Camps Derivatives. a Minireview Over Synthetic Medicinal Chemistry T Kjell Undheim

Bioorganic Chemistry 91 (2019) 103152

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Bioorganic Chemistry

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cAMPS derivatives. A minireview over synthetic medicinal chemistry T Kjell Undheim

Department of Chemistry, University of Oslo, N-0315 Oslo, Norway

ARTICLE INFO ABSTRACT

Keywords: Cyclic nucleosides belonging to the cAMPS system can exert antagonistic or agonistic effects on the cAMP/PKA (Rp)-cAMP-phosphorothioates type 1 inhibitory pathway affecting T-lymphocyte replications. The stereochemistry at the phosphorus atom in cAMPS antagonists the phosphate group is important for the expressed selectivity. The two stereoisomers at the phosphorus atom in cAMPS agonists the phosphate arise by selective replacements of one of the oxygens pendant from the phosphorus atom by a Stereoselective phosphorus-thiation sulfur atom. Methods for the preparation of cAMPS derivatives as stereochemical mixtures at the phosphorus Pd-effected trans-coupling atom and separation of stereoisomers have been developed into highly stereoselective syntheses. Methods for Arylation Hetarylation halogenation in the purine 8-position afford corresponding halides. Heteronucleophilic substitution of the halides afford corresponding amines, ethers or sulfides. Transition metal catalysis for carbylation of the8-halides affords simple and efficient routes for the preparation of 8-aryl, 8-hetaryl or 8-alkyl cAMPS derivatives. Preparations of prodrugs for improved cell membrane penetration are described. The prodrugs are S-alkylated derivatives which are constructed for selective cleavage of the SeP bond by an esterase to regenerate the bioactive cAMPS species at the site of the desired action.

1. Introduction 2. Synthesis

The purine framework is widely incorporated in essential biological Two main procedures are available for the preparation of the target molecular systems. Purine chemistry and construction of purine derived cAMPS derivatives. In the first approach, appropriately substituted structures constitute an important part of medicinal chemistry. Hence adenosines are prepared as intermediates for cyclothiophosphorylation the purine ring system has been an object for intense chemical studies. (Scheme 1). In the second approach, cyclic phosphoramidates are pre- This report describes a summary of synthetic chemical work on purines pared in a stereoselective manner and subsequently thiated with steric for the construction of thiated adenosine derived analogues. The basis retention (Schemes 7 and 8). for this review is cyclic adenosine monophosphate (cAMP) which is an important second messenger that regulates a broad range of cellular 2.1. Adenosine substrates functions in response to various hormones [1–4]. One of the oxygen atoms pendant from the phosphorus atom in cAMP has been replaced The cAMPS parent compound was first synthesized in 1974 [7].A by a sulfur atom in attempts to modify the biological responses of subsequent stereoselective synthesis and configurational assignment cAMP. By this change a new stereogenic center is created at the phos- have been worked out. The configuration at the phosphorus atom can phorus atom. The resultant adenosine-3,5-cyclic monophosphorothioic be correlated with 31P NMR shifts [8,9]. acid (cAMPS) is stereochemically stable for isolation of the (Rp)- and In the first synthetic approach, cAMPS and derivatives are prepared (Sp)-diastereomers [5,6]. The pharmacological effect of the (Rp)- and in a one-pot procedure from unprotected nucleosides by thiopho- the (Sp)-isomers may differ at the protein kinase A effector insucha sphorylation with thiophosphoryl chloride followed by cyclization in way that (Rp)-cAMPS acts as a competitive antagonist and (Sp)-cAMPS the presence of sodium hydroxide in aqueous acetonitrile (Scheme 1). acts as an agonist [5,6]. The products are diastereomeric cAMPS mixtures that afford the pure General structures of target molecules (Rp)-cAMPS and (Sp)-cAMPS stereoisomers after chromatographic separations [10]. Scheme 1 shows are shown in Fig. 1. The target molecules differ in the configuration at that adenosine reacts with thiophosphoryl chloride in a triethyl phos- the phosphorus atom and may differ in the nature of the 8-R sub- phate solution followed by alkali promoted cyclization to afford a stituent. diastereomeric mixture of cAMPS (4). The 8-(2-furyl) substrate 5 in dry pyridine reacts in the same manner via a thiophosphoryl intermediate

E-mail address: [email protected]. https://doi.org/10.1016/j.bioorg.2019.103152 Received 27 November 2018; Received in revised form 27 May 2019; Accepted 24 July 2019 Available online 26 July 2019 0045-2068/ © 2019 The Author. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). K. Undheim Bioorganic Chemistry 91 (2019) 103152

atom. When the reaction is carried out using a stereoisomeric mixture as substrate, the diastereomers of the phosphorothioic acid 14 can be separated by chromatography [13]. 3′,5′-Cyclic phosphorothioate nucleotides are available by cyclothiophosphorylation of unprotected nucleosides 15.(Scheme 4). Initial phosphorylation presumably takes place at the 5′-OH group in the sugar. The reaction between adenosine and thiophoshoryl chloride proceeds well in dry pyridine at low temperature for 10 min. The product is a 1:1-mixture of the (Rp)- and (Sp)-8-substituted adenosine-3′,5′- cyclic phosphorothioic acid (17). The isomers are separated by reverse- Fig. 1. Target molecules. phase chromatography on a C18 functionalized silica gel column. The method delivers almost exclusively the desired 3′,5′-cyclic phosphor- to afford a (Rp/Sp)-diastereomeric mixture of the stereoisomers 6 in the othioates 17, but as stereochemical mixtures [13]. ratio 2:3 in favour of the (Sp)-isomer (6b) [11]. A simple method for the preparation of 8-alkyl substrates for the Alternative thiophosphoryl chloride reagents include bis(p-ni- cyclothiophosphorylation reactions is shown in Scheme 5. Similar re- trophenyl)phosphorochlorothioate (8) which reacts with N-benzoyl- actions are possible for aryl and hetaryl derivatives (vide infra). 8- adenosine 7 in pyridine at room temperature without protection of the Bromo derivatives are used as substrates for the trans-coupling reaction hydroxyl groups to afford the adenosine 5-bis(p-nitrophenyl)phos- [13]. Pd-promoted catalytic reactions with tetraethyltin provides the phorothioate 9 (Scheme 2). Potassium tbutoxide as base is used for the ethyl substrate 19. Trimethylaluminum is used for the methyl analogue cyclization in dry DMF. The phenolic ester 10 is cleaved by aq. am- 21. The vinyl ether 22, a protected acyl derivative, is prepared similarly monia at 50 °C to afford the thiophosphoric acid isomers 11 which can by Pd-catalysis. The resultant products 23 are subsequently used as be separated by ionic chromatography [12]. substrates for cyclothiophosphorylation reactions [13]. Adenosine reacts with trivalent phosphorus reagents to afford cyclic phosphites as shown for adenosine-3′,5′-cyclic methyl phosphite 13 2.2. cAMP substrate for stereoselective amidation and thiation (Scheme 3). Both the cis- and trans-cyclophosphite esters 13 are formed. At elevated temperature, the trans-ester 13b is inverted to the cis-isomer A general method for introduction of carbon substituents into the 13. The isomerization is promoted by 1H-tetrazole. Each isomer, or the purine 8-position is available employing stereoselectively prepared product as a stereoisomeric mixture 13, is thiated by oxidative addition amidates (vide infra). 8-Bromo or 8-chloro derivatives are substrates for of sulfur at the site of the lone pairs of electrons on the phosphorus 8-carbylation reactions promoted by palladium or other transition atom with retention of the relative configuration at the phosphorus metals for catalysis [11,13]. This approach is especially useful for the

Scheme 1. Reagents and reaction conditions.

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Scheme 2. Reagents and reaction conditions.

Scheme 3. Reagents and reaction conditions.

Scheme 4. Reagents and reaction conditions.

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Scheme 5. Reagents and reaction conditions. preparation of aryl and hetaryl targets but includes also alkyl deriva- facilitates isolation of the thioic acid by precipitation of the product tives. The intermediate substrates are available from adenosine or de- from the complex aqueous mixture (vide infra). The bromine in the rivatives via stereoselective amidation reactions. Scheme 6 shows the electrophilic 8-position may be displaced by another halide ion during reaction between N,N,O2-tribenzoyladenosine cyclic 3′5′-phosphate the coupling procedure. The exchange is reduced or avoided when the (24) and aniline in the presence of triphenylphosphine-carbon tetra- silylation is effected under relatively mild conditions. Alternatively, chloride which affords the cyclic 3′,5′-phosphoranilidate nucleosides tbutyldimethylsilyl triflate (TBDMS-OTf) can be used as a silylating 25. A subsequent separation affords the individual diastereomers 25a reagent instead of TBDMS-Cl. Oxalyl chloride in the presence of DMF in and 25b. A stereorententive mode for the PN → PS conversion [9,14] THF or dichloromethane at low temperature (−60 °C to −20 °C) can be converts the amidates isomers 25 into the corresponding thionucleo- used for the amidation step. The actual chlorinating agent of phos- sides, the cyclic ammonium salts 26 of 3′,5′-phosphorothioic acids 1a phorus is probably the Vilsmeyer reagent, (chloromethylene)dimethy- and 1b. lammonium chloride, which is generated in situ from oxalyl chloride Scheme 7 shows silyl protection of the 2′-OH group and cross-cou- and DMF. The crude halide is used directly in the amidation step with pling in the 8-position followed by stereoselective constructions of primary amines. Aniline affords the N-phenyl phosphoramidate (SP)-29. amidates. The low solubility often experienced with cyclic nucleotides Only a very minor amount of the other diastereomer is observed. in organic solvents is overcome for 8-Br-cAMP (27) by conversion into a Benzylamine provides the corresponding amidate 30. Only the (SP)- tributylammonium salt that is soluble in DMF. The reactions with isomer is seen. The intermediate phosphoryl chloride in the reaction tbutyldimethylsilyl chloride (TBDMS-Cl) in DMF in the presence of may consist of a stereochemical mixture. A main signal in 31P NMR of imidazole as base at 20–60 °C, affords the silyl protected compound 28. the mixture is seen at −1.73 ppm. Selective formation of the amidate

A bulky silyl protecting group avoids interference with other functional with (Sp)-configuration from a mixture of acid chlorides would require groups under the conditions employed in the construction of the some configurational equilibrations. In the (Sp)-isomer, the anilino or phosphoryl chloride or an equivalent. A bulky substituent will also af- benzylamino group in the amidates occupies an equatorial position as fect conformational preferences in the reacting species and hence pre- shown by single crystal X-ray analyses (vide infra) [11]. ferential selectivity. The hydrophobic and bulky silyl group also

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Scheme 6. Reagents and reaction conditions.

Scheme 7. Reagents and reaction conditions.

2.3. Carbylation in 8-position a nucleophile and a cyclisation reaction subsequently occurs where the sulfur nucleophile adds to the phosphorus atom with a concurrent 2.3.1. 8-Aryl substituents cleavage of the PeN bond with retention of the true configuration at the Cross-coupling reactions affording 8-aryl or 8-hetaryl reactants by phosphorus atom. There is, however, an apparent inversion from the palladium catalysis are effected as shown in Scheme 8. Stannane re- expression of the configuration at the phosphorus atom because ofthe agents are normally used in the cross-coupling with the 8-amidate 29. nomenclature priority rules. This process generates a phosphorothioic The silyl protected amidates are sufficiently soluble in organic solvents acid 32 from the amidate 31 with retention of the true configuration. for cross-coupling reactions (Scheme 8). If not commercially available, The benzyl amidate 30 delivers the trans-coupled product 33 and sub- the stannanes are prepared via the corresponding lithiated species. Pd- sequently the phosphothioic acid 34 (Scheme 8). Thiation is effected in catalysis is used in the substitution reactions of the 8-bromo sub- reactions with carbon disulfide after deprotonation using a strong base. stituent. The solvent in these reactions is either DMF or N-methyl-2- For the N-phenylamidates 31, potassium tbutoxide salt or the corre- pyrrolidone (NMP), or a mixture thereof depending on relative solu- sponding sodium salt is used. For the benzylamine 33 n-BuLi in THF is bility of the reactants at elevated temperatures. After deprotonation of the base. The hydrophobic nature of the bulky TBDMS-protecting group the amide hydrogen with a strong base, the metallated amido-nitrogen of the 2′-hydroxy group leads to precipitation of the thioic acid 32 from reacts with carbon disulfide. A sulfur atom in the intermediate becomes the aqueous mixture after the thiation reaction is completed (Scheme

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Scheme 8. Reagents and reaction conditions.

Scheme 9. Reagents and reaction conditions.

8). The benzylamidate (Sp)-30 reacts in the same manner. 1-Bromo-4- tributylammonium salt 35 and converted to the sodium salt 36. fluorobenzene is lithiated, stannylated and cross-coupled to afford the Tributylammonium salts are formed by addition of tributylamine to 4-fluorophenyl product 33 which is thiated (34) and desilylated to the thioic acid 33. The ammonium salts are soluble in polar organic provide the corresponding thioic acid which is isolated as the solvents and can be purified by recrystalization or chromatography

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Scheme 10. Reagents and reaction conditions.

Scheme 11. Reagents and reaction conditions.

(Scheme 9). The tributylammonium salts 37 are transformed into so- derivatives 39 are formed from the phenylamidate (Sp)-29 by Pd-pro- dium salts 38 by dissolution in methanolic sodium hydroxide and moted trans-coupling reactions (Scheme 10). Subsequent thiation (40) precipitation by addition of hexane. and desilylation by ammonium fluoride afford the target compounds 41 which are converted into the sodium salts 42 by methanolic sodium hydroxide.

2.3.2. 8-Hetaryl substituents With 2-thienylzinc chloride and the N-benzylamidate (Sp)-30, the 8- Both π-electron rich and π-electron deficient heterocyclic

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Scheme 12. Reagents and reaction conditions.

(2-thienyl) product 43 is obtained under the influence of Pd-catalysis. fully suitable for conversion of a phosphoramidate because of compe- The amidate 43 is reacted further to afford the thioic acid 44. titive alkylation at the phosphoramidate amino nitrogen whereby the Preparation of products with an 8-substitent containing hydroxy amino group loses its essential hydrogen for the thiylation reaction (vide groups is illustrated by synthesis of the fully protected 8-furyl deriva- infra). In the literature some information on 6-amino derivatives is tive 46 (Scheme 11). The substrate is furan-2-methanol that is protected available in a patent [17]. More recently, reference is made to a highly as a TBDMS-derivative and lithiated in the vacant heterocyclic α-posi- workable procedure for selective substitutions in the 6-position that can tion before stannylation on treatment with a stannyl chloride. Other be effected by reacting a 6-(1,2,4-triazole) derivatives of adenine and furan compounds are similarly lithiated and subsequently stannylated. nucleosides. A π-deficient triazole ring that is hinged at an π-electron Deprotection is effected by ammonium fluoride in DMF to afford the deficient heterocyclic position is replaced under nucleophilic conditions target compound 47. The desilylation using ammonium fluoride in DMF [18]. Thus the primary 6-amino group in substrate 48 (Scheme 12) is solution is run at room temperature over 2–5 days after which the target transformed into the 6-(1,2,4-triazol-4-yl) derivative 47 in 50% yield products are isolated in excellent yields. In most cases the reaction when substrate 48 is reacted with 1,2-bis[(dimethylamino)methylene] sequence is taken further without isolation of the product. Addition of hydrazine and its dihydrochloride at 95 °C in pyridine [15]. The sub- n-tributylamine to the acid or ammonium salts affords the corre- sequent nucleophilic displacements of the triazole moiety afford N- sponding tributylammonium salts that are soluble in organic solvents substituted 6-amino products such as dimethylamino, methylamino and and are purified by flash chromatography on silica gel. benzylamino derivatives 50a-c in high chemical yields. For the preparation of the phosphorothioic acid derivatives 51 by the Stec proce- 2.4. Hetero substituents dure (vide supra), the reaction is initiated by proton abstraction from the amidate 50 by a strong base, in this case nBuLi at −78 °C. The lithium 2.4.1. 6-Amino derivatives amide reacts with carbon disulfide to form an adduct with a subsequent Changes of the 6-amino function in the heterocyclic moiety are thiation at phosphorus and cleavage of the phosphorus-amino bond. presented in Scheme 12. The primary 6-amino group has been trans- The product, without any further purification is desilylated using am- formed into N-alkylated or N-acylated groups [15]. In the bioscreening, monium fluoride in DMF at room temperature. The reaction is complete only small differences in bioactivity are observed. The methods known after 5 days. The product as its n-tributylammonium salt 52 is purified for the conversion of cAMP to 6-alkylamino derivatives [16] are not by flash chromatography on silica gel. Addition of sodium hydroxide to

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Scheme 13. Reagents and reaction conditions.

Scheme 14. Reagents and reaction conditions.

a solution of the ammonium salt in methanol, furnishes the corre- 2.4.2. 8-Halogeno derivatives sponding sodium salts 53. The 31P NMR shifts are diagnostic for ste- 8-Bromo- and 8-chloro-cAMPS generally show higher bioactivity reochemical purity (see above). All phosphorus NMR spectra are than the parent compound [19]. 8-Iodo-cAMPS is less active and it is characterised by a single line absorption. All tributylammonium salts chemically less readily available [17]. 8-F-cAMPS is not available, 50 in deuteriomethanol resonate at 57.9 ppm, and the same chemical presumably because the fluorine substituent in the electrophilic 8-po- shift values are observed for the sodium salts 53. The shift values of the sition is activated for ready nucleophilic displacement and would ex- amidates 50 in deuteriomethanol vary slightly but are close to 8.0 ppm, clude the use of an 8-fluoro derivatives as a drug. 8-Bromides or8- and so is the triazol intermediate 49. chlorides are convenient intermediates for introduction of other sub- The acidity of the NeH monosubstituted 6-amino group may in- stituents into the 8-position. Both heteronucleophilic and carbonu- fluence the antagonistic activity. An N-acyl derivative has been pre- cleophilic substitutions can take place. The halides become available by pared (Scheme 13). Butyryl chloride is used for acylation with DMAP as direct electrophilic substitution. A simple bromination of cAMP is base. When the initial acylation is on the sulfur in the phosphorothioic shown in Scheme 14 [11]. acid, the product will be a highly reactive intermediate for acylation of 8-Cl-cAMP 59 can be prepared by the reaction between cAMP so- the amino group to form a stable amide 55a. The DMAP salt of the dium salt and tetrabutylammonium iodotetrachloride in DMF at room acylated product 55a is subjected to flash chromatography on silica gel temperature (Scheme 15) [20]. The 8-Cl analogue is subsequently re- using CH2Cl2:NBu3. The product eluted is the tributylammonium salt acted further to afford 8-Cl-cAMPS targets by analogy to the reactions 55b. The silyl group is removed by the ammonium fluoride reagent, affording 8-Br-cAMPS derivatives. and the product 56 purified by flash chromatography. The sodium salt In the preparation of 8-substituted (Rp)-cAMPS partial exchange of 57 is available by dissolution of the ammonium salt 56 in methanol the 8-bromine substituent with chlorine may occur. The exchange can followed by addition of sodium hydroxide [15]. be significantly reduced, or even prevented, by the choice of specific reagents and reaction conditions producing a substantially pure product

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Scheme 15. Reagents and reaction conditions. which may be further substituted in a controlled manner at the 8- products may become admixed with the corresponding chloro com- postion. The halogen exchange can be adapted to preparative conver- pounds due to halogen exchange reactions. Perhaps a warning. In vivo, sion of a bromide to a chloride. Thus treatment of tri-nbutylammonium the 8-halogeno derivatives may become involved in covalent bonding 8-bromoadenosine-2‘-OTBDMS-3‘,5‘-cyclic monophosphate (60) in dry with heteronucleophiles such as thiols, exemplified by cysteine deri- CH2Cl2 with a mixture of DMF in dry CH2Cl2 affords the chemoselec- vatives, which may affect the biodata and toxicity. tively pure 8-chloro intermediate 61 which is reacted further with Most hetero-substituents in the 8-cAMPS position are sulfenyl de- benzylamine to afford the 8-chloro benzylamidate 61 (Scheme 15) rivatives, in particular phenylthio and p-substituted phenylthio deri- [21]. vatives [5]. The ease of substitution with thiophenols as nucleophiles Lithiation of the benzylamidate 62 takes place selectively at the may in part explain why phenyl derivatives are commonly prepared amidate nitrogen (Scheme 16). Subsequent treatment with carbon dis- according to the literature. The substrates are 8-Br- or 8-Cl-cAMPS ulfide affords the phosphorothioic acid 63. Desilylation with ammo- derivatives. The amidate 66 (Scheme 17) is appropriately silyl pro- nium fluoride followed by treatment with tributylamine affords the tected and substituted to afford the corresponding sulfide followed by ammonium salt 64. The sodium salt 65 is generated by treatment of the thiation to afford the cAMPS derivative 68. A subsequent fluoride ef- ammonium salt with methanolic sodium hydroxide [21]. fected desilylation provides the 8-sulfide 69 [21]. Little information on 8-oxygeno derivatives is available. Hydrolytic reactions from corresponding 8-bromo or 8-chloro afford oxygenated

2.4.3. 8-Amino, 8-hydroxy and 8-sulfenyl derivatives tautomeric derivatives 70 (Scheme 18). The (Sp)- and (Rp)-8-OH-cAMPS Purine derivatives such as cAMP are in general attacked by elec- stereoisomers 70 possess bioactivities similar to those observed for trophiles in the 8-position (vide supra). The 8-position is also a π-elec- (Rp)-cAMPS, but the hydroxyl function in (Rp)-8-OH-cAMPS and (SP)-8- tron deficient site for nucleophilic substitution reactions. Hence aha- OH-cAMPS makes these isomers less membrane permeable than (Rp)- logen substituent at this site may be displaced by nucleophilic reactants cAMPS [22]. such as thiols or by other halogen atoms depending on the reaction 8-Amino derivatives are available from corresponding bromides or conditions chosen. Partial halogen exchange may become a problem chlorides by simple amine substitutions. Thus treatment of an 8-epi- when elementally pure 8-halogeno derivatives are required. In the meric mixture of 8-Cl/Br-cAMPS 71 with piperidine at 50 °C affords presence of chloride ions in the reaction mixture targeted 8-bromo

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Scheme 16. Reagents and reaction conditions.

Scheme 17. Reagents and reaction conditions.

(Rp)-8-piperidino-adenosine 3′,5-(cyclic)thiophosohates (72a) and the alkylation will provide neutral molecules with improved lipophilic (Sp)-8-piperidino-cAMPS analogue 72b [23]. Ready azide substitution properties. Once inside the cell, the ester function needs to be cleaved followed by a reductive transformation into an amine or an N-hetero- chemoselectively at the alkyl-sulfur bond, not at the SeP bond, in order cycle may afford an alternative solution. to generate the thiophosphoric pharmacophore. A cleavage between the sulfur and the phosphorus atom results in formation of the corre- 2.5. Prodrugs sponding phosphate which is a cAMP derivative without the desired antagonistic activity. Several structural types of prodrugs can be pre- The target molecules in Fig. 1 possess significant hydrophilic pared [24–26]. In order to favor chemical cleavage of the SeP bond, the properties and hydrogen bonding properties. The passive transport of S-alkyl group must be activated for ready cleavage. In phosphates and these molecules across the cell membrane is relatively poor. The acidic phosphonates acyloxy esters are frequently used [26]. Pivalates are properties of the phosphorothioic group in cAMPS analogues may also useful since they are stable against hydrolysis but are readily cleaved interfere with the cell membrane passage and make cell membrane enzymatically by esterases. In Scheme 19 is shown synthesis of the more difficult to penetrate. To improve membrane passage several pivalates 73 and 75 by alkylation with chloromethyl pivalate of the structural types of prodrugs can be prepared. Ester formation by S- sodium salts of the cAMPS substrates 38 as a racemate and 73 under

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Scheme 18. Reagents and reaction conditions.

Scheme 19. Reagents and reaction conditions. reflux in methanol. Sodium iodide is present in the reaction mixtureto hydroxymethyl intermediate 78 that rapidly loses formaldehyde to af- promote the alkylation by chloride-iodide exchange in the methyl ford the drug 79 [26]. group prior to the alkylation reaction. In this manner the diaster- Similarly, membrane permeable p-acetoxybenzyl ester prodrugs of eomeric mixture of 8-phenyl substrate 38 as sodium salt affords a (Rp)-cAMPS are formed from 8-Br-cAMPS derivatives when alkylated diastereomeric mixture of (Rp)-73a and (Sp)-73b phenyl prodrugs that with p-acetoxybenzyl chloride in the presence of ethyldiisopropylamine can be resolved by chromatography (Scheme 19). The (Rp)-8-(2- as base in MeCN (Scheme 21). The prodrugs 82 are quickly bioactivated thienyl) substrate 74 reacts in the same way to provide the (Rp)-prodrug to the parent drug 84 by cytostolic esterases that are ubiquitously ex- 75. pressed in mammalian cells [4]. The intermediate 83 after the ester A structurally different prodrug is shown in Scheme 20. The p-iso- cleavage reacts further by autocatalysis and hydrolysis to afford the butyryloxybenzyl prodrug (Rp)-77 is prepared from (Rp)-8-(3-furyl) desired drug 84. adenosine-3′,5′-cyclic phosphorothioic acid triethylamine salt (76) and 4-(isobutyryloxy)benzyl iodide. The alkylation is run in DMF at room temperature. The methylene moiety with its carbonyl function is re- 3. Conclusion placed by a benzyl function (vide supra) as in structure 77. The benzene ring acts in the main as a spacer between an esterase-labile acyl group The chemistry associated with the important group of cAMPS de- and the phosphate. When treated with an esterase the prodrug 77 rivatives is reviewed and summarized. Stereochemistry at phosphorus is mediates reversibility to generate the parent phosphorothioate and a introduced by stereoselective amidation on appropriately silylated small amount of desulfurized product. The predominant enzymatic cAMP derivatives. The phosphoroamidates with the (Sp)-configuration cleavage of the carboxylate or carbonate ester generates a transient are converted by thiylation into phosphorothioic acids with (Rp)-configuration by2 CS under strongly basic conditions. From AMP

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Scheme 20. Reagents and reaction conditions.

Scheme 21. Reactions and conditions.

13 K. Undheim Bioorganic Chemistry 91 (2019) 103152 derivatives, corresponding stereochemical mixtures are obtain re- [7] F. Eckstein, L.P. Simonson, H.-P. Bär, Biochemistry 13 (1974) 3806–3810. quiring chiral resolutions. Carbon substituents are introduced into the [8] J. Baraniak, R.W. Kinas, K. Lesiak, W. Stec, J. Chem. Soc. Chem. Commun. (1979) 940–941. purine 8-position via corresponding halides by Pd-catalyzed trans- [9] J. Baraniak, W. Stec, J. Chem. Soc, Perkin. Trans. I (1987) 1645–1656. coupling reactions. Both alkyl, aryl and hetaryl derivatives become [10] H.-G. Genieser, W. Dostmann, U. Bottin, E. Butt, B. Jastorff, Tetrahedron Lett. 23 available. 8-Aza, 8-oxa and 8-thia-substituents are introduced by nu- (1988) 2803–2804. [11] M. Andrei, V. Bjørnstad, G. Langli, C. Rømming, J. Klaveness, K. Taskén, cleophilic substitutions from corresponding halides. Changes in the 6- K. Undheim, Org. Biomol. Chem. 5 (2007) 2070–2080. amino group are effected via a 6-(4-triazolo)-intermediate for nucleo- [12] F. Eckstein, U. Kutzke, Tetrahedron Lett. 27 (1986) 1657–1660. philic substitutions. The relatively poor transport ability of the target [13] K. Undheim, K. Taskén, J. Klaveness, G. Langli, V. Bjørnstad, WO 2005123755, drugs across the cell membrane can be improved by S-alkylation pro- 2005 (Chem. Abstract 144 (2006), 70065). [14] J. Baraniak, W.J. Stec, Tetrahedron Lett. 26 (1985) 4376–4382. viding activated lipophilic prodrugs. [15] M. Andrei, T. Bakkebø, J. Klaveness, K. Tasken, K. Undheim, Eur. J. Med. Chem. 46 (2011) 5935–5940. Acknowledgement [16] S. Kataoka, J. Imai, N. Yamaji, M. Kato, T. Kawada, S. Imai, Chem. Pharm. Bull. Jpn 38 (1990) 1596–1600. [17] B. Jastorff, H.-G. Genieser, Y. S. Cho-Chung, PCT Int. Appl. (1993) WO 9321929A1. This work did not receive any specific grant from funding agencies [18] R.W. Miles, V. Samano, M.J. Robins, J. Am. Chem. Soc. 117 (1995) 5951–9557. in the public, commercial, or not-for-profit sectors. [19] B.T. Gjertsen, G. Mellgren, A. Otten, E. Maronde, H.-G. Genieser, B. Jastorff, O.K. Vintermyr, G.S. McKnight, S.O. Døskeland, J. Biol. Chem. 270 (1995) 20599–20607. References [20] E.M. Weissinger, K. Oetrich, C. Evans, H.-G. Genieser, F. Schwede, M. Dangers, E. Dammann, H.-J. Kolb, H. Mischak, A. Ganser, W. Kolch, Brit. J. Cancer 91 (2004) 186–192. [1] S.H. Francis, M.A. Blount, R. Zoraghi, J.D. Corbin, Front. Biosci. 10 (2005) [21] K. Undheim, M. Andrei, PCT Int. Appl. (2008) WO 2008032103 A2. 2097–2117. [22] N. Otmakhov, J.E. Lisman, J. Neurophysiol. 87 (2002) 3018–3032. [2] K. Taskén, E.M. Aandahl, Physiol. Rev. 84 (2004) 137–167. [23] D. Øgreid, W. Dostmann, H.-G. Genieser, P. Niemann, S.O. Døskeland, B. Jastorff, [3] F. Schwede, E. Maronde, H.-G. Genieser, B. Jastorff, Pharmacol. Therapeut. 87 Eur. J. Biochem. 221 (1994) 1089–1094. (2000) 199–226. [24] C. Schultz, Bioorg. Med. Chem. 11 (2003) 885–898. [4] F. Schwede, O.G. Chepurny, M. Kaufholz, D. Bertinetti, C.A. Leech, O. Cabrera, [25] H.J. Jessen, C. Schultz, J. Balzarini, C. Meier, Angew. Chem. Int. Ed. 47 (2008) Y. Zhu, F. Mei, X. Cheng, J.E.M. Fox, P.E. Macdonald, H.-G. Genieser, F.W. Herberg, 8719–8722. G.G. Holz, Mol. Endocrinol. 29 (2015) 988–1005. [26] R.P. Iyer, D. Yu, T. Devlin, N.-H. Ho, S. Agrawal, Bioorg. Med. Chem. Lett. 6 (1996) [5] W.R. Dostman, S.S. Taylor, H.-G. Genieser, B. Jastorff, S.O. Døskeland, D. Øgreid, J. 1917–1922. Biol. Chem. 265 (1990) 10484–10491. [6] W.R. Dostman, S.S. Taylor, Biochemistry 30 (1991) 8710–8716.