar cyclic strategy was used to prepare purine derivatives, the synthesis of which was difficult using the acyclic approach.The addition of 2-chloroadenine was investigated, as the presence of a halogen at the two position of the nucleobase renders analogues, such as Clofarabine, more stable to deamination, a major mechanism of metabolic clearance in vivo [29].Similar to the addition of thymine, the activation of thiofuranoside 62 with DMTSF followed by the addition of 2-chloroadenine or 2,6-dichloropurine resulted in a mixture of compounds with the major products identified as a 1:1 mixture of N9-β:α anomers (results not shown).The nucleobase addition with thiofuranosides 63a,b bearing an anomeric acetate was next investigated (Table 5).A similar cyclic strategy was used to prepare purine derivatives, the synthesis of which was difficult using the acyclic approach.The addition of 2-chloroadenine was investigated, as the presence of a halogen at the two position of the nucleobase renders analogues, such as Clofarabine, more stable to deamination, a major mechanism of metabolic clearance in vivo [29].Similar to the addition of thymine, the activation of thiofuranoside 62 with DMTSF followed by the addition of 2-chloroadenine or 2,6-dichloropurine resulted in a mixture of compounds with the major products identified as a 1:1 mixture of N9-β:α anomers (results not shown).The nucleobase addition with thiofuranosides 63a,b bearing an anomeric acetate was next investigated (Table 5).Similar to glycosylation of thiofuranoside 62, the addition of 2-chloroadenine to 63a,b resulted in a mixture of compounds with the major products identified as a 1:1 mixture of N9-β:α anomers 64a,b at RT or 84 °C (entries 1 and 2).However, a 5:1 ratio of N9-products 65a,b in favor of the desired β-anomer was obtained with 2,6-dichloropurine using DBU and TMSOTf at 84 °C versus a 1:1 ratio at room temperature (entries 3 and 4), indicative of a thermodynamic equilibrium favoring the β-anomer.The removal of the C5′-silyl ether of anomers 65a,b followed by debenzoylation and displacement of the 6-chloro moiety with ammonia provided the corresponding 6-amino derivatives 24a and 24b (Scheme 11).Scheme 11.Synthesis of final 4′-thionucleoside analogues bearing a 2-chloroadenine nucleobase.
In conclusion, nucleoside analogue synthesis is a research field of great interest in medicinal chemistry.The intramolecular cyclization of acyclic thioaminals has been used to synthesize nucleosides as well as 4′-thioanalogues.Herein, this approach was evaluated in the context of a novel family of 4′-thionucleosides bearing a quaternary stereogenic center at C2′.The challenge of this study resides in the SN2-like S1′-to-C4′ cyclization Similar to glycosylation of thiofuranoside 62, the addition of 2-chloroadenine to 63a,b resulted in a mixture of compounds with the major products identified as a 1:1 mixture of N9-β:α anomers 64a,b at RT or 84 • C (entries 1 and 2).However, a 5:1 ratio of N9-products 65a,b in favor of the desired β-anomer was obtained with 2,6-dichloropurine using DBU and TMSOTf at 84 • C versus a 1:1 ratio at room temperature (entries 3 and 4), indicative of a thermodynamic equilibrium favoring the β-anomer.The removal of the C5 ′ -silyl ether of anomers 65a,b followed by debenzoylation and displacement of the 6-chloro moiety with ammonia provided the corresponding 6-amino derivatives 24a and 24b (Scheme 11).
A similar cyclic strategy was used to prepare purine derivatives, the synthesis of which was difficult using the acyclic approach.The addition of 2-chloroadenine was investigated, as the presence of a halogen at the two position of the nucleobase renders analogues, such as Clofarabine, more stable to deamination, a major mechanism of metabolic clearance in vivo [29].Similar to the addition of thymine, the activation of thiofuranoside 62 with DMTSF followed by the addition of 2-chloroadenine or 2,6-dichloropurine resulted in a mixture of compounds with the major products identified as a 1:1 mixture of N9-β:α anomers (results not shown).The nucleobase addition with thiofuranosides 63a,b bearing an anomeric acetate was next investigated (Table 5).Similar to glycosylation of thiofuranoside 62, the addition of 2-chloroadenine to 63a,b resulted in a mixture of compounds with the major products identified as a 1:1 mixture of N9-β:α anomers 64a,b at RT or 84 °C (entries 1 and 2).However, a 5:1 ratio of N9-products 65a,b in favor of the desired β-anomer was obtained with 2,6-dichloropurine using DBU and TMSOTf at 84 °C versus a 1:1 ratio at room temperature (entries 3 and 4), indicative of a thermodynamic equilibrium favoring the β-anomer.The removal of the C5′-silyl ether of anomers 65a,b followed by debenzoylation and displacement of the 6-chloro moiety with ammonia provided the corresponding 6-amino derivatives 24a and 24b (Scheme 11).

Scheme 11. Synthesis of final 4′-thionucleoside analogues bearing a 2-chloroadenine nucleobase.
In conclusion, nucleoside analogue synthesis is a research field of great interest in medicinal chemistry.The intramolecular cyclization of acyclic thioaminals has been used to synthesize nucleosides as well as 4′-thioanalogues.Herein, this approach was evaluated in the context of a novel family of 4′-thionucleosides bearing a quaternary stereogenic center at C2′.The challenge of this study resides in the SN2-like S1′-to-C4′ cyclization Scheme 11.Synthesis of final 4 ′ -thionucleoside analogues bearing a 2-chloroadenine nucleobase.
In conclusion, nucleoside analogue synthesis is a research field of great interest in medicinal chemistry.The intramolecular cyclization of acyclic thioaminals has been used to synthesize nucleosides as well as 4 ′ -thioanalogues.Herein, this approach was evaluated in the context of a novel family of 4 ′ -thionucleosides bearing a quaternary stereogenic center at C2 ′ .The challenge of this study resides in the S N 2-like S1 ′ -to-C4 ′ cyclization combined with stereoselective formation of the desired thioaminal typically dependent on the presence of an electron-withdrawing group at C2 ′ , which in this case is absent.The lack of diastereoselectivity for thioaminal formation turned out to be significant, with a modest stereoselectivity (1.4:1) favoring the 1 ′ ,2 ′ -anti isomer.Interestingly, an original solution arose from these challenges.A kinetic bias favoring cyclization of the 1 ′ ,2 ′ -anti thioaminal was observed, with the desired β-anomer being obtained in a 7:1 ratio.DFT calculations suggest that this kinetic resolution favors the 1 ′ ,4 ′ -cis product due to significant steric clashes arising in the S N 2 TS of the 1 ′ ,4 ′ -trans isomer.These unfavorable interactions increase the activation energy, resulting in a slower rate of cyclization as compared to the corresponding 1 ′ ,4 ′ -cis isomers.An alternative approach, in which the nucleobase was added onto an already formed thiofuranoside, allowed for the synthesis of novel 2-chloroadenine 4 ′ -thionucleoside analogues.These proprietary molecules and other derivatives are currently being evaluated both in vitro and in vivo for their biological profiles, more specifically in the context of cardioprotection.These novel nucleoside scaffolds could potentially also find interesting applications in synthetic vaccine development.

General Information-Synthesis
All reactions requiring anhydrous conditions were carried out under an atmosphere of nitrogen or argon in flame-dried glassware using standard syringe techniques.All anhydrous solvents were dried with 3 Å molecular sieves prior to use.The 3 Å molecular sieves (1-2 mm beads) were activated by being heated at 180 • C for 48 h under vacuum prior to being added to new bottles of solvent purged with nitrogen.Commercially available reagents were used as received.Flash chromatography was performed on silica gel 60 (0.040-0.063 mm) using forced flow (flash chromatography) of the indicated solvent system or an automated flash purification system.Analytical thin-layer chromatography (TLC) was carried out on precoated (0.25 mm) silica gel aluminum plates.Visualization was performed with short-wavelength UV and/or revealed with potassium permanganate solutions. 1H NMR spectra were recorded at room temperature at 500 MHz and 13 C were recorded at 126 MHz.The data are reported as follows: chemical shift in parts per million referenced to residual solvent (CDCl 3 δ 7.26 ppm, CD 3 OD δ 3.31 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 (hertz), and integration. 13C{ 1 H}MR spectra were recorded at room temperature using 126 MHz.The data are reported as follows: chemical shift in parts per million referenced to residual solvent (CDCl 3 δ 77.16 ppm, CD 3 OD δ 49.00 ppm).Infrared spectra were recorded on a Fourier-transform infrared spectrophotometer with a single-reflection diamond ATR module, and signals were reported in cm −1 .Mass spectra were recorded through electrospray ionization positive-ion mode.A Hybrid Quadrupole-Orbitrap mass analyzer was used for HRMS measurements.Optical rotations were measured at room temperature from the sodium D line (589 nm) using CHCl 3 as solvent unless otherwise noted, and calculated using the following formula: [α] D = (100)α obs /(l•c)), where c = (g of substrate/100 mL of solvent) and l = 1 dm.Diol 31, methylester S1 and aldehyde 32 were prepared using previously reported procedures [17][18][19].

(-)-Methyl (S)-2-(benzyloxy)-3-((tert-butyldiphenylsilyl)oxy)propanoate (S2).
on the presence of an electron-withdrawing group at C2′, which in this case is absent.The lack of diastereoselectivity for thioaminal formation turned out to be significant, with a modest stereoselectivity (1.4:1) favoring the 1′,2′-anti isomer.Interestingly, an original solution arose from these challenges.A kinetic bias favoring cyclization of the 1′,2′-anti thioaminal was observed, with the desired β-anomer being obtained in a 7:1 ratio.DFT calculations suggest that this kinetic resolution favors the 1′,4′-cis product due to significant steric clashes arising in the SN2 TS of the 1′,4′-trans isomer.These unfavorable interactions increase the activation energy, resulting in a slower rate of cyclization as compared to the corresponding 1′,4′-cis isomers.An alternative approach, in which the nucleobase was added onto an already formed thiofuranoside, allowed for the synthesis of novel 2-chloroadenine 4′-thionucleoside analogues.These proprietary molecules and other derivatives are currently being evaluated both in vitro and in vivo for their biological profiles, more specifically in the context of cardioprotection.These novel nucleoside scaffolds could potentially also find interesting applications in synthetic vaccine development.

General Information-Synthesis
All reactions requiring anhydrous conditions were carried out under an atmosphere of nitrogen or argon in flame-dried glassware using standard syringe techniques.All anhydrous solvents were dried with 3 Å molecular sieves prior to use.The 3 Å molecular sieves (1-2 mm beads) were activated by being heated at 180 °C for 48 h under vacuum prior to being added to new bottles of solvent purged with nitrogen.Commercially available reagents were used as received.Flash chromatography was performed on silica gel 60 (0.040-0.063 mm) using forced flow (flash chromatography) of the indicated solvent system or an automated flash purification system.Analytical thin-layer chromatography (TLC) was carried out on precoated (0.25 mm) silica gel aluminum plates.Visualization was performed with short-wavelength UV and/or revealed with potassium permanganate solutions. 1H NMR spectra were recorded at room temperature at 500 MHz and 13 C were recorded at 126 MHz.The data are reported as follows: chemical shift in parts per million referenced to residual solvent (CDCl3 δ 7.26 ppm, CD3OD δ 3.31 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 (hertz), and integration. 13C{ 1 H}MR spectra were recorded at room temperature using 126 MHz.The data are reported as follows: chemical shift in parts per million referenced to residual solvent (CDCl3 δ 77.16 ppm, CD3OD δ 49.00 ppm).Infrared spectra were recorded on a Fourier-transform infrared spectrophotometer with a single-reflection diamond ATR module, and signals were reported in cm −1 .Mass spectra were recorded through electrospray ionization positive-ion mode.A Hybrid Quadrupole-Orbitrap mass analyzer was used for HRMS measurements.Optical rotations were measured at room temperature from the sodium D line (589 nm) using CHCl3 as solvent unless otherwise noted, and calculated using the following formula: [α]D = (100)αobs/(l•c)), where c = (g of substrate/100 mL of solvent) and l = 1 dm.Diol 31, methylester S1 and aldehyde 32 were prepared using previously reported procedures [17][18][19].

General Information-DFT Calculations
Quantum mechanics calculations were conducted in Gaussian 16 [23] using the M06-2X [24,25] density functional in conjunction with the 6-31G* basis set, the LANLDZpd [30,31] effective core potential for Iodide, and using the polarizable continuum solvation model for 2,6-lutidine (PCM) [26].Frequency calculations were carried out on all optimized geometries to distinguish minima (no imaginary frequencies) or transition structures (one imaginary frequency).The geometry and transition state optimizations (Berny algorithm) were achieved with tight SCF convergence and an ultrafine integral.The different conformations of the rotamers and the C2 ′ -endo and C2 ′ -exo ring conformations were evaluated.

Scheme 3 .
Scheme 3. (a) This work focuses on the synthesis of 4′-thionucleoside analogues bearing a C2′ a carbon quaternary center formed from intermediate 28.(b) Potential to reach both C2′ stereochem istries.

Scheme 8 .
Scheme 8. Relative Gibbs free energy (kcal/mol) profile in 2,6-lutidine using the continuum sol tion model (PCM) for (a) cyclization of 1′,2′-syn thioaminal 54 as a model of 46b, and (b) cyclizat of 1′,2′-syn thioaminal 58 as a model of 49.U = Uracil, ∆Gact corresponds to the TS free ene Scheme 8. Relative Gibbs free energy (kcal/mol) profile in 2,6-lutidine using the continuum solvation model (PCM) for (a) cyclization of 1 ′ ,2 ′ -syn thioaminal 54 as a model of 46b, and (b) cyclization of 1 ′ ,2 ′ -syn thioaminal 58 as a model of 49.U = Uracil, ∆G act corresponds to the TS free energy difference with the lowest corresponding acyclic thioaminal free energy 54 and 58.Otherwise, all free energies are arbitrarily referenced to the lowest energy acyclic conformer of compound 51 at 423.2 K (refer to Scheme 7).CYLview structures for TS are shown [27].‡ : The symbol is used to denote transition states.
c Isolated yields.
c Isolated yields.
a Reaction conditions: c Isolated yields.
a Reaction conditions: c Isolated yields.
a Reaction conditions: c Isolated yields.
a Determined by1H NMR analysis of the crude reaction mixture.b Isolated yields.
a Determined by1H NMR analysis of the crude reaction mixture.b Isolated yields.