Regiospecific N9 alkylation of 6-(heteroaryl)purines: shielding of N7 by a proximal heteroaryl C-H1

Purine alkylations have been plagued with formation of mixtures of N9 (usually desired), N7, and other regioisomers. We have developed methods for synthesis of 6-(azolyl)purine derivatives whose X-ray crystal structures show essentially coplanar conformations of the linked azole-purine rings. Such ring orientations position the C-H of the azole above N7 of the purine, which results in protection of N7 from alkylating agents. Treatment of 6-(2-butylimidazol-1-yl)-2-chloropurine (9) with sodium hydride in DMF followed by addition of ethyl iodide resulted in exclusive formation of 6-(2-butylimidazol-1-yl)-2-chloro-9-ethylpurine (10), whereas identical treatment of 2-chloro-6-(4,5-diphenylimidazol-1-yl)purine (11) produced a regioisomeric mixture 12/13 (N9/N7, approximately 5:1). The linked imidazole and purine rings are coplanar in 9 (the butyl side chain is extended away from the purine ring and C-H is over N7) but are rotated approximately 57 degrees in 11, and the more bulky azole substituent in 11 did not prevent formation of the minor N7 regioisomer 13. Access to various regioisomerically pure 9-alkylpurines is now readily available.     

Mild and efficient functionalization at C6 of purine 2'-deoxynucleosides and ribonucleosides

[reaction: see text] Treatment of sugar-protected inosine and 2'-deoxyinosine derivatives with a cyclic secondary amine or imidazole and I(2)/Ph(3)P/EtN(i-Pr)(2)/(CH(2)Cl(2) or toluene) gave quantitative conversions into 6-N-(substituted)purine nucleosides. S(N)Ar reactions with 6-(imidazol-1-yl) derivatives gave 6-(N, O, or S)-substituted products. The 6-(benzylsulfonyl) group underwent S(N)Ar displacement with an arylamine at ambient temperature.     

Regioselective Sonogashira cross-coupling reactions of 6-chloro-2,8-diiodo-9-THP-9H-purine with alkyne derivatives

Lithiation of 6-chloro-9-(tetrahydro-2H-pyran-2-yl)-9H-purine with LiTMP, gave access to 6-chloro-2,8-dihalogenated purine derivatives. In particular, the 6-chloro-2,8-diiodopurine derivative is an interesting new intermediate which gave regioselectively various 2-alkynylated compounds or 2,8-dialkynylated purines by using an excess of alkyne.     

Regiospecific and highly stereoselective coupling of 6-(substituted-imidazol-1-yl)purines with 2-deoxy-3,5-di-O-(p-toluoyl)-alpha-D-erythro-pentofuranosyl chloride. Sodium-salt glycosylation in binary solvent mixtures: improved synthesis of cladribine

Glycosylation of 6-(substituted-imidazol-1-yl)purine sodium salts with 2-deoxy-3,5-di-O-(p-toluoyl)-alpha-D-erythro-pentofuranosyl chloride proceeds with regiospecific formation of the N9 isomers. Base substrates with lipophilic substituents on the C6-linked imidazole moiety are more soluble in organic solvents, and the solubility is further increased with binary solvent mixtures. Selective solvation also diminishes the extent of anomerization of the chlorosugar. Stirred reaction mixtures of the modified-purine sodium salts generated in a polar solvent and cooled solutions of the protected 2-deoxysugar chloride in a nonpolar solvent give 2'-deoxynucleoside derivatives with N9 regiochemistry and enhanced beta/alpha configuration ratios. Application of the binary-solvent methodology with 2-chloro-6-(substituted-imidazol-1-yl)purine salts in cold acetonitrile and the chlorosugar in cold dichloromethane gives essentially quantitative yields of the N9 isomers of beta-anomeric 2'-deoxynucleoside intermediates. Direct ammonolysis (NH(3)/MeOH) of such intermediates or benzylation of the imidazole ring followed by milder ammonolysis of the imidazolium salt gives high yields of the clinical anticancer drug cladribine (2-chloro-2'-deoxyadenosine).     

Nucleic acid related compounds. 118. Nonaqueous diazotization of aminopurine derivatives. Convenient access to 6-halo- and 2,6-dihalopurine nucleosides and 2'-deoxynucleosides with acyl or silyl halides

Treatment of 9-(2,3,5-tri-O-acetyl-beta-d-ribofuranosyl)-2-amino-6-chloropurine (1) with TMS-Cl and benzyltriethylammonium nitrite (BTEA-NO2) in dichloromethane gave the crystalline 2,6-dichloropurine nucleoside 2, and acetyl chloride/BTEA-NO2 was equally effective ( approximately 85%, without chromatography). TMS-Br/tert-butyl nitrite/dibromomethane gave crystalline 2-bromo-6-chloro analogue 3 (85%). (Chloro or bromo)-dediazoniation of 3',5'-di-O-acetyl-2'-deoxyadenosine (4) gave the 6-[chloro (5, 63%) or bromo (6, 80%)]purine deoxynucleosides, and 2',3',5'-tri-O-acetyladenosine (8) was converted into the 6-chloropurine nucleoside 9 (71%).     

Unusual deoxygenation and reactivity studies related to O6-(benzotriazol-1-yl)inosine derivatives

New and unusual developments related to the chemistry of O6-(benzotriazol-1-yl)inosine derivatives are reported. First, a simple, scalable method for their syntheses via the use of PPh3/I2/HOBt has been developed and has been mechanistically investigated by 31P(1H) NMR. Studies were then conducted into a unique oxygen transfer reaction between O6-(benzotriazol-1-yl)inosine nucleosides and bis(pinacolato)diboron (pinB-Bpin) leading to the formation of C-6 (benzotriazol-1-yl)purine nucleoside derivatives and pinB-O-Bpin. This reaction has been investigated by 11B(1H) NMR and compared to pinB-O-Bpin obtained by oxidation of pinB-Bpin. The structures of the C-6 (benzotriazol-1-yl)purine nucleosides have been unequivocally established via Pd-mediated C-N bond formation between bromo purine nucleosides and 1H-benzotriazole. Finally, short and extremely simple synthesis of 1,N6-ethano- and 1,N6-propano-2'-deoxyadenosine are reported in order to demonstrate the synthetic versatility of the O6-(benzotriazol-1-yl)inosine nucleoside derivatives for the assembly of relatively complex compounds.     

Synthesis and structure of 6-substituted 9-(2-ethoxy-1,3-dioxan-5-yl)purines

The reaction of 4-chloro-5-amino-6-(1,3-dihydroxy-2-propyl)aminopyrimidine with excess ethyl orthoformate gave a cyclic acetal, viz., 6-chloro-9-(2-ethoxy-1,3-dioxan-5-yl)purine, amination of which yielded 6-amino-9-(2-ethoxy-1,3-dioxan-5-yl)purine. The presence of two configurational isomers with a diaxial orientation of the purine ring and the ethoxy group in the trans isomer and an equatorial orientation of the ethoxy group in the cis isomer was established for these compounds by ¹H and ¹³C NMR and IR spectroscopy. The three-dimensional structure of trans-6-chloro-9-(2-ethoxy-1,3-dioxan-5-yl)purine was determined by an x-ray difraction study, and the trans-diaxial orientation of the purine ring and the ethoxy group was confirmed; it is shown that the dioxane ring is in an anti conformation relative to the purine ring.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 976–983, July, 1979.     

Nucleic acid related compounds. 116. Nonaqueous diazotization of aminopurine nucleosides. Mechanistic considerations and efficient procedures with tert-butyl nitrite or sodium nitrite

Nonaqueous diazotization-dediazoniation of two types of aminopurine nucleoside derivatives has been investigated. Treatment of 9-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl)-2-amino-6-chloropurine (1) with SbCl(3)/CH(2)Cl(2) was examined with benzyltriethylammonium (BTEA) chloride as a soluble halide source and tert-butyl nitrite (TBN) or sodium nitrite as the diazotization reagent. Optimized yields (>80%) of the 2,6-dichloropurine derivative were obtained with SbCl(3). Combinations with SbBr(3)/CH(2)Br(2) gave the 2-bromo-6-chloropurine product (>60%), and SbI(3)/CH(2)I(2)/THF gave the 2-iodo-6-chloropurine derivative (>45%). Antimony trihalide catalysis was highly beneficial. Mixed combinations (SbX(3)/CH(2)X'(2); X/X' = Br/Cl) gave mixtures of 2-(bromo, chloro, and hydro)-6-chloropurine derivatives that were dependent on reaction conditions. Addition of iodoacetic acid (IAA) resulted in diversion of purine radical species into a 2-iodo-6-chloropurine derivative with commensurate loss of other radical-derived products. This allowed evaluation of the efficiency of SbX(3)-promoted cation-derived dediazoniations relative to radical-derived reactions. Efficient conversions of adenosine, 2'-deoxyadenosine, and related adenine nucleosides into 6-halopurine derivatives of current interest were developed with analogous combinations.     

N-oxides of adenosine-type nucleosides undergo pyrimidine ring opening and closure to give 5-amino-4-(1,2,4-oxadiazol-3-yl)imidazole derivatives

Treatment of acylated adenosine N-oxides with carboxylic anhydrides and thiophenol resulted in pyrimidine ring opening followed by exocyclic ring closure. Ammonolysis gave 5-amino-4-(5-substituted-1,2,4-oxadiazol-3-yl)-1-(beta-d-ribofuranosyl)imidazole derivatives, whereas iodine in methanol selectively unmasked the 5-amino group. Related flexible nucleoside analogues can be prepared from adenine-type precursors.     

Synthesis of (purin-6-yl)methylphosphonate bases and nucleosides

Three approaches to the synthesis of the title (purin-6-yl)methylphosphonates were investigated and compared. While, the Arbuzov reaction of 6-(iodomethyl)purines with triethyl phosphite did not work, Michaelis-Becker alkylation of the sodium salt of diethyl phosphonate with 6-(mesyloxymethyl)purines gave the desired products in good yields. The best method was based on Rh- or Pd-catalyzed cross-coupling reactions of 6-iodopurines with (diisopropoxyphosphorylmethyl)zinc bromide. In this way a small series of 6-(diisopropoxyphosphorylmethyl)purine bases and nucleosides was prepared in high yields.     

Synthesis of nucleoside analogs of 1,4-oxathiane, 1,4-dithiane,and 1,4-dioxane

The two regioisomers 6-chloro-9-(1, 4-oxathian-3-yl)-9H-purine ( 5 ) and 6-chloro-9-(1,4-oxathian-2-yl)-9H-purine ( 6 ) were obtained when 3-acetoxy-1,4-oxathiane ( 3 ) was subjected to the acid-catalyzed fusion procedure; compound 3 was prepared by a Pummerer reaction with 1,4-oxathiane 4-oxide ( 2 ). The nucleoside analog 6 could he converted into the adenine derivative 7 and 9-(1,4-oxathian-2-yl)-9H-purine-6(1H)thione ( 8 ). The following nucleoside analogs have also been synthesized: 6-chloro-9-(1,4-dithian-2-yl)-9H-purine ( 13 ), 9-(1,4-dithian-2-yl)adenine ( 14 ), 9-(1,4-dithian-2-yl)-9H-purine-6(1H)thione ( 15 ), and 6-chloro-9-(1,4-dioxan-2-yl)-9H-purine ( 18 ).     

Synthesis of 9-(2,3-dideoxy-2-fluoro-beta-D-threo-pentofuranosyl)adenine (FddA) via a purine 3'-deoxynucleoside.

A synthesis of 9-(2,3-dideoxy-2-fluoro-beta-D-threo-pentofuranosyl)adenine (1, FddA) via a 6-chloro-9-(3-deoxy-beta-D-erythro-pentofuranosyl)-9H-purine (9), which was readily obtained from inosine (5), is described. Fluorination at the C2'-beta position of the purine 3'-deoxynucleoside with diethylaminosulfur trifluoride was improved by the introduction of a 6-chloro group and proceeded in moderate yield. Purine 3'-deoxynucleoside derivatives were also subjected to nucleophilic reactions with triethylamine trihydrofluoride and gave the desired fluorinated nucleoside in good yield. The safety and yield of the fluorination process were greatly improved by the use of triethylamine trihydrofluoride. The influence of the sugar ring conformation and 6-chloro group on the rate of the nucleophilic reaction against elimination are also discussed.     

Synthetic studies directed towards asmarines; construction of the tetrahydrodiazepinopurine moiety by ring closing metathesis

Asmarines are tetrahydro[1,4]diazepino[1,2,3-g,h]purine derivatives isolated from marine sponges (Raspailia sp). They possess profound cytotoxic activity towards cancer cell lines, and are thus attractive synthetic targets. The tetrahydrodiazepinopurine ring skeleton has been prepared employing the RCM reaction on Boc-protected 6-allylamino-7-(propen-1-yl)purine as the key step for the construction of the seven-membered ring. 7-(Propen-1-yl)purines were formed by a novel rearrangement of 7-allylpurines under basic conditions. Boc-protected N⁶,7-diallylpurine also participated in RCM to give the eight-membered ring analog of the diazepinopurine.     

Synthesis of diverse 6-(1,2-disubstituted ethyl)purine bases and nucleosides via 6-(oxiran-2-yl)purines

Dihydroxylation of 6-vinylpurines with t-BuOOH and OsO₄ gave 6-(1,2-dihydroxyethyl)purines 2, while the epoxidation with H₂WO₄ and t-BuOOH afforded 6-(oxiran-2-yl)purines 3. Oxirane ring-opening reactions of 3 with diverse nucleophiles gave a series of title 6-(1,2-disubstituted ethyl)purine bases and nucleosides, which were tested for cytostatic and antiviral activities.     

Synthesis and biological activity of 1-(aryloxy)-3-(6-chloro-4,4-dimethyl-1,2,3,4-tetrahydroisoquinolin-2-yl)propan-2-ols

Treatment of 1-(4-chlorobenzylamino)-2-methylpropan-2-ol with concentrated sulfuric acid at 0°C gave 6-chloro-4,4-dimethyl-1,2,3,4-tetrahydroisoquinoline which reacted with (aryloxymethyl)oxiranes to afford new propan-2-ol derivatives of the tetrahydroisoquinoline series, 1-(aryloxy)-3-(6-chloro-4,4-dimethyl-1,2,3,4-tetrahydroisoquinolin-2-yl)propan-2-ols. Some of the synthesized compounds or their hydrochlorides showed moderate adrenergic blocking and sympatholytic activities.     

Cyclization and rearrangement products from coupling reactions between terminal o-alkynylphenols or o-ethynyl(hydroxymethyl)benzene and 6-halopurines

Cyclization reactions on 6-[(2-hydroxyphenyl)ethynyl]purines, 6-[(2-hydroxymethylphenyl)ethynyl]purines and 6-[(2-hydroxyphenyl)propyn-1-yl]purines have been studied. 6-(2-Benzofuryl)purines are readily available via a one-pot Sonogashira coupling-cyclization between 6-iodopurine and 2-ethynylphenol. When the same reaction was performed with o-(hydroxymethyl)ethynylbenzene, 6-[isobenzofuran-1(3H)-ylidenemethyl]purine was formed, mainly as the (E)-isomer. Acid catalyzed isomerization of the (E)-compound afforded the (Z)-isomer. The latter compound was also formed from a two-step reaction; Sonogashira coupling with O-silylated alkyne followed by deprotection and subsequent 5-exo cyclization. Sonogashira coupling between 6-halopurines and 2-propynylphenol gave only the alkyne coupling product and no cyclization took place. However, the Sonogashira product was unexpectedly rearranged to 6-(3-phenoxypropa-1,2-dienyl)purines under basic conditions. Theoretical calculations demonstrated that the allenes are more stable than their alkyne isomers.     

Azoles as Suzuki cross-coupling leaving groups: syntheses of 6-arylpurine 2'-deoxynucleosides and nucleosides from 6-(imidazol-1-yl)- and 6-(1,2,4-triazol-4-yl)purine derivatives

[reaction: see text] 6-(Imidazol-1-yl)-, 6-(benzimidazol-1-yl)-, and 6-(1,2,4-triazol-4-yl)purine nucleosides undergo a nickel-mediated C-C cross-coupling of azole-substituted purine derivatives with arylboronic acids to give good yields of 6-arylpurine nucleosides.     

Synthesis of di(Imdazolium) and di(Pyrazolium) Salts as Precursors for N-heterocyclic Dicarbene Complexes

Alpha,omega-bis(pyrazol-1-yl)alkanes and alpha,omega-bis(imidazol-1-yl)alkanes with spacers consisting of four to ten methylene groups have been prepared from pyrazole, 3,5-dimethylpyrazole or imidazole and corresponding dibromoalkanes in a superbasic medium KOH-DMSO. The proposed method of synthesis allowed the preparation of new flexible bidentate ligands without the need to use toxic solvents and tedious workup procedures. Bis(pyrazol-1-yl)alkanes were further functionalized for their use as precursors for “non-classical” mesoionic N-heterocyclic carbene ligands. One the first step, iodine atoms were introduced to positions 4 of pyrazole rings by oxidative iodination using I₂-HIO₃ system. On the next step, nitrogen atoms in positions 2 of pyrazole rings were alkylated using several agents. Reaction with methyliodide unexpectedly led to the formation of only mono-alkylated products even after 7 days of refluxing in a neat alkyliodide. Methylation by trimethyloxonium tetrafluoroborate or methyltriflate led to dimethylated products in high yields. Bis(imidazol-1-yl)alkanes were easily alkylated by methyliodide to give di(imidazolium) salts – precursors to “classic” N-heterocyclic dicarbenes.     

Modular and practical synthesis of 6-substituted pyridin-3-yl C-nucleosides

A novel modular and practical methodology for preparation of 6-substituted pyridin-3-yl C-nucleosides was developed. The Heck reaction of 2-chloro-5-iodopyridine with a 3'-TBDMS-protected glycal gave a 6-chloropyridin-3-yl nucleoside analogue, which was then desilylated, selectively reduced, and reprotected to give the TBDMS-protected 6-chloropyridin-3-yl C-2'-deoxyribonucleoside as a pure beta-anomer in a total yield of 39% over four steps. This key intermediate was then subjected to a series of palladium-catalyzed cross-coupling reactions, aminations, and alkoxylations to give a series of protected 1beta-(6-alkyl-, 6-aryl-, 6-hetaryl, 6-amino-, and 6-tert-butoxypyridin-3-yl)-2'-deoxyribonucleosides. 6-Unsubstituted pyridin-3-yl C-nucleoside was prepared by catalytic hydrogenation of the chloro derivative and 6-oxopyridine C-nucleoside by treatment of the 6-tert-butoxy derivative with TFA. Deprotection of all the silylated nucleosides by Et3N.3HF gave a series of free C-nucleosides (10 examples).