WO2021140471A1 - Method for synthesis of 2'-alkyl- or 2'-alkenyl- or 2'-alkynyl-4'-fluoro-adenosine derivatives and intermediates thereof - Google Patents

Abstract

A method of making nucleoside 1, which has a 3-hydrocarbyl-5-fluoro-5-(hydroxy-methyl)tetrahydrofuran-3,4-diol ring system, where hydrocarbyl group R1 is lower alkyl, lower alkenyl, or lower alkynyl and R2 may be an adenine moiety having an exocyclic amino group protected as an imidodicarbonate or as a carbamate. Nucleoside 1 is prepared from an intermediate 4 by adding iodine fluoride across the exocyclic double bond to produce a 5-fluoro-5-iodomethyl derivative 5. The iodine atom is displaced with a carboxylic acid nucleophile in the presence of an oxidizing agent to produce a 5-fluoro-5-(acyloxy)methyl ester derivative 7, where R³ is an acyl group. Ester 7 may be converted into compound 1 by cleavage of the ester.

Description

METHOD FOR SYNTHESIS OF 2'-ALKYL- OR 2'-ALKENYL- OR '-ALKYNYL-4'-FLUORO-ADENOSINE DERIVATIVES AND INTERMEDIATES THEREOF

TECHNICAL FIELD

[001] This disclosure relates generally to preparation of nucleoside analogs having a 5- hydroxymethyl-5-fluorotetrahydrofuran-3,4-diol ring. More particularly, this disclosure relates to preparation of nucleoside analogs having a 5-hydroxymethyl-5-fluorotetrahydrofuran-3,4-diol ring in high yield using a 5-methylenetetrahydrofuran-3,4-diol intermediate.

BACKGROUND

[002] Many nucleoside analogs have exhibited antiviral activity through inhibition of DNA or RNA polymerases in the replication of viral DNA. Nucleoside analogs with fluorinated ribose or deoxyribose rings are known in the art. Such nucleoside analogs include 2'-fluoro-2'-methyl cytosine; 1-methyl-5-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)uracil (FMAU); and 3'a-fluoro- 2',3'-dideoxyuridine (FddU). The naturally occurring fluorinated nucleoside nucleocidin is an antibacterial 4’-fluoroadenosine derivative.

[003] Thus, biologically active fluorinated nucleoside analogs are of great interest in the medical and pharmaceutical arts. There is a need in the art for methods of preparing such fluorinated nucleosides.

SUMMARY

[004] Various embodiments disclosed herein are directed to methods of preparing a nucleoside compound of Formula 1 having a fluorinated ribose group:

or a salt thereof; wherein:

R¹ is a substituted or unsubstituted hydrocarbyl group; R² is an adenin-9-yl group of the formula:

wherein protecting group P³ is an alkylcarbonyl group, an arylcarbonyl group, an alkoxycarbonyl group, or an aryloxycarbonyl group; and A is a substituent selected from the group consisting of an alkyl group, an alkoxy group, an aryl group, an aryloxy group, a halogen, a substituted or unsubstituted amino group, or a protected amino group; n is 0 to 2; and x is 0 or 1.

[005] In various embodiments of Formula 1, R¹ is a substituted or unsubstituted hydrocarbyl group. In various embodiments, the hydrocarbyl is a lower hydrocarbyl group selected from the group consisting of a C1-C6 alkyl group, a C2-C6 alkenyl group, and a C2-C6 alkynyl group. In some embodiments, R¹ is a C2-C6 alkynyl group; x is 0; and P³ is an alkoxycarbonyl group. In some embodiments, R¹ is an ethynyl group.

[006] Various embodiments disclosed herein are directed to methods of preparing a nucleoside compound of Formula la having a fluorinated ribose group:

or a salt thereof; wherein R¹ is a substituted or unsubstituted hydrocarbyl group. In some embodiments, R¹ is a lower hydrocarbyl group selected from the group consisting of a C1-C6 alkyl group, a C2-C6 alkenyl group, and a C2-C6 alkynyl group. In some embodiments, R¹ is a C2-C6 alkynyl group. In some embodiments, R¹ is an ethynyl group.

[007] Disclosed herein are methods of preparing a compound of Formula 1:

or a salt thereof; wherein:

R¹ is selected from the group consisting of a C1-C6 alkyl group, a C2-C6 alkenyl group, and a C2- C6 alkynyl group;

R² is an adenin-9-yl group of formula:

where protecting group P³ is an an alkylcarbonyl group, an arylcarbonyl group, an alkoxycarbonyl group, or an aryloxycarbonyl group;

A is an alkyl group, an alkoxy group, an aryl group, an aryloxy group, a halogen atom, a substituted or unsubstituted amino group, or a protected amino group; n is 0 to 2; and x is 0 or 1 ; comprising any one or more of the following steps:

(1) converting the OH of the hydroxymethyl group on a compound of Formula 2 to a leaving group L¹ to produce a compound of Formula 3;

wherein L¹ is a leaving group selected from the group consisting of a halide leaving group, a sulfonate leaving group, and a carboxylate leaving group; and each P¹ is independently selected from the group consisting of H and a protecting group P²; (2) eliminating the leaving group L¹ from the compound of Formula 3 to produce a compound of Formula 4;

(3) iodo-fluorinating the compound of Formula 4, optionally with triethylamine trihydrofluoride and an iodine source, to produce a compound of Formula 5,

(4) when each P¹ in the compound of Formula 5 is H (a compound of Formula 5a), protecting the hydroxyl groups with a protecting group P² to form a compound of Formula 5b,

(5) reacting the compound of Formula 5b with a nucleophile of formula R³OH, wherein R³ is alkylcarbonyl, arylcarbonyl, alkylsulfonyl, or arylsulfonyl, in the presence of an oxidizing agent to produce the compound of Formula 7; and

(6) deprotecting the compound of Formula 7 to produce the compound of Formula 1.

[008] In some embodiments, the method comprises step 1, step 2, step 3, step 4, or step 5, or comprises steps 4, 5, and optionally 6, or comprises steps 3, 4, 5, and optionally 6, or comprises 2,

3, 4, 5, and optionally 6, or comprises steps 2, 3, 4, 5, and optionally 6, or comprises steps 1, 2, 3,

4, 5, and optionally 6, or comprises steps 2, 3, and 4, or comprises 2, 3, and 5, or comprises 1, 2, 3, 5, and optionally 6.

[009] In light of the present need for improved methods of antiviral nucleoside analogues, a brief summary of various exemplary embodiments is presented. Detailed descriptions of various embodiments adequate to allow those of ordinary skill in the art to make and use the concepts disclosed herein will follow in later sections.

DEFINITIONS

[0010] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art. In the event that there are a plurality of definitions for a term herein, those in this section prevail unless stated otherwise. [0011] As described herein, the terms “optionally substituted” and “substituted or unsubstituted” are synonyms. Whenever a group is described as being substituted, optionally or otherwise, by various indicated substituents, the group may be substituted with one or more of the indicated substituents.

[0012] As used herein, “alkyl” refers to a fully saturated linear, branched, or cyclic hydrocarbon group. The alkyl group may be a lower alkyl, having 1 to 6 carbon atoms. The alkyl group may be designated as “C1 to C6 alkyl” or similar designations, indicating that the alkyl group is a linear or branched alkyl group having up to six carbon atoms, such as methyl, ethyl, propyl, isopropyl. butyl, isobutyl, tertiary butyl, pentyl and hexyl. The alkyl group may be substituted or unsubstituted.

[0013] As used herein, “alkenyl” refers to a linear, branched, or cyclic hydrocarbon group having one or more double bonds. The double bond may be at any position, unless otherwise indicated. An alkenyl group may be unsubstituted or substituted.

[0014] As used herein, “alkynyl” refers to a linear or branched hydrocarbon group having one or more triple bonds. The triple bond may be at any position, unless otherwise indicated. An alkynyl group may be unsubstituted or substituted.

[0015] Unless otherwise indicated, “hydrocarbyl” refers to an alkyl, alkenyl, or alkynyl group. [0016] As used herein, “aryl” refers to a monocyclic or bicyclic aromatic ring system having carbocyclic rings, unless otherwise indicated. Examples of aryl groups include, but are not limited to, benzene and naphthalene. An aryl group may be substituted or unsubstituted.

[0017] As used herein, “heteroaromatic” and “heteroaryl” refer to a monocyclic, bicyclic or tricyclic aromatic ring system that contain(s) one or more heteroatoms, including but not limited to, nitrogen, oxygen and sulfur. Furthermore, the term “heteroaromatic” and “heteroaryl” include fused ring systems where two rings, such as at least one aryl ring and at least one heteroaryl ring, or at least two heteroaryl rings, share a chemical bond. Examples of heteroaryl rings include, but are not limited to, a pyrrole ring, an imidazole ring; a pyrazole ring, an indole ring system, a benzimidazole ring system, an indazole ring system, or a purine ring system. A heteroaryl group may be substituted or unsubstituted.

[0018] As used herein, “arylalkyl” refers to an aryl group connected, as a substituent, to a lower alkylene group. The lower alkylene and aryl group of an aryl(alkyl) may be substituted or unsubstituted. Examples include but are not limited to benzyl, 2-phenyl(alkyl), 3-phenyl(alkyl), diphenylmethyl, and triphenylmethyl.

[0019] As used herein, “acyl” refers to an alkyl, alkenyl, alkynyl, or aryl group connected, as a substituent, to a carbonyl group. Examples include acetyl, propanoyl, and benzoyl. An acyl may be substituted or unsubstituted. [0020] A “sulfonyl” group refers to an -SO₂R group, in which R can be alkyl, alkenyl, alkynyl, or aryl, heteroaryl. A sulfonyl may be substituted or unsubstituted.

[0021] The term “ester,” as used herein, refers to a -OCOR or -OSO₂R group in, which R can be alkyl, alkenyl, alkynyl, aryl, heteroaryl, or aryl(alkyl). An ester may be substituted or unsubstituted.

[0022] The term “nucleoside” is used herein refers to a compound composed of an optionally substituted ribose or deoxyribose moiety attached to a heterocyclic base via a N-glycosidic bond, such as attached via the 9-position of a purine base or the 1 -position of a pyrimidine base. In some instances, the nucleoside can be a nucleoside analog.

[0023] As used herein, the term “heterocyclic base” refers to an optionally substituted nitrogen- containing heterocyclic ring compound that can be attached to a ribose or deoxyribose moiety. In some embodiments, the heterocyclic base can be selected from an optionally substituted purine base or an optionally substituted pyrimidine base. A non-limiting list of optionally substituted purine bases includes purine, adenine, guanine, hypoxanthine, xanthine, alloxanthine, theobromine, caffeine, uric acid and isoguanine. A non-limiting list of optionally substituted pyrimidine bases includes cytosine, thymine, uracil, and 5,6-dihydrouracil. Where a heterocyclic base has a ring carbonyl, an exocyclic amino substituent, or other functional groups, these groups may be protected with a protecting group by methods known in the art.

[0024] The term “protecting group” as used herein refer to any atom or group of atoms that is added to a molecule in order to prevent existing groups in the molecule from undergoing unwanted chemical reactions. Examples of protecting group moieties are described in T. W. Greene and P. G. M Wuts, Protective Groups in Organic Synthesis, 3. Ed, John Wiley & Sons, 1999, incorporated by reference for the limited purpose of disclosing suitable protecting groups. A non- limiting list of protecting groups includes:

Hydroxy protecting groups, such as methoxymethyl, ethoxymethyl, tetrahydropyran-2-yl, tetrahydrofuran-2-yl, t-butyl, allyl, benzyl, trimethylsilyl, t-butyldimethylsilyl, t- butyldiphenylsilyl, acetyl, pivaloyl, and benzoyl; 1,2-Diol protecting groups, such as acetonide and benzylidene; and Amino protecting groups, such as 9-fluorenylmethoxycarbonyl (Fmoc), t-butoxy carbonyl (Boc), benzyloxycarbonyl, phthalimide, benzyl, triphenylmethyl, and benzylidene.

[0025] The term “protected hydroxy group” as used herein refers to a moiety derived from a hydroxy group by replacing the hydroxyl hydrogen with a hydroxy protecting group. The term “protected amino group” as used herein refers to a moiety derived from an amino group by replacing at least one amino hydrogen with an amino protecting group.

[0026] Unless otherwise indicated, a person of ordinary skill in the art would understand that protecting groups can be replaced with other protecting groups which serve a similar protective function. For example, in the protection of a hydroxy group, methoxymethyl may be replaced with tetrahydropyran-2-yl, allyl, or benzoyl. In the protection of an amino group, t-butoxycarbonyl may be replaced with phthalimide, benzyl, or triphenylmethyl. Diols may be individually protected with separate hydroxy protecting groups, or protected as a cyclic acetal or ketal, e.g., as an acetonide.

[0027] Unless indicated otherwise, IUPAC numbering will be used herein. When referring to a compound of Formula 1 or a derivative thereof, the ribose ring will be numbered as a tetrahydrofuran derivative. Thus, the R² group is normally identified as attached to the carbon atom in the 2-position, and fluorine is attached to the carbon atom in the 5-position, marked with an asterisk, although the numbering about the ribose ring may be reversed in some chemical names. In some cases, a compound of Formula 1 or a derivative thereof may be named as a nucleoside derivative, e.g., 2’-ethynyl-4’-fluoroadenosine, where R² is adenine and R¹ is ethynyl. When a compound of Formula 1 is named as a nucleoside derivative, the R² group is attached to the carbon atom in the 1 ’-position, and fluorine is attached to the carbon atom in the 4’ -position, marked with an asterisk. Both numbering systems are known in the art and should be understood as synonymous.

[0028] Terms and phrases used in this application and variations thereof, especially in the appended claims, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of any of the foregoing, the terms “including,” “containing,” and “comprising” are synonymous, and should be read to mean “including but not limited to” or “including at least,” and do not exclude additional, unrecited elements or method steps. The term “example” is used to provide exemplary instances of the item in discussion, not an exhaustive or limiting list thereof. When used in the context of a process, the term “comprising” means that the process includes at least the recited steps but may include additional steps. When used in the context of a compound or composition, the term “comprising” means that the compound or composition includes at least the recited features or components but may also include additional features or components. Likewise, a group of items linked with the conjunction “and” should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as “and/or” unless expressly stated otherwise. Similarly, a group of items linked with the conjunction “or” should not be read as requiring mutual exclusivity among that group but should be read as “and/or” unless expressly stated otherwise.

[0029] Where a range of values is provided, it is understood that the upper and lower limit is encompassed within the range.

DETAILED DESCRIPTION

[0030] The present application is directed to a method of preparing a nucleoside compound of Formula 1, having a fluorinated ribose group:

or a salt thereof; wherein: R¹ is a substituted or unsubstituted hydrocarbyl group; and may be a lower hydrocarbyl group selected from the group consisting of a C1-C6 alkyl group, a C2-C6 alkenyl group, and a C2- C6 alkynyl group; and

R² is an adenin-9-yl group having the following structure:

wherein protecting group P³ is an alkylcarbonyl group, an arylcarbonyl group, an alkoxycarbonyl group, or an aryloxycarbonyl group;

A is an alkyl group, an alkoxy group, an aryl group, an aryloxy group, a halogen atom, a substituted or unsubstituted amino group, or a protected amino group; n is 0 to 2; and x is 0 or 1.

[0031] In various embodiments, R¹ is methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec- butyl, a linear or branched pentyl group, a linear or branched hexyl group, ethenyl, propen- 1-yl, propen-2-yl, 2-methylpropen-1-yl, butene- 1-yl, buten-2-yl, a linear or branched pentenyl group, a linear or branched hexenyl group, ethynyl, propyn-1-yl, propyn-3-yl, 1-butyne-1-yl, 2-butyn-1-yl, a linear or branched pentynyl group, or a linear or branched hexynyl group. In some embodiments, R¹ is an alkynyl group selected from the group consisting of ethynyl, propyn-1-yl, propyn-3-yl, 1- butyne-1-yl, 2-butyn-1-yl, a linear or branched pentynyl group, and a linear or branched hexynyl group. In various embodiments, R¹ is ethynyl.

[0032] In various embodiments shown in Scheme 1, the method involves using the compound of Formula 2 as a starting material. In the compound of Formula 2, R¹ and R² are as described above for Formula 1. The 3,4-diol moiety on the compound of Formula 2 may be used without protection, e.g., P¹ may be hydrogen. Alternatively, the hydroxyl groups may each be protected with a hydroxyl protecting group P², where P² may be alkoxyalkyl, such as methoxymethyl or tetrahydropyran-2-yl group; an allyl group; benzyl substituted or unsubstituted benzyl group, such as benzyl or diphenylmethyl; t-butyl; a trialkyl-, dialkylaryl-, diarylalkyl-, or triarylsilyl group; a substituted or unsubstituted benzoyl group; or a substituted or unsubstituted alkanoyl group; or both P² groups may together be a bivalent acetal or ketal group, forming part of a substituted or unsubstituted dioxolane ring. If both P² groups together are a bivalent acetal or ketal group, the P² groups may form an acetonide or benzylidene group.

[0033] As shown in Scheme 1 , methods disclosed herein comprise converting the OH of the hydroxymethyl group on a ribose derivative of Formula 2, e.g., where R¹ is lower alkyl, lower alkenyl, or lower alkynyl, to a leaving group L¹ to produce a compound of Formula 3, wherein L¹ is a leaving group selected from the group consisting of a halide leaving group, a sulfonate leaving group, and a carboxylate leaving group, and in some embodiments is iodide; and each P¹ is independently selected from the group consisting of H and a protecting group P², or both P² groups may together form a cyclic acetal or ketal protective group, such as a dioxolane ring.

[0034] In various embodiments of the synthesis disclosed herein, the starting material compound of Formula 2 is characterized in that R¹ is: a C1-C6 alkyl group, such as methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, isobutyl, pentyl, or hexyl; a C2-C6 alkenyl group, such as vinyl, allyl, 1 -methylethenyl, butenyl, 3-methylallyl, pentenyl, or hexenyl; or a C2-C6 alkynyl group, such as a group of formula -(CH₂)_nC≡C(CH₂)_mH, where m+n = between 0 and 4.

[0035] In various embodiments of the synthesis disclosed herein, the starting material compound of Formula 2 is characterized in that R¹ is an ethynyl group. In various embodiments, the exocyclic amino group -NH_x(P³)_2-x is -NHP³ or -N(P³)2, where P³ is an alkoxy carbonyl group. [0036] The introduction of the leaving group may be accomplished under conditions known in the art. In some embodiments, the method comprises converting the OH of the hydroxymethyl group on ribose derivative 2 to leaving group L¹, wherein L¹ is an iodide leaving group, by reacting the compound of Formula 2 with iodine and a triarylphosphine such as triphenylphosphine, optionally in the presence of a base, to produce the compound of Formula 3 In various embodiments where P¹ is a hydroxy protecting group P², the OH of the hydroxymethyl group on compound of Formula 2 may be converted to a leaving group L¹, where L¹ is alkylsulfonate or arylsulfonate, by reacting the compound of Formula 2 with an alkylsulfonyl chloride or an arylsulfonyl chloride, as appropriate, to produce a sulfonate ester of Formula 3 Similarly, the OH of the hydroxymethyl group on the compound of Formula 2 may be converted to an arylcarboxylate leaving group L¹, where L¹ is arylcarboxylate, by reacting the compound of Formula 2 with a substituted or unsubstituted arylcarbonyl chloride to produce an acyl ester of compound of Formula 3

[0037] In various embodiments disclosed herein, the OH of the hydroxymethyl group on the protected compound of Formula 2 may be converted into a sulfonate ester as disclosed above, and the sulfonate group may then be displaced with a halogen anion to produce a 3-hydrocarbyl-5- halomethyltetrahydrofuran-3,4-diol compound of Formula 3, where L¹ is a halogen atom.

[0038] In various embodiments, preparation of the compound of Formula 4 as shown in Scheme 1 involves eliminating the leaving group L¹, such as an iodide group, from the compound of Formula 3 by reacting the compound of Formula 3 with a non-nucleophilic base to produce the 5- methylenetetrahydrofuran-3,4-diol derivative of Formula 4 Suitable non-nucleophilic bases include: amine bases, such as N,N-diisopropylethylamine, l,8-diazabicycloundec-7-ene (DBU), l,5-diaza-bicyclo(4.3.0)non-5-ene (DBN), and 2,6-di-tert-butylpyridine; phosphazene bases; amide salts, such as lithium diisopropylamide (LDA), sodium or potassium bis(trimethylsilyl)amide (NaHMDS and KHMDS, respectively), and lithium tetramethylpiperidide (LiTMP or harpoon base); metal hydride bases, such as sodium hydride and potassium hydride; and alkoxide bases, such as sodium or potassium tert-butoxide. In various embodiments, L¹ is chloride, bromide, iodide, p-toluenesulfonate, methanesulfonate, benzoyl, or 3-chlorobenzoyl. In some embodiments, L¹ is iodide, and conversion of the compound of Formula 2 to the compound of Formula 4 as shown in Scheme 1 proceeds in an overall yield of from 60% to 80%.

[0039] In some embodiments, P² is an alkylcarbonyl (e.g., acetyl), benzoyl, benzyl, alkoxyalkyl, or allyl group, or 2 P² groups in combination form a dioxolane ring substituted with one or two alkyl groups (e.g., 2 P² groups taken together with the oxygens to which they are attached form -OC(R^x)(R^y)0-, wherein R^xand R^y are each H or Cl-4alkyl).

[0040] In the synthesis described in Scheme 1, each P¹ may be independently selected from the group consisting of H and a hydroxy protecting group P². The hydroxy protecting group P² may be acetyl, benzoyl, benzyl, tetrahydropyran-2-yl, tetrafuran-2-yl, or alkoxyalkyl. Alternatively, both P² groups in combination may together form a cyclic acetal or ketal protective group. In various embodiments, both P² groups in combination may be a substituted or unsubstituted methylene group, e.g., >C(CH₃)₂ or >CH — C₆H₅, thereby completing a dioxolane ring. In various embodiments, each P¹ is H. In various embodiments, each P¹ is a protecting group P², and each P² is acetyl, benzoyl, benzyl, or alkoxyalkyl; or both P² groups in combination form a dioxolane ring as described herein. In various embodiments, each P¹ is H. In various embodiments, each P¹ is independently selected from the group consisting of H and P², where P² is an alkylcarbonyl (e.g., acetyl), benzoyl, benzyl, alkoxyalkyl, or allyl group. In various embodiments, each P¹ is P², where both P² groups together form a cyclic acetal or ketal protective group as a dioxolane ring as described herein. In some embodiments, each P² is acetyl. In some embodiments, each P¹ is H in compounds 2, 3, and 4, and are converted to P² protecting groups prior to or after introduction of the leaving group. [0041] The 5-methylenetetrahydrofuran-3,4-diol derivative of Formula 4 is fluoro-iodinated, for example, by reaction with triethylamine trihydrofluoride and an iodinating agent sue has N- iodosuccinimide, to produce a 5-fluoro-5-(iodomethyl) tetrahydrofuran-3,4-diol derivative of Formula 5, where P¹ is H or a hydroxy protecting group P². In various embodiments, fluoro- iodinating the 5-methylenetetrahydrofuran-3,4-diol derivative of Formula 4, optionally by reacting with triethylamine trihydrofluoride and N-iodosuccinimide (NIS), produces 5-fluoro-5- (iodomethyl) tetrahydrofuran-3,4-diol derivative of Formula 5 in a yield of between from about 50% to about 99%, from about 55% to about 95%, or from about 60% to about 90%. In various embodiments, reacting the 5-methylenetetrahydrofuran-3,4-diol derivative of Formula 4 with triethylamine trihydrofluoride and N-iodosuccinimide produces an α-fluoro-β-iodomethyl anomer of Formula 5 in excess over an a-iodomethyl-β-fluoro anomer, by a ratio of from 5: 1 to 10:1, from 6:1 to 9:1, or from 6.5:1 to 9:1.

[0042] In various embodiments, P¹ may be a hydroxy protecting group P² (Formula 5b), where P² may be an acyl protecting group, e.g., acetyl or benzoyl, which is removable under acidic or basic conditions, or by reaction with a nucleophile. In various embodiments, P¹ may be a hydroxy protecting group P², where P² may be a monovalent acetal or ketal protecting group, e.g., an alkoxyalkyl group such as methoxymethyl or ethoxymethyl, tetrahydropyran-2-yl, or tetrahydrofuran-2-yl. Alternatively, each P¹ may be a hydroxy protecting group P², where the P² groups may be taken in combination to form a cyclic acetal or ketal protecting group, e.g., acetonide or benzylidene groups. Such monovalent and cyclic acetal and ketal protecting groups are stable under basic conditions and labile to acid. Other protecting groups known in the art may be used as P². Suitable iodinating agents include N-iodosuccinimide; N-iodosaccharine, and 1,3- diiodo- 5 , 5 -dimethy lhy dantoin.

[0043] In various embodiments, the hydroxyl groups on the 5-iodomethyl-5- fluorotetrahydrofuran-3,4-diol compound of Formula 5a are then protected with a protecting group P² to produce the compound of Formula 5b, where P² is as defined above. In various embodiments, the 3,4-diol moiety may be protected by conversion into an acetal or ketal moiety, such as an acetonide ring. In various embodiments, the 3,4-diol moiety may be protected by conversion into an ester by reaction with an acid chloride, such as benzoyl chloride or acetyl chloride, or an anhydride, such as benzoic anhydride or acetic anhydride.

[0044] In some cases, it may be desirable to convert 5-methylenetetrahydrofuran compound of Formula 4, where P¹ is a first protecting group P², to compound 5b, where P² is a second protecting group different from P¹, as shown in Schemes 1 and 2. In such cases, after conversion of compound 4 to compound 5, where each P¹ is a first protecting group P², the 3,4-diol moiety on the compound of Formula 5 is deprotected to remove the P¹ protecting groups to form a compound of Formula 5a. The 3,4-diol moiety is then protected with a hydroxyl protecting group P², which is different from the first protecting group P². This may allow replacement of a protective group P² which is stable under basic conditions, e.g., an acetal or ketal protecting group, with an acyl protecting group which may be removed under basic conditions, for example.

[0045] The 5-fluoro-5-(iodomethyl) tetrahydrofuran-3,4-diol derivative of Formula 5 (e.g., 5a or 5b as shown in Scheme 2) is then converted into a carboxylate or sulfonate ester of Formula 7, where P² is a hydroxy protecting group and R³ is alkylcarbonyl, arylcarbonyl, alkylsulfonyl, or arylsulfonyl, as shown in Schemes 2 and 3. When P¹ in the compound of Formula 5 is H, shown as Formula 5a, conversion of the compound of Formula 5a to the ester of Formula 7 comprises protecting the hydroxyl groups on the 5-iodomethyl-5-fluorotetrahydrofuran-3,4-diol derivative of Formula 5a with a protecting group P² to produce the compound of Formula 5b, and reacting the compound of Formula 5b with a nucleophile of formula R³OH in the presence of an oxidizing agent to produce the ester of Formula 7. If P¹ is a hydroxy protecting group P² (P¹ = P²), reacting the compound of Formula 4 with triethylamine trihydrofluoride and N-iodosuccinimide produces the compound of Formula 5b directly. The compound of Formula 5b may then be reacted with a nucleophile of formula R³OH in the presence of an oxidizing agent to produce the ester of Formula 7. In various embodiments, reacting the compound of Formula 5b with a nucleophile comprises reacting the compound of Formula 5b with a nucleophile of formula R³OH in the presence of a peracid oxidizing agent of formula R³O-OH, where R³ is a substituted or unsubstituted acyl group, e.g., acetyl or benzoyl.

[0046] In an embodiment, the iodide in compound 5b may be displaced by a nucleophile of formula R³OH, where R³ is alkylcarbonyl, arylcarbonyl, alkylsulfonyl, or arylsulfonyl, in the presence of an oxidizing agent to produce ester 7. In various embodiments, the iodomethyl group in compound 5b is reacted with a nucleophile in the presence of an oxidizing agent capable of oxidizing iodine, as shown in Scheme 3. In various embodiments, oxygen nucleophiles of formula HOR³ may be used, where -R³ is alkylcarbonyl, arylcarbonyl, alkylsulfonyl, or arylsulfonyl, as shown in Scheme 2. If a carboxylic acid nucleophile is used, the reaction may be carried out in the presence of a base.

[0047] A variety of oxidizing agents suitable for oxidizing iodides may be used. Suitable oxidizing agents may, for example, include hydrogen peroxide, peroxy acids, and permanganate salts. In various embodiments, the oxidizing agent is a peroxy acid of formula HO-OR³, and the nucleophile is a carboxylic acid of formula HOR³, where each R³ is alkylcarbonyl or arylcarbonyl. In some embodiments, the oxidizing agent has formula HO-OR³, and the nucleophile has formula HOR³, where both R³ groups are identical. Thus, peroxybenzoic acid may be used as the oxidant, and benzoic acid may be used as the nucleophile. Similarly, peroxyacetic acid may be used as the oxidant, and acetic acid may be used as the nucleophile. In some embodiments, inorganic permanganate salts, such as KMn04, or organic permanganate salts, such as tetrabutylammonium permanganate, may be used as an oxidizing agent. In various embodiments, a peracid and a carboxylic acid may be separately added to the reaction mixture. In various embodiments, a carboxylic acid anhydride R³ ₂O, where R³ is acyl, may be reacted with H₂O₂ to produce a mixture of the oxidant R³O-OH and the nucleophile R³OH, which may be combined with iodide 5b. [0048] In various embodiments disclosed herein, compound 2 may be converted to compound 7 in the synthesis of Scheme 2, where P¹ for compound 2 is H, by sequentially: converting the OH of the hydroxymethyl group on compound 2 to leaving group L¹, wherein L¹ is an iodide leaving group, by reacting compound 2 with iodine and a triarylphosphine, to form compound 3; eliminating the leaving group L¹ from compound 3 by reacting compound 3 with a non- nucleophilic base to form compound 4; reacting compound 4 with triethylamine trihydrofluoride and an iodine source, such as N- iodosuccinimide, to produce compound 5 (5a); protecting compound 5a with a protecting group P² to produce compound 5b; and reacting compound 5b with a nucleophile of formula R³OH, wherein R³ is arylcarbonyl, optionally in the presence of an oxidizing agent, to produce ester 7.

Ester 7 may then be converted into compound 1, if desired, by removing the protecting groups P² on the diol moiety and the R³ group on compound 7, where the protecting groups P² and the R³ group may be removed simultaneously or sequentially.

[0049] In various embodiments disclosed herein, compound 2 may be converted to compound 7 in the synthesis of Scheme 2, where P¹ in compound 2 is a protecting group P², by sequentially: converting the OH of the hydroxymethyl group on compound 2 to leaving group L¹, wherein L¹ is an iodide leaving group, by reacting compound 2 with iodine and a triarylphosphine to form compound 3; eliminating the leaving group L¹ from compound 3 by reacting compound 3 with a non- nucleophilic base to form compound 4; reacting compound 4 with triethylamine trihydrofluoride and an iodine source, such as N- iodosuccinimide, to produce compound 5b; and reacting compound 5b with a nucleophile of formula R³OH, wherein R³ is arylcarbonyl, optionally in the presence of an oxidizing agent, to produce ester 7. Ester 7 may then be converted into compound 1.

[0050] In various embodiments, the R³ group on the ester of Formula 7 is removed to produce the compound of Formula 1. In various embodiments, the method may further include a step of removing a protecting group P³ from the adenin-9-yl moiety in the R² group of the compound of Formula 1. In various embodiments shown in Schemes 1 or 2, each P² is an acetyl or benzoyl group, and deprotecting an ester of Formula 7 comprises deprotecting the exocyclic amino group -NH_X(P³)_2-X by removing the alkoxycarbonyl group P³ under acidic conditions; and, either before or after deprotecting the exocyclic amino group, removing the acetyl or benzoyl groups P² from the ester of Formula 7 by reaction with a nucleophilic base, and converting the -OR³ group on ester 7 into a hydroxyl group. In various embodiments, the nucleophilic base is a primary amine. [0051] In various embodiments, each P² is an acetyl or benzoyl group, R³ is a substituted benzoyl group, and deprotecting the ester of Formula 7 comprises deprotecting the exocyclic amino group -NH_X(P³)_2-X by removing the alkoxycarbonyl group P³ under acidic conditions; and, either before or after deprotecting the exocyclic amino group, simultaneously removing each P² group and the R³ group from ester 7 by reaction with a nucleophilic base.

[0052] As shown in Scheme 4, in various embodiments where P¹ is H; the ribose derivative of Formula 2 may be produced by protecting the hydroxyl groups on adenosine derivative 2-1 with silyl groups to produce a compound of Formula 2-2, where R³ is linear or branched lower alkyl or aryl (e.g., C1 -4 alkyl, such as methyl, ethyl, or tert-butyl, or phenyl, or a combination thereof, such as tert-butyl-dimethylsilyl, trimethylsilyl, or tert-butyl-diphenylsilyl). Next, the exocyclic amino group on the compound of Formula 2-2 may be protected with protecting group P³ to produce a compound of Formula 2-3. The silyl groups on the compound of Formula 2-3 may then be removed to produce a compound of Formula 2. Conversion of compound 2-1 to a compound of Formula 2 may proceed in an overall yield of from 60% to 80%.

[0053] Various embodiments disclosed herein are directed to a protected nucleoside of Formula

X:

or a salt thereof; wherein R⁴ is a linear or branched lower alkyl or aryl;

A is an alkyl group, an alkoxy group, an aryl group, an aryloxy group, a halogen atom, a substituted or unsubstituted amino group, or a protected amino group; x is 0 or 1 ; n is 0 to 2; and

P³ is a C1-C6 alkoxy carbonyl group or a C1-C6 acyl group.

[0054] In various embodiments, compound 2 is an adenosine analogue, where P¹ is H; R¹ is a lower hydrocarbyl group selected from the group consisting of a C1-C6 alkyl group, a C2-C6 alkenyl group, and a C2-C6 alkynyl group ; and R² is an adenine group with a protected exocyclic amino group of formula -NH_X(P³)_2-X, shown as compound 11 of Scheme 5. In various embodiments, the P³ protecting group on compound 11 may be a 9-fluorenylmethoxy carbonyl (Fmoc) group; a t-butoxycarbonyl (Boc) group; a benzyloxycarbonyl group; or a C1-C6 alkylcarbonyl group. If x is 0, then -N(P³)2 may also be a phthalimide or benzylideneamino group. In some embodiments, P³ may be a Boc group and R³ may be ethynyl.

[0055] According to Scheme 5, the adenosine analogue 11 may be produced by: protecting the hydroxyl groups on compound 8 with trimethylsilyl groups to produce compound 9; protecting the exocyclic amino group on adenine with an P³ protecting group to produce compound 10; and removing the trimethylsilyl groups from compound 10 to produce compound 11.

[0056] In various embodiments disclosed herein, compound 11 may be converted into an adenosine analogue of formula la, by following the procedure of Scheme 6. The OH of the hydroxymethyl group on N-protected adenosine nucleoside 11 may be converted to iodomethyl compound 12 by reacting compound 11 with iodine and a phosphine compound PAr₃ where Ar is aryl. Next, the iodine atom is eliminated by using a non-nucleophilic base, e.g., DBU, DBN, or an amide salt, to convert nucleoside 12 to nucleoside 13, having a 5-methylenetetrahydrofuran-3,4- diol ring.

[0057] Next, nucleoside 13 is reacted with triethylamine trihydrofluoride and N- iodosuccinimide to add iodine and fluorine across the double bond, producing 2’-hydrocarbyl-4’- fluoro-5’-iodo adenosine 14 When nucleoside 13 with is reacted with triethylamine trihydrofluoride and N-iodosuccinimide (NIS), nucleoside 14 may be produced as the α-fluoro-β- iodomethyl anomer in an overall yield of from about 50% to about 99%, from about 55% to about 95%, or from about 60% to about 90%. In various embodiments, the α-fluoro-β-iodomethyl anomer 14 is produced in excess over the corresponding a-iodomethyl-β-fluoro anomer, by a ratio of from 5:1 to 10:1, from 6:1 to 9:1, or from 6.5:1 to 9:1.

[0058] The nature of the protected amino group -NH_X(P³)_2-X on compound 13 may influence the yield and stereochemistry of iodine and fluorine addition to compound 13 If x=1, e.g., if the exocyclic amino group on 13 is protected with a single protecting group such as Boc, Fmoc, or a substituted or unsubstituted trityl group, nucleoside 14 may be produced as the α-fluoro-β- iodomethyl anomer in an overall yield of from about 50% to about 75%, where the α-fluoro-β- iodomethyl anomer is produced in excess over the corresponding a-iodomethyl-β-fluoro anomer by a ratio of from 8:1 to 10:1. If x=2, e.g., if the exocyclic amino group on 13 is protected with a two protecting groups, e.g., Boc, or a cyclic protecting group, e.g., phthalamide, the α-fluoro-β- iodomethyl anomer of nucleoside 14 may be produced in an overall yield of from about 75% to about 99%, or about 80% to about 95%, where the α-fluoro-β-iodomethyl anomer is produced in excess over the α-iodomethyl-β-fluoro anomer by a ratio of from 5:1 to 8:1, or from 6:1 to 7.5:1. Thus, while the yield and stereoselectivity of iodofluorination are both good, judicious selection of the protecting groups P³ on compound 13 either allows the overall reaction yield, as a mixture of anomers, to be enhanced, or allows the stereoselectivity of the reaction to be enhanced.

[0059] The vicinal diol moieties on the ribose ring of compound 14 are then protected as -OP² groups to form compound 14a, as seen in Scheme 6. Each P² group may individually be a hydroxyl protecting group selected from a group including an acetyl group, a benzoyl group, an alkylcarbonyl group, a benzyl group, an alkoxyalkyl group, or an allyl group; or where both P² groups may together complete a dioxolane ring. After protection of the hydroxyl groups, compound 14a is reacted with a nucleophile of formula R³OH, wherein R³ is arylcarbonyl, in the presence of an oxidizing agent to produce ester 15. In various embodiments, the oxidizing agent may be a peroxy compound or a permanganate salt. In various embodiments, the oxidizing agent may be a peracid of formula R³O-OH, where nucleophile R³OH and peracid R³O-OH may have the same R³ acyl group.

[0060] Reaction of compound 14 with a nucleophile and an oxidizing agent may not produce the desired ester 15 by iodide substitution unless the vicinal diol 14 is first protected as compound 14a, as shown in Scheme 6.

[0061] While it is possible to use nucleophile R³OH and a peracid R⁴O-OH to produce ester 15, where R³ is different from R⁴, oxidation of the iodide reduces the peracid to a carboxylic acid. This may cause cross reaction of the nucleoside with the two acids, as shown in Scheme 6. As seen in Scheme 7, reaction of iodide 5b with R³OH and R⁴O-OH, where R³ is different from R⁴, may result in a mixture of esters 7 and 17.

[0062] Ester 15 is converted into compound 1a by deprotecting the vicinal diol moiety of ester

15 and removing the R³ group on ester 15 to produce compound 16, as shown in Scheme 6. The protecting groups P¹ on the diol moiety and the R³ group may be removed simultaneously or sequentially. For example, if each P¹ is a protecting group P², where P² is acetyl or benzoyl, the P¹ groups and the R³ acyl group may be removed simultaneously by basic hydrolysis or reaction with a nucleophilic base, such as a primary amine. If the P¹ groups are each protecting groups P² which together form a cyclic acetal or ketal moiety, e.g., an acetonide group, the P¹ groups and the R³ group may be removed sequentially, where the acetonide may be cleaved with mild acid and the R³ acyl group may be removed with base.

[0063] Compound 16 may then be converted into the desired 2’-hydrocarbyl-4’-fluoro adenosine analog la by removing the protecting group P³ to deprotect the exocyclic amino group -NH_X(P³)₂ on compound 16. This deprotection step may be carried out with strong acid, e.g., trifluuoroacetic acid (TFA), as shown in Scheme 5.

[0064] In various embodiments disclosed herein, the exocyclic amino group -NH_X(P³)_2-X on compound 16 is deprotected by removing the alkoxycarbonyl protecting group P³ under acidic conditions. The acetyl or benzoyl protecting groups P² and the arylcarbonyl R³ group on compound

16 are removed from the fluorinated ribose ring by reaction with a nucleophilic base, such as a primary amine. Although Scheme 6 shows deprotection of the hydroxyl groups on the fluorinated ribose ring of compound 16 before deprotecting the exocyclic amino group -NH_X(P³)_2-X, the exocyclic amino group may be deprotected either before or after removing the protecting groups P² and the R³ group from the fluorinated ribose ring.

[0065] Referring to Scheme 1 , this synthesis offers several advantages. First, an adenosine derivative 2 with an exocyclic amino group protected as a carbamate or as an imidodicarbonate may be prepared as a starting material in high yield, e.g., in a yield of 60% to 80%, 65% to 80%, or 65% to 75%. Further, the 5-hydroxmethyltetrahydrofuran derivative 2 may be converted to the 5-methylenetetrahydrofuran derivative 4 in two steps with an overall yield of from 60% to 80%. If R² on compound 2 is an adenine moiety where the exocyclic amino moiety is protected as an imidodicarbonate, e.g., the exocyclic amino moiety is -N(P³)2, compound 2 may be converted to compound 4 with an overall yield of 70% to 80%. If R² on compound 2 is an adenine moiety with an exocyclic amino moiety protected as a carbamate, e.g., the exocyclic amino moiety is -NH(P³), compound 2 may be converted to compound 4 with an overall yield of 60% to 70%.

EXAMPLES

[0066] Additional embodiments are described in the following examples, which are not in any way intended to limit the scope of the claims. New compounds prepared by the procedures described below have been characterized by ¹H NMR spectroscopy, Liquid Chromatography Mass Spectrometry (LC-MS) using electrospray ionization, and, where appropriate, ¹⁹F NMR spectroscopy. The following examples show synthesis of (2S,3S,4R,5R)-5-(6-amino-9H-purin-9- yl)-4-ethynyl-2-fluoro-2-(hydroxymethyl)tetrahydrofuran-3,4-diol 18, although the procedures used are applicable to other adenosine analogs.

Example 1. Synthesis of (2R,3S,4R,5R)-5-(6-(bis(tert-butoxycarbonyl)amino)-9H-purin-9- yl)-4-ethynyl-2-((3-chlorobenzoyl)oxy)methyl-2-(iodomethyl)tetrahydrofuran-3,4-diyl dibenzoate. [0067] This example is directed to the synthesis of (2R,3S,4R,5R)-5-(6-(bis(tert- butoxycarbonyl)amino)-9H-purin-9-yl)-4-ethynyl-2-((3-chlorobenzoyl)oxy)methyl-2- (iodomethyl)-tetrahydrofuran-3,4-diyl dibenzoate 27 by the process of Scheme 8.

A. Synthesis of 9-((2R,3R,4R,5R)-3-ethynyl-3,4-bis((trimethylsilyl)oxy)-5-(((trimethyl- silyl)oxy)methyl)tetrahydrofuran-2-yl)-9H-purin-6-amine 20.

[0074] Hexamethyldisilazane (26 g, 163 mmol) was added to a suspension of 2-b- ethynyladenosine 19 (14.3 g, 49.3 mmol) in 100 mL of anhydrous dichloromethane (DCM). A catalytic amount of trimethylsilyl trifluoromethanesulfonate (550 mg, 5 mol%) was added at room temperature, and the resulting solution was stirred for two hours. The reaction was quenched by addition of a sodium bicarbonate solution. After partitioning between water and DCM, the organic DCM solution was concentrated to furnish compound 20 as a foamy crude product. ¹H NMR (400 MHz, CDCI₃) δ 8.43 (s, 1 H), 8.36 (s, 1 H), 6.21 (s, 1 H), 5.64 (s, 2 H), 4.58 (d, J= 8.8 Hz, 1 H), 4.10-4.06 (m, 1 H), 4.01 (dd, J= 11.6, 2.4 Hz, 1 H), 3.78 (dd, J= 11.6 Hz, 2 Hz, 1 H), 2.14 (s, 1 H), 0.27 (s, 9 H), 0.21 (s, 9 H), 0.20 (s, 9 H). LCMS: m/z = 508.2 [M + H]⁺.

B. Synthesis of 1,3-bis(1,1-dimethylethyl) (9-((2R,3R,4R,5R)-3-ethynyl-3,4- bis((trimethylsilyl)oxy)-5-(((trimethylsilyl)oxy)methyl)tetrahydrofuran-2-yl)-9H- purin-6-yl) imidodicarbonate 21.

[0075] Di-tert-butyl pyrocarbonate (29 g, 2.7 eq.) was added to a solution of crude 2-b- ethynyladenosine 20 (49.3 mmol) in 98 mL of anhydrous acetonitrile. Then N,N-dimethylpyridin- 4-amine (600 mg, 10 mol%) was added at room temperature. The reaction was quenched via addition of methanol after 3 h. After aqueous work-up, the organic layer was concentrated under reduced pressure to afford crude 1,3-bis(1,1-dimethylethyl) (9-((2R,3R,4R,5R)-3-ethynyl-3,4- bis((trimethylsilyl)oxy)-5-(((trimethylsilyl)oxy)methyl)-tetrahydrofuran-2-yl)-9H-purin-6-yl) imidodicarbonate 21, which was used in the next step without further purification. LCMS: m/z = 708.3 [M + H]⁺.

C. Synthesis of 1,3-bis(1,1-dimethylethyl) (9-((2R,3R,4R,5R)-3-ethynyl-3, 4-dihydroxy- 5- (hydroxymethyl)tetrahydrofuran-2-yl)-9H-purin-6-yl) imidodicarbonate 22.

[0076] HF-pyridine (6 g, 4 eq.) was added to a solution of crude imidodicarbonate 21 (49.3 mmol) in 100 mL of acetonitrile at 0 °C. The mixture was stirred at room temperature for one hour and quenched via addition of 80 mL of 1 M sodium phosphate monobasic solution. Methyl t-butyl ether (MTBE; 100 mL) was added to the reaction mixture, and two layers were separated. The aqueous layer was extracted with 20 mL of MTBE. The combined MTBE organic layers were washed with saturated sodium bicarbonate and brine, and dried over sodium sulfate. The organic layer was concentrated to an amorphous solid. The amorphous solid was purified by column chromatography (silica gel, 0-10% MeOH in DCM) to afford compound 22 as a foamy solid (16.2 g, 33 mmol, 67% over 3 steps), which was crystallized from a mixture of wet EtOAc in hexane. ¹H NMR (400 MHz, CDCI₃) δ 8.85 (s, 1 H), 8.57 (s, 1 H), 6.32 (s, 1 H), 5.13 (d, J= 5.6 Hz, 1 H), 4.86-4.76 (m, 3 H), 4.19-4.13 (m, 2 H), 3.97-3.93 (m, 1 H), 2.10 (s, 1 H), 1.38 (s, 18 H). LCMS: m/z = 492.2 [M + H]⁺. D. Synthesis of 1,3-bis(1,1-dimethylethyl) (9-((2R,3R,4R,5R)-3-ethynyl-3, 4-dihydroxy- 5- (iodomethyl)tetrahydrofuran-2-yl)-9H-purin-6-yl) imidodicarbonate 23.

[0077] Compound 22 (3 g, 6.1 mmol) dissolved in ethyl acetate, and solvent was removed via evaporation. The residue was co-evaporated with toluene twice and then dissolved in 35 mL of anhydrous THF. To the solution, triphenylphosphine (2.4 g, 1.5 eq.) and imidazole (0.75 g, 1.8 eq.) were added followed by iodine (1.9 g, 7.6 mmol). The reaction was quenched via addition of

1 mL of methanol after 7 hrs at room temperature. After aqueous work-up, the organic layer was concentrated under reduced pressure to afford compound 23 as an oily crude product, which was used in next step directly. ¹H NMR (400 MHz, CDCI₃) δ 8.83 (s, 1 H), 8.38 (s, 1 H), 6.24 (s, 1 H), 5.86 (s, 1 H), 4.52 (t, J= 4.8 Hz, 1 H), 4.31-4.27 (m, 1 H), 3.89 (d, J= 4 Hz, 1 H), 3.61-3.53 (m,

2 H), 2.21 (s, 1 H), 1.44 (s, 18 H). LCMS: m/z = 602.1 [M + H]⁺.

E. Synthesis of 1,3-bis(1,1-dimethylethyl) N-(9-((2R,3R,4S)-3-ethynyl-3,4-dihydroxy-5- methylenetetrahydrofuran-2-yl)-9H-purin-6-yl) imidodicarbonate 24.

[0078] The base l,8-Diazabicyclo[5.4.0]undec-7-ene (DBU; 1.4 g; 1.5 eq.) was added to a solution of crude iodide 23 in 25 mL of 2-methyltetrahydrofuran. The mixture was heated at 70° C for two hours. The reaction mixture was cooled to room temperature, and a 1 M solution of NaH₂PO₄ (7 mL) was added. After aqueous work-up, the organic layer was concentrated to furnish an oily residue. After column chromatography (silica gel, 5-50% EtOAc in hexane), compound 24 was collected as a white foamy solid (2.2 g, 4.6 mmol, 76% over 2 steps). ¹H NMR (400 MHz, CDCI₃) δ 8.85 (s, 1 H), 8.29 (s, 1 H), 6.45 (s, 1 H), 5.69 (br s, 1 H), 5.02 (s, 1 H), 4.78-4.77 (m, 1 H), 4.61- 4.60 (m, 1 H), 3.83 (br s, 1 H), 2.26 (s, 1 H), 1.43 (s, 18 H). LCMS: m/z = 474.2 [M + H]⁺.

F. Synthesis of 1,3-bis(1,1-dimethylethyl) (9-((2R,3R,4S,5R)-3-ethynyl-3, 4-dihydroxy- 5- fluoro-5-(iodomethyl)tetrahydrofuran-2-yl)-9H-purin-6-yl) imidodicarbonate 25.

[0079] The 5-methylenetetrahydrofuran compound 24 (3.34 g, 7.05 mmol) was dissolved in DCM (28 mL), and cooled to -25° C. Triethylamine trihydrofluoride (1.165 g, 7.05 mmol, 1 eq) was immediately added to the cold DCM solution, immediately followed by addition of N- iodosuccinimide (1.746 g, 7.76 mmol, 1.1 eq). The reaction temperature was raised to -20° C, and reaction was complete in 40 mins. The reaction mixture was warmed to 0 °C and quenched with aqueous sodium bicarbonate/sodium thiosulfate solution (0.4 M/0.4 M; 20 mL). The mixture was stirred for 30 mins, and the organic layer DCM layer was separated from the aqueous layer. The aqueous layer was extracted with DCM (two 5 mL aliquots), and the DCM layers were washed with 0.5 M monosodium phosphate (20 mL) and dried over sodium sulfate. The dry solution was concentrated to obtain compound 25. Compound 25 was purified by column chromatography (silica, 5-60% ethyl acetate in hexanes). Purified compound 25 was collected as a white foamy solid (3.99 g, 6.43 mmol, 91%). The stereochemistry about the fluorinated C5 carbon was determined. The 5R isomer of compound 25 (the α-fluoro-β-iodomethyl anomer) was produced along with the 5S isomer (the a- iodomethyl -b- fluoro anomer, structure not shown). The ratio of the α-fluoro-β-iodomethyl anomer to the a-iodomethyl-β-fluoro anomer was 6.6:1, based on HPLC analysis. ¹H NMR (400 MHz, CDCI₃) δ 8.84 (s, 1 H), 8.29 (s, 1 H), 6.48 (s, 1 H), 5.17 (dd, J= 16.8 Hz, 10.4 Hz, 1 H), 5.00 (s, 1 H), 4.00-3.97 (m, 1 H), 3.76-3.68 (m, 2 H), 2.19 (s, 1 H) 1.44 (s, 18 H). LCMS: m/z = 620.1 [M + H]⁺.

G. Synthesis of (2R,3S,4R,5R)-5-(6-(bis(tert-butoxycarbonyl)amino)-9H-purin-9-yl)-4- ethynyl-2-fluoro-2-(iodomethyl)tetrahydrofuran-3,4-diyl dibenzoate 26.

[0080] The α-fluoro-β- iodomethyl compound 25 (2.75 g, 4.44 mmol) was dissolved in 80 mL of anhydrous THF at 0° C. Triethylamine (4.5 g, 10 eq) was added to this solution, followed by dimethylaminopyridine (DMAP; 3 mg, 0.6 mol%). Benzoyl chloride (1.31 g, 2.1 eq) was then added via a syringe dropwise, and a cloudy reaction mixture was formed. After addition of benzoyl chloride was complete, the mixture was stirred at room temperature for 20 mins. The reaction was quenched by addition of methanol (0.1 mL). The reaction mixture was concentrated to remove most of the solvent, and the residue was diluted with ethyl acetate (50 mL). After aqueous workup, the organic layer was concentrated to obtain compound 26. Compound 26 was purified via column chromatography (silica gel, 5-40% ethyl acetate in hexanes) and recovered as a white foamy solid (3.5 g, 4.22 mmol, 95%). ¹H NMR (400 MHz, CDCI₃) δ 8.99 (s, 1 H), 8.38 (s, 1 H), 8.16-8.12 (m, 4 H), 7.67-7.61 (m, 2 H), 7.61-7.46 (m, 4 H), 6.96 (d, J= 15.2 Hz, 1 H), 6.95 (s, 1 H), 4.00-3.91 (m, 1 H), 3.96-3.81 (m, 1 H), 2.37 (s, 1 H), 1.46 (s, 18 H). LCMS: m/z = 828.2 [M + H]⁺. H. Synthesis of (2R,3S,4R,5R)-5-(6-(bis(tert-butoxycarbonyl)amino)-9H-purin-9-yl)-4- ethynyl-2-((3-chlorobenzoyl)oxy)methyl-2-(iodomethyl)tetrahydrofuran-3,4-diyl dibenzoate 27.

[0081] Dibenzoate 26 (3.42 g, 4.13 mmol) was added to a stirred mixture of tetra -n- butylammonium hydrogensulfate (1.54 g, 4.54 mmol, 1.1 eq), dipotassium hydrogenphosphate (2.16 g, 12.4 mmol, 3 eq), and meta-chlorobenzoic acid (mCBA; 1.616 g, 10.32 mmol, 2.5 eq) in DCM (18 mL) and water (10 mL). Next, meta-chloroperoxybenzoic acid (mCPBA; -77%, 2.85 g, 16.51 mmol, 4 eq) was added. After 21 h at room temperature, the reaction mixture was cooled to 0° C and quenched by slow addition of sodium sulfite (2.5 g, 19.8 mmol) in 12.5 mL water, while maintaining the temperature below 10 °C. The mixture was stirred until the iodine color disappeared. Solid mCBA was filtered off and rinsed with DCM. The filtrate was diluted with DCM and washed with saturated aqueous NaHCO₃ and water and dried over sodium sulfate. The dried filtrate was concentrated, and the residue was purified by column chromatography (silica gel, 5-40% ethyl acetate in hexane) to yield compound 27 as a white foamy solid (2.84 g, 3.31 mmol, 80%). ¹H NMR (400 MHz, CDCI₃) δ 8.89 (s, 1 H), 8.36 (s, 1 H), 8.16-8.09 (m, 4 H), 7.99- 7.98 (m, 1 H), 7.89-7.86 (m, 1 H), 7.67-7.59 (m, 2 H), 7.52-7.41 (m, 5 H), 7.22 (t, J= 7.6 Hz, 1 H), 7.11 (d, J= 15.2 Hz, 1 H), 7.01 (s, 1 H), 5.11-5.04 (m, 1 H), 4.90 (dd, J= 12 Hz, 10.4 Hz, 1 H), 2.32 (s, 1 H), 1.44 (s, 18 H). ¹⁹F NMR (376 MHz, CDCI₃) δ -117.25. LCMS: m/z = 857.2 [M + H]⁺.

Example 2. Deprotection of (2R,3S,4R,5R)-5-(6-(bis(tert-butoxycarbonyl)amino)-9H-purin-

9-yl)-4-ethynyl-2-((3-chlorobenzoyl)oxy)methyl-2-(iodomethyl)tetrahydrofuran-3,4-diyl dibenzoate.

[0082] This example is directed to the synthesis of (2S,3S,4R,5R)-5-(6-amino-9H-purin-9-yl)-4- ethynyl-2-fluoro-2-(hydroxymethyl)tetrahydrofuran-3,4-diol 18 by the process of Scheme 9.

A. Synthesis of tert-butyl (9-((2R,3R,4S,5S)-3-ethynyl-5-fluoro-3, 4-dihydroxy- 5-

(hydroxymethyl)tetrahydrofuran-2-yl)-9H-purin-6-yl)carbamate 28.

[0083] The 3-chlorobenzoate ester 27 (2.8 g, 3.27 mmol) was mixed with 12 mL of «-propylamine at room temperature for one hour. After concentration of the reaction mixture, the residue was purified via column chromatography (silica gel, 1-12% MeOH in DCM) and to obtain compound 28 as a foamy solid (877 mg, 2.14 mmol, 66%). ¹H NMR (400 MHz, CD₃OD) δ 8.59 (s, 1 H), 8.58 (s, 1 H), 6.58 (s, 1 H), 3.88-3.80 (m, 2H), 2.67 (s, 1 H), 1.59 (s, 9H). ¹⁹F NMR (376 MHz, CD₃OD) δ -124.42. LCMS: m/z = 410.2 [M + H]⁺.

B. Synthesis of (2S,3S,4R,5R)-5-(6-amino-9H-purin-9-yl)-4-ethynyl-2-fluoro-2- (hydroxy-methyl)tetrahydrofuran-3,4-diol 18.

[0084] The N- Boc protected nucleoside 28 (5.31 g, 12.97 mmol) was suspended in DCM (200 mL). Trifluoroacetic acid (TFA; 24 mL) was added to the mixture at 0° C and stirred at room temperature for 4 hrs. The reaction solution was concentrated, and the residue was purified via column chromatography (silica gel, 0-20% MeOH in DCM) to furnish diol 18 (3.9 g, 12.6 mmol, 97%) as an amorphous solid. ¹H NMR (400 MHz, CD₃OD) δ 8.38 (s, 1 H), 8.21 (s, 1 H), 6.48 (s, 1 H), 4.81 (d, J= 18.8 Hz, 1 H), 3.88-3.80 (m, 2 H), 2.70 (s, 1 H). ¹⁹F NMR (376 MHz, CD₃OD) δ -124.36. LCMS: m/z = 310.1 [M + H]⁺.

Example 3. Synthesis of tert-butyl (9-((2R,3R,4S,5R)-3-ethynyl-5-fluoro-3, 4-dihydroxy- 5- (iodomethyl)tetrahydrofuran-2-yl)-9H-purin-6-yl)carbamate. [0085] This example is directed to the synthesis of tert-butyl (9-((2R,3R,4S,5R)-3-ethynyl-5- fluoro-3,4-dihydroxy-5-(iodomethyl)tetrahydrofuran-2-yl)-9H-purin-6-yl)carbamate 25a by the process of Scheme 10.

A. Synthesis of tert-butyl (9-((2R,3R,4R,5R)-3-ethynyl-3, 4-dihydroxy- 5-

(hydroxymethyl)-tetrahydrofuran-2-yl)-9H-purin-6-yl)carbamate 22a.

[0086] Compound 22 (32.8 g, 66.8 mmol), prepared as described in Example 1C, was mixed with «-propylamine (16.8 g, 4.2 eq.) at 0° C. The reaction mixture was stirred at room temperature for one hour and concentrated under reduced pressure. The residue was crystallized from acetonitrile to afford carbamate 22a as a solid (20.5 g, 52.5 mmol, 79%). ¹H NMR (400 MHz, DMSO-d₆) δ 9.96 (br s, 1 H), 8.72 (s, 1 H), 8.60 (s, 1 H), 6.46 (s, 1 H), 6.16 (s, 1 H), 5.76 (d, J= 7.2 Hz, 1 H), 5.24 (t, J= 4.8 Hz, 1 H), 4.47-4.42 (m, 1 H), 3.95-3.92 (m, 1 H), 3.85-3.80 (m, 1 H), 3.73-3.67 (m, 1 H), 3.10 (s, 1 H), 1.47 (s, 9 H). LCMS: m/z = 392.2 [M + H]⁺.

B. Synthesis of tert-butyl (9-((2R,3R,4R,5R)-3-ethynyl-3,4-dihydroxy-5-(iodomethyl)- tetrahydrofuran-2-yl)-9H-purin-6-yl)carbamate 23a.

[0087] Carbamate 22a (22.6 g, 57.8 mmol) was dissolved in anhydrous THF (315 mL), and imidazole (7.1 g, 1.8 eq.) and triphenylphosphine (22.7 g, 1.5 eq.) were added to the solution. Iodine (18.3 g, 1.25 eq.) was added at 0° C. The reaction mixture was stirred at room temperature overnight. The reaction was quenched by addition of methanol, and solids were removed by filtration. The filtrate was concentrated to afford the 5-iodomethyl carbamate 23a as an oily residue. The oily product was used in the next step without further purification.

C. Synthesis of tert-butyl (9-((2R, 3R,4S)-3-ethynyl-3, 4-dihydroxy- 5-methylene- tetrahydrofuran-2-yl)-9H-purin-6-yl)carbamate 24a.

[0088] The 5-iodomethyl carbamate 23a was dissolved in acetonitrile (200 mL) and DBU (22 g) was added. The mixture was stirred at room temperature overnight. Most of the solvent was removed under reduced pressure, and ethyl acetate (200 mL) was added. The solution was washed with water and brine, and concentrated. After concentration, the residue was purified via column chromatography (silica gel, 0~6% MeOH in DCM) to afford the unsaturated product 24a (14.1 g, 37.8 mmol) as a solid. The two step conversion of compound 22a to compound 24a took place in a total yield of 65%. ¹H NMR (400 MHz, DMSO-d₆) δ 10.16 (s, 1 H), 8.82 (s, 1 H), 8.60 (s, 1 H), 6.57 (s, 1 H), 6.36 (s, 1 H), 6.09 (d, 1 H), 5.38-5.35 (m, 1 H), 4.46-4.45 (m, 1 H), 4.27-4.26 (m, 1 H), 3.28 (s, 1 H), 1.47 (s, 9 H). LCMS: m/z = 374.1 [M + H]⁺.

D. Synthesis of tert-butyl (9-((2R,3R,4S,5R)-3-ethynyl-5-fluoro-3, 4-dihydroxy- 5- (iodomethyl)tetrahydrofuran-2-yl)-9H-purin-6-yl)carbamate 25a.

[0089] Compound 24a (13.4 g, 35.9 mmol) was dissolved in anhydrous acetonitrile and cooled to - 25° C. Triethylamine trihydrofluoride (5.8 g, 1 eq.) was added at - 25° C, and NIS (8.9 g, 1.1 eq.) was added immediately. The reaction mixture was slowly warmed up to 0 °C over 30 mins. Reaction was quenched with sodium bicarbonate/sodium thiosulfate solution (0.4 M/0.4 M; 100 mL). Two layers separated, and organic layer was concentrated. The fluorinated nucleoside 18a was recovered as a product and purified by column chromatography. Nucleoside 25a was recovered as a foamy solid (11.3 g, 21.7 mmol, 61%). The 5R isomer of compound 25a (the a- fluoro-β-iodom ethyl anomer) was produced along with the 5S isomer. The 5R isomer of compound 25a (the α-fluoro-β-iodomethyl anomer) is favored over the 5S isomer by a ratio of 9:1, based on proton NMR of the crude product. ¹H NMR (400 MHz, CDCI₃) δ 8.70 (s, 1 H), 8.25 (br s, 1 H), 8.18 (s, 1 H), 6.48 (s, 1 H), 5.18 (d, J= 16.8 Hz, 1 H), 3.77 (s, 1 H), 3.74 (d, J= 3.6 Hz, 1 H), 2.13 (s, 1 H), 1.56 (s, 9 H). ¹⁹F NMR (376 MHz, CDCI₃) δ -109.40. LCMS: m/z = 520.1 [M + H]⁺.

Example 4. Synthesis of (2S,3S,4R,5R)-5-(6-amino-9H-purin-9-yl)-4-ethynyl-2-fluoro-2- (hydroxymethyl)tetrahydrofuran-3,4-diol.

[0090] This example is directed to the synthesis of (2S,3S,4R,5R)-5-(6-amino-9H-purin-9-yl)-4- ethynyl-2-fluoro-2-(hydroxymethyl)tetrahydrofuran-3,4-diol 18 by the process of Scheme 11.

A. Synthesis of (2R,3S,4R,5R)-5-(6-((tert-butoxycarbonyl)amino)-9H-purin-9-yl)-4- ethynyl-2-fluoro-2-(iodomethyl)tetrahydrofuran-3,4-diyl diacetate 29.

[0091] The α-fluoro-β-iodomethyl compound 25a (14.6 g, 28.2 mmol) was dissolved in anhydrous acetonitrile. The solution was cooled in an ice- water bath and triethylamine (22.8 g, 8 eq.) was added, followed by catalytic amount of DMAP. Acetic anhydride (5.8 g, 2.02 eq.) was added and the mixture was stirred at room temperature for two hours. After aqueous work-up, the organic solution was concentrated to furnish compound 29 as a solid residue which was used for the next step without further purification. ¹H NMR (400 MHz, CDCI₃) δ 8.77 (s, 1 H), 8.15 (s, 1 H), 8.14 (s, 1 H), 6.67 (s, 1 H), 6.55 (d, J= 14.8 Hz, 1 H), 3.86-3.77 (m, 1 H), 3.71-3.66 (m, 1 H), 2.38 (s, 1 H), 2.24 (s, 3 H), 2.20 (s, 3 H), 1.55 (s, 9 H). LCMS: m/z = 604.3 [M + H]⁺.

B. Synthesis of (2S,3S,4R,5R)-5-(6-((tert-butoxycarbonyl)amino)-9H-purin-9-yl)-2-(((3- chlorobenzoyl)oxy)methyl)-4-ethynyl-2-fluorotetrahydrofuran-3,4-diyl diacetate 30.

[0092] The procedure for preparation of 3-chlorobenzoic acid ester 30 from compound 29 was substantially similar to the procedure for preparation of compound 27 in Example 1H. ¹H NMR (400 MHz, CDCI₃) δ 8.66 (s, 1 H), 8.14 (s, 1 H), 8.11 (s, 1 H), 8.05 (t, J= 1.6 Hz, 1 H), 8.00-7.97 (m, 1 H), 7.57-7.54 (m, 1 H), 7.40 (t, J = 7.6 Hz, 1 H), 6.74 (s, 1 H), 6.61 (d, J= 14.4 Hz, 1 H), 4.88 (t, J= 12 Hz, 1 H), 4.79 (t, J= 12 Hz, 1 H), 2.38 (s, 1 H), 2.21 (s, 3 H), 2.20 (s, 3 H), 1.55 (s, 9 H). ¹⁹F NMR (376 MHz, CDCI₃) δ -117.87. LCMS: m/z = 633.2 [M + H]⁺.

C. Synthesis of (2S,3S,4R,5R)-5-(6-amino-9H-purin-9-yl)-2-(((3-chlorobenzoyl)oxy)- methyl)-4-ethynyl-2-fluorotetrahydrofuran-3,4-diyl diacetate 31.

[0093] To a solution of 3-chlorobenzoic acid ester 30 (8.72 g, 13.8 mmol) in 200 mL of DCM was added 37 g of TFA (24 eq.). The mixture was stirred at room temperature for three hours and subjected to aqueous workup. The 6-amino compound 31 was recovered, and purified by column chromatography (silica gel, 1~6% MeOH in DCM) to obtain a solid product (6.28 g, 11.8 mmol, 86%). ¹H NMR (400 MHz, CDCI₃) δ 8.30 (s, 1 H), 8.07 (t, J= 2 Hz, 1 H), 8.02 (s, 1 H), 8.01-7.98 (m, 1 H), 7.58-7.55 (m, 1 H), 7.41 (t, J= 7.6 Hz, 1 H), 6.71 (s, 1 H), 6.68 (d, J= 14 Hz, 1 H), 5.67 (br s, 2 H), 4.93-4.78 (m, 2 H), 2.43 (s, 1 H), 2.21 (s, 6 H). ¹⁹F NMR (376 MHz, CDCI₃) δ -117.44. LCMS: m/z = 532.3 [M + H]⁺.

D. Synthesis of (2S,3S,4R,5R)-5-(6-amino-9H-purin-9-yl)-4-ethynyl-2-fluoro-2- (hydroxy-methyl)tetrahydrofuran-3,4-diol 18.

[0094] The 6-amino compound 31 (6.28 g, 11.8 mmol) was combined with 31 g of propylamine at 0 °C. The mixture was stirred at ambient temperature for one hour and then excess propylamine was evaporated. The product was purified via column chromatography, and compound 18 was recovered as a foamy solid (3.5 g, 11.3 mmol, 96%) following the chromatographic procedure of Example 2B.

[0095] Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be affected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.

Claims

WHAT IS CLAIMED IS:

1. A method of preparing a nucleoside compound of Formula 1 having a fluorinated ribose group, wherein:

or a salt thereof; wherein:

R¹ is selected from the group consisting of a C1-C6 alkyl group, a C2-C6 alkenyl group, and a C2- C6 alkynyl group;

R² is an adenin-9-yl group of formula:

where protecting group P³ is an an alkylcarbonyl group, an arylcarbonyl group, an alkoxycarbonyl group, or an aryloxycarbonyl group;

A is an alkyl group, an alkoxy group, an aryl group, an aryloxy group, a halogen atom, a substituted or unsubstituted amino group, or a protected amino group; n is 0 to 2; and x is 0 or 1 ; comprising: converting the OH of the hydroxymethyl group on a compound of Formula 2 to a leaving group L¹ to produce a compound of Formula 3;

wherein L¹ is a leaving group selected from the group consisting of a halide leaving group, a sulfonate leaving group, and a carboxylate leaving group; and each P¹ is independently selected from the group consisting of H and a protecting group P²; eliminating the leaving group L¹ from the compound of Formula 3 to produce a compound of Formula 4;

iodo-fluorinating the compound of Formula 4, optionally with triethylamine trihydrofluoride and an iodine source, to produce a compound of Formula 5,

when each P¹ in the compound of Formula 5 is H (a compound of Formula 5a), optionally protecting the hydroxyl groups with a protecting group P² to form a compound of Formula reacting the compound of Formula 5a or 5b with a nucleophile of formula R³OH, wherein R³ is alkylcarbonyl, arylcarbonyl, alkylsulfonyl, or arylsulfonyl, in the presence of an oxidizing agent to produce the compound of Formula 7; and

Optionally deprotecting the compound of Formula 7 to produce the compound of Formula 1.

2. The method of claim 1, further comprising removing the protecting group P³ from the adenin-9-yl group R² in the compound of Formula 7.

3. The method of claim 1, wherein iodo-fluorinating the compound of Formula 4 produces the compound of Formula 5 in a yield of between 50% and 99%.

4. The method of claim 1, wherein iodo-fluorinating the compound of Formula 4 produces the compound of Formula 5 with an α-fluoro-β-iodomethyl anomer in at least a 5: 1 excess over an a-iodomethyl-β-fluoro anomer.

5. The method of claim 1, wherein L¹ is iodide, and conversion of the compound of Formula 2 to the compound of Formula 4 proceeds in an overall yield of from 60% to 80%.

6. The method of claim 1, wherein P¹ is H; and comprising producing a compound of Formula 2:

by protecting the hydroxyl groups on adenosine derivative 2-1 with silyl groups to produce compound 2-2, where each R³ is a linear or branched lower alkyl or aryl;

protecting the exocyclic amino group on compound 2-2 with protecting group P³ to produce compound 2-3; and

2-3 removing the silyl groups from compound 2-3 to produce compound 2.

7. The method of claim 6, wherein conversion of the compound of Formula 2-1 to the compound of Formula 2 proceeds in an overall yield of from 60% to 80%.

8. The method of claim 1, wherein reacting the compound of Formula 5b with a nucleophile of formula R³OH comprises reacting the compound of Formula 5b with a nucleophile of formula R³OH in the presence of a peracid oxidizing agent of formula R³O-OH, and R³ is a substituted or unsubstituted benzoyl group.

9. The method of claim 1 , wherein L¹ is chloride, bromide, iodide, para-toluenesulfonyl, methanesulfonyl, benzoyl, or 3-chlorobenzoyl.

10. The method of claim 1, wherein the compound of Formula 4 is produced by: converting the OH of the hydroxymethyl group of the compound of Formula 2 to leaving group

L¹, wherein L¹ is an iodide leaving group, by reacting the compound of Formula 2 with iodine and a triarylphosphine to produce the compound of Formula 3; and eliminating the iodide leaving group L¹ from the compound of Formula 3 by reacting the compound of Formula 3 with a non- nucleophilic base to produce the compound of Formula 4.

11. The method of claim 1 , wherein R¹ is a C2-C6 alkynyl group; x is 0; and P³ is an alkoxycarbonyl group.

12. The method of claim 1l, wherein R¹ is an ethynyl group.

13. The method of claim 1, wherein each P¹ on the compound of Formula 5 is H or a protecting group P², wherein P² is selected from the group consisting of acetyl, benzoyl, benzyl, and alkoxyalkyl, or both P² groups in combination form a dioxolane ring.

14. The method of claim 13, wherein each P¹ on the compound of Formula 5 is a protecting group P², wherein each P² is independently selected from the group consisting of acetyl, benzoyl, benzyl, and alkoxyalkyl.

15. The method of claim 1, wherein each P¹ on the compound of Formula 5 is H.

16. The method of claim 13, wherein each P¹ is a hydroxy protecting group P², wherein P² is selected from the group consisting of an acetyl or benzoyl group,

P³ is an alkoxcarbonyl group; and deprotecting the compound of Formula 7 comprises: deprotecting the exocyclic amino group -NH_X(P³)_2-X by removing the alkoxy carbonyl group P³ under acidic conditions; and either before or after deprotecting the exocyclic amino group, removing the acetyl or benzoyl groups P² from ester 7 by reaction with a nucleophilic base, and converting the -OR³ group on the compound of Formula 7 into a hydroxyl group.

17. The method of claim 16, wherein the nucleophilic base is a primary amine.

18. The method of claim 13, wherein each P¹ is a hydroxy protecting group P², wherein P² is selected from the group consisting of an acetyl or benzoyl group,

R³ is a substituted benzoyl group, and deprotecting the compound of Formula 7 comprises: deprotecting the exocyclic amino group -NH_X(P³)_2-X by removing the protecting group P³; and, either before or after deprotecting the exocyclic amino group, simultaneously removing each P² group and the R³ group from the compound of Formula 7 by reaction with a nucleophilic base.

19. A protected nucleoside of Formula X:

or a salt thereof; wherein:

R⁴ is a linear or branched lower alkyl or aryl; A is an alkyl group, an alkoxy group, an aryl group, an aryloxy group, a halogen atom, a substituted or unsubstituted amino group, or a protected amino group; x is 0 or 1 ; n is 0 to 2; and

P³ is a C1-C6 alkoxy carbonyl group or a C1-C6 acyl group.