Nucleotides. Part L. Aglycone protection by the (2-dansylethoxy)carbonyl (= {2-{[5-(dimethylamino)naphthalen-l-yl]sulfonyl}ethoxy}carbonyl; dnseoc) group a new variation in oligodeoxyribonucleotide synthesis

The (2-dansylethoxy)carbonyl (= {2-{[5-(dimethylamino)naphthalen-l-yl]sulfonyl}ethoxy}carbonyl; dnseoc) group was employed for protection of the amino functions of the aglycone residues. The lactam function of 2′-deoxyguanosine was on the one hand unprotected and on the other hand alkylated at O⁶ of the aglycone with the 2-(4-nitrophenyl)ethyl (npe) and 2-(phenylsulfonyl)ethyl (pse) group, respectively. The syntheses of monomeric building blocks, both phosphoramidites and nucleoside- functionalized supports, are described for the three common 2′-deoxynucleosides (2′-deoxycytidine, 2′-deoxyadenosine, 2′-deoxyguanosine). As kinetic studies with the tritylated nucleosides showed, the dnseoc group is more labile towards DBU cleavage than the corresponding 2-(4-nitrophenyl)ethyl-(npe) and [2-(4-nitrophenyl)ethoxy]carbonyl(npeoc)-protected analogues (see Table 2). These results were confirmed by the very fast deprotection rate of the dnseoc groups at some oligonucleotides.     

Nucleotides. Part XLV. Synthesis of new (2′–5′)adenylate trimers,containing 5′-amino-5′-deoxyadenosine residues at the 5′-end of the oligoadenylate chain,and of its analogues,carrying a 9-[(2-hydroxyethoxy)methyl]adenine residue at the 2′-terminus

The 5′-amino-5′-deoxy-2′,3′-O-isopropylideneadenosine ( 4 ) was obtained in pure form from 2′,3′-O-isopropylideneadenosine ( 1 ), without isolation of intermediates 2 and 3 . The 2-(4-nitrophenyl)ethoxycarbonyl group was used for protection of the NH₂ functions of 4 (→7) . The selective introduction of the palmitoyl (= hexadecanoyl) group into the 5′-N-position of 4 was achieved by its treatment with palmitoyl chloride in MeCN in the presence of Et₃N (→ 5 ). The 3′-O-silyl derivatives 11 and 14 were isolated by column chromatography after treatment of the 2′,3′-O-deprotected compounds 8 and 9 , respectively, with (tert-butyl)dimethylsilyl chloride and 1H-imidazole in pyridine. The corresponding phosphoramidites 16 and 17 were synthesized from nucleosides 11 and 14 , respectively, and (cyanoethoxy)bis(diisopropylamino)phosphane in CH₂Cl₂. The trimeric (2′–5′)-linked adenylates 25 and 26 having the 5′-amino-5′-deoxyadenosine and 5′-deoxy-5′-(palmitoylamino)adenosine residue, respectively, at the 5′-end were prepared by the phosphoramidite method. Similarly, the corresponding 5′-amino derivatives 27 and 28 carrying the 9-[(2-hydroxyethoxy)methyl]adenine residue at the 2′-terminus, were obtained. The newly synthesized compounds were characterized by physical means. The synthesized trimers 25–28 were 3-, 15-, 25-, and 34-fold, respectively, more stable towards phosphodiesterase from Crotalus durissus than the trimer (2′–5′)ApApA.     

Nucleic-Acid Analogues with Constraint Conformational Flexibility in the Sugar-Phosphate Backbone (‘Bicyclo-DNA’). Part 1. Preparation of (3S,5′R)-2′-Deoxy-3′,5′-ethano-αβ-D-ribonucleosides (‘Bicyclonucleosides’)

We describe the synthesis of 2′-deoxy-3′,5′-ethano-D -ribonucleosides 1 – 8 (= (5′,8′-dihydroxy-2′-oxabicyclo-[3.3.0]oct-3′-yl)purines or -pyrimidines) of the nucleobases adenine, thymine, cytosine, and guanine. They differ from natural 2′-deoxyribonucleosides only by an additional ethylene bridge between the centers C(3′) and C(5′). The configuration at these centers (3S,5′R) was chosen as to match the geometry of a repeating nucleoside unit in duplex DNA as close as possible. These nucleosides were designed to confer, as constituents of an oligonucleotide chain, a higher degree of preorganization of a single strand for duplex formation with respect to natural DNA, thus leading to an entropic advantage for the pairing process. The synthesis of these ‘bicyclonucleosides’ was achieved by construction of an enantiomerically pure carbohydrate precursor 18 / 19 (Schemes 1), which was then converted to the corresponding nucleosides by known methods in nucleoside synthesis (Schemes 2 and 3). In all cases, both anomeric forms of the nucleosides were obtained in pure crystalline form, the relative configuration of which was established by ¹H-NMR-NOE spectroscopy. A conformational analysis of the nucleosides with β-configuration at the anomeric center by means of X-ray and ¹H-NMR (including NOE) spectroscopy show the furanose part of the molecules to adopt uniformly a 1′exo-conformation with the base substituents preferentially in the anti-range in the pyrimidine nucleosides (anti/syn ca. 2:1) distribution in the purine nucleosides (in solution).     

Synthesis of various 5-substituted 2′-deoxy-3′-5′-di-O-acetyluridines

The Pd(0)-catalyzed coupling reaction of β-5-iodo-2′-deoxy-3′,5′-di-O-acetyluridine with various heteroaryltrimethylstannyl compounds gave the corresponding β-5-heteroaryl-2′-deoxy-3′,5′-di-O-acetyluridines in moderate yields. This direct coupling approach for nucleosides represented an interesting alternative to the 5-heteroaryl functionalization of pyrimidines followed by the Hilbert-Johnson glycosylation reaction which often yields mixtures of the α and β anomers.     

Nucleosides. Part LI The 2-(4-nitrophenyl)ethoxycarbonyl (npeoc) and 2-(2,4-dinitrophenyl)ethoxycarbonyl (dnpeoc) groups for protection of hydroxy functions in ribonucleosides and 2′ -deoxyribonucleosides

The Common 2′ -deoxypyrimidine and -purine nucleosides, thymidine ( 4 ), O⁴-[2-(4-nitrophenyl)ethyl]-thymidine ( 17 ), 2′-deoxy-N⁴-[2-(4-nitrophenyl)ethoxycarbonyl]cytidine ( 26 ), 2′-deoxy-N⁶-[2-(4-nitrophenyl)-ethoxycarbonyl]adenosine- 39 , and 2′-deoxy-N²-[2-(4-nitrophenyl)(ethoxycarbonyl]-O⁶-[2–4-nitrophenyl)ethyl]-guanosine ( 52 ) were further protected by the 2-(4-nitrophenyl)ethoxycarbonyl (npeoc) and the 2-(2,4-dinitrophenyl)ethoxycarbonyl (dnpeoc) group at the OH functions of the sugar moiety to form new partially and fully blocked intermediates for nucleoside and nucleotide syntheses. The corresponding 5′-O-monomethoxytrityl derivatives 5 , 18 , 30 , 40 , and 56 were also used as starting material to synthesize some other intermediates which were not obtained by direct acylations. In the ribonucleoside series, the 5′ -O-monomethoxytrityl derivatives 14 , 36 , 49 , and 63 reacted with 2-(4-nitrophenyl) ethyl chloroformate ( 1 ) to the corresponding 2′,3′-bis-carbonates 15 , 37 , 50 , and 64 which were either detriylated to 16 , 38 , 51 , and 65 , respectively, or converted by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) treatment to the 2′,3′-cyclic carbonates 66 – 69 . The newly synthesized compounds were characterized by elemental analyses and UV and ¹H-NMR spectra.     

Nucleoside und Nucleotide. Teil 16. Verhalten von 1-(2′-Desoxy-β-D-ribofuranosyl)-2(1 H)-pyrimidinon-5′-triphosphat, 1-(2′-Desoxy-β-D-ribofuranosyl)-2(1 H)pyridinon-5′-triphosphat und 4-Amino-1-(2′-Desoxy-β-D-ribofuranosyl)-2(1 H)-pyridinon-5′-triphosphat gegenüber DNA-Polymerase

Nucleosides and Nucleotides. Part 16. The Behaviour of 1-(2′-Deoxy-β-D -ribofuranosyl)-2(1H)-pyrimidinone-5′-triphosphate, 1-(2′-Deoxy-β-D -ribofuranosyl-2(1H))-pyridinone-5′-triphosphate and 4-Amino-1-(2′-desoxy-β-D -ribofuranosyl)-2(1H)-pyridinone-5′-triphosphate towards DNA Polymerase The behaviour of nucleotide base analogs in the DNA synthesis in vitro was studied. The investigated nucleoside-5′-triphosphates 1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyrimidinone-5′-triphosphate (pppM_d), 1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyridinone-5′-triphosphate (pppII_d) and 4-amino-1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyridinone-5′-triphosphate (pppZ_d) can be considered to be analogs of 2′-deoxy-cytidine-5′-triphosphate. However, their ability to undergo base pairing to the complementary guanine is decreased. When pppM_d, pppII_d or pppZ_d are substituted for pppC_d in the enzymatic synthesis of DNA by DNA polymerase no incorporation of these analogs is observed. They exhibit only a weak inhibition of the DNA synthesis. The mode of the inhibition is uncompetitive which shows that these nucleotide analogs cannot serve as substrates for the DNA polymerase.     

Nucleosides and nucleotides. 2. Synthesis of both anomers of 1-(5'-O-phosphoryl-2'-deoxy-D-ribofuranosyl)-2(1H)-pyridone

The two anomeric 1-(2′-deoxy-D -ribofuranosyl)-2(1H)-pyridones 6 and 7 were synthesized from 2-pyridone and 3,5-di-(O-p-toluoyl)-2-deoxy-D -ribofuranosyl chloride ( 2 ) via the di-O-p-toluoyl derivatives 3 and 4 using the mercuric halide procedure. Phosphorylation of the nucleosides 6 and 7 by bis-(2,2,2-trichloroethyl)-chlorophosphate gave the phosphate esters 8 and 9 together with some 2-(bis-[2,2,2-trichloroethyl]-phosphoryloxy)-pyridine 10 , which proved to be very labile. Structure and configuration of compounds 6 to 9 were established by spectral methods, the configurations being derived from the chemical shifts of the sugar protons and the splitting patterns of the anomeric protons (‘triplet-quartet rule’). The specific rotations of 3 , 4 , 6 , 7 , 8 and 9 show that the three pairs of anomers represent exceptions to Hudson's rule of isorotation. Reductive removal of the trichloroethyl groups in 8 and 9 with zinc proceeds stepwise, yielding the phosphoro-diesters 13 and 14 and the two desired anomeric 5′-nucleotides 15 and 16 . These latter were purified and characterised as the ammonium salts. Enzymatic cleavage by the 5′-nucleotidase of Crotalus adamanteus venom took place only in the ‘natural’ β-series. The ‘unnatural’ α-anomers were resistent to the enzyme. The structure of 10 was established by spectral methods and confirmed by synthesis.     

Nucleotides. Part XLIII. Solid-phase synthesis of oligoribonucleotides using the 2-dansylethoxycarbonyl (  2-{[5-(dimethylamino)naphthalen-1-yl]sulfonyl}ethoxycarbonyl; dnseoc) group for 5′-hydroxy protection

A new efficient method for solid-phase synthesis of oligoribonucleotides via the phosphoramidite approach is described. The combination of the base-labile 2-dansylethoxycarbonyl (Dnseoc) group for 5′-OH protection with the acid-labile tetrahydro-4-methoxy-2H-pyran-4-yl (Thmp) group as 2′-OH blocking group is orthogonal regarding cleavage reactions and fulfills the requirements of an automated synthesis in an excellent manner if the phosphoramidite function carries the N,N-diethyl-O-[2-(4-nitrophenyl)ethyl] substitution.     

Nucleotide. IX. Synthese und Eigenschaften von 1-(2′-Desoxy-D-ribofuranosyl)-lumazin-3′-monophosphaten

Nucleotides. IX. Synthesis and properties of 1-(2′-deoxy-D -ribofuranosyl)-lumazin-3′-monophosphates The synthesis of various 1-(2′-deoxy-α-[and β-]D -ribofuranosyl)-lumazine-3′-monophosphates 25--30 starting from the corresponding pteridine nucleosides 1--6 is described. Monomethoxytritylation in 5′-position to 7--12 , phosphorylation by cyanoethylphosphate to 13--18 , and deprotection by acid and base treatment afforded the lumazine nucleotides 25--30 in good overall yield. The various reaction products have been characterized by physical means, such as UV. spectra, pK-values and their chromatographical and electrophoretical behaviour. Enzymatic dephosphorylations by alkaline phosphatase led to the starting material 1--6 with a 3--4 times slower hydrolysis rate in comparison to Tp.     

Nucleotides,Part LXV,Synthesis of 2′‐Deoxyribonucleoside 5′‐Phosphoramidites: New Building Blocks for the Inverse (5′‐3′)‐Oligonucleotide Approach

The syntheses of the 3′‐O‐(4,4′‐dimethoxytrityl)‐protected 5′‐phosphoramidites 25 – 28 and 5′‐(hydrogen succinates) 29 – 32 , which can be used as monomeric building blocks for the inverse (5′‐3′)‐oligodeoxyribonucleotide synthesis are described (Scheme). These activated nucleosides and nucleotides were obtained by two slightly different four‐step syntheses starting with the base‐protected nucleosides 13 – 20 . For the protection of the aglycon residues, the well‐established 2‐(4‐nitrophenyl)ethyl (npe) and [2‐(4‐nitrophenyl)ethoxy]carbonyl (npeoc) groups were used. The assembly of the oligonucleotides required a slightly increased coupling time of 3 min in application of the common protocol (see Table 1). The use of pyridinium hydrochloride as an activator (instead of 1H‐tetrazole) resulted in an extremely shorter activation time of 30 seconds. We established the efficiency of this inverse strategy by the synthesis of the oligonucleotide 3′‐conjugates 33 and 34 which carry lipophilic caps derived from cholesterol and vitamin E, respectively, as well as by the formation of (3′‐3′)‐ and (5′‐5′)‐internucleotide linkages (see Table 2).     

Nucleoside und Nucleotide. Teil 10. Synthese von Thymidylyl-(3′-5′) -thymidylyl-(3′-5′)-1-(2′-desoxy-β-D-ribofuranosyl)-2(1 H)-pyridon

Nucleosides and Nucleotides. Part 10. Synthesis of Thymidylyl-(3′-5′)-thymidylyl-(3′-5′)-1-(2′-deoxy-β-D - ribofuranosyl)-2(1 H)-pyridone The synthesis of 5′-O-monomethoxytritylthymidylyl-(3′-5′)-thymidylyl-(3′-5′)-1-(2′-deoxy-β-D -ribofuranosyl)-2(1H)-pyridone ((MeOTr)T_dpT_dp∏_d, 5 ) and of thymidylyl-(3′-5′)-thymidylyl-(3′-5′)-1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyridone (T_dpT_dp∏_d, 11 ) by condensing (MeOTr) T_dpT_d ( 3 ) and p∏_d(Ac) ( 4 ) in the presence of DCC in abs. pyridine is described. Condensation of (MeOTr) T_dpT_dp ( 6 ) with Π_d(Ac) ( 7 ) did not yield the desired product 5 because compound 6 formed the 3′-pyrophosphate. The removal of the acetyl- and p-methoxytrityl protecting group was effected by treatment with conc. ammonia solution at room temperature, and acetic acid/pyridine 7 : 3 at 100°, respectively. Enzymatic degradation of the trinucleoside diphosphate 11 with phosphodiesterase I and II yielded T_d, pT_d and p∏_d, T_dp and Π_d, respectively, in correct ratios.     

Synthesis of 5′‐O‐(2‐Azido‐2‐deoxy‐α‐D‐glycosyl)nucleosides and Their Antitumor Activities

The 1,3,4,6‐tetra‐O‐acetyl‐2‐azido‐2‐deoxy‐β‐D ‐mannopyranose ( 4 ) or the mixture of 1,3,6‐tri‐O‐acetyl‐2‐azido‐2‐deoxy‐4‐O‐(2,3,4,6‐tetra‐O‐acetyl‐β‐D ‐galactopyranosyl)‐β‐D ‐mannopyranose ( 10 ) and the corresponding α‐D ‐glucopyranose‐type glycosyl donor 9 / 10 reacted at room temperature with protected nucleosides 12 – 15 in CH₂Cl₂ solution in the presence of BF₃?OEt₂ as promoter to give 5′‐O‐(2‐azido‐2‐deoxy‐α‐D ‐glycosyl)nucleosides in reasonable yields (Schemes 2 and 3). Only the 5′‐O‐(α‐D ‐mannopyranosyl)nucleosides were obtained. Compounds 21, 28, 30 , and 31 showed growth inhibition of HeLa cells and hepatoma Bel‐7402 cells at a concentration of 10 μM in vitro.     

2′‐Ethynyl‐DNA: Synthesis and Pairing Properties

2‐Ethynyl‐DNA was developed as a potential DNA‐selective oligonucleotide analog. The synthesis of 2′‐arabino‐ethynyl‐modified nucleosides was achieved starting from properly protected 2′‐ketonucleosides by addition of lithium (trimethylsilyl)acetylide followed by reduction of the tertiary alcohol. After a series of protecting‐group manipulations, phosphoramidite building blocks suitable for solid‐phase synthesis were obtained. The synthesis of oligonucleotides from these building blocks was successful when a fast deprotection scheme was used. The pairing properties of 2′‐arabino‐ethynyl‐modified oligonucleotides can be summarized as follows: 1) The 2′‐arabino‐ethynyl modification of pyrimidine nucleosides leads to a strong destabilization in duplexes with DNA as well as with RNA. The likely reason is that the ethynyl group sterically influences the torsional preferences around the glycosidic bond leading to a conformation not suitable for duplex formation. 2) If the modification is introduced in purine nucleosides, no such influence is observed. The pairing properties are not or only slightly changed, and, in some cases (deoxyadenosine homo‐polymers), the desired stabilization of the pairing with a DNA complementary strand and destabilization with an RNA complement is observed. 3) In oligonucleotides of alternating deoxycytidine‐deoxyguanosine sequence, the incorporation of 2′‐arabino‐ethynyl deoxyguanosine surprisingly leads to the formation of a left‐handed double helix, irrespective of salt concentration. The rationalization for this behavior is that the ethynyl group locks such duplexes in a left‐handed conformation through steric blockade.     

Nucleosides and Nucleotides. Part 13. Synthesis and spectral properties of 1- (3′-O-Phosphoryl-2′-deoxy-β -D-ribofuranosyl)-2(1H)-pyridone-(3′-5′)-1-(2′-deoxy-β-D-ribofuranosyl)-2 (1H)-pyridone

The dinucleoside phosphate Π_dpΠ_d ( 4 ) was synthesized from the monomers 1-(5′-O-monomethoxytrityl - 2′ - deoxy - β - D - ribofuranosyl) - 2 (1 H) - pyridone ((MeOTr) Π_d, 2 ) and 1-(5′-O-phosphoryl-3′-O-acetyl-2′-deoxy-β-D -ribofuranosyl)-(1H)-pyridone (pΠ_d(Ac), 3 ). Its 6.4% hyperchromicity and an analysis of the ¹H-NMR. spectra indicate that the conformation and the base-base interactions in 4 are similar to those in natural pyrimidine dinucleoside phosphates.     

The synthesis of some purine nucleosides from 4,6-di-O-acetyl-3-deoxy-3-(ethoxycarbonylamino)-D-glucal

The acid catalyzed reaction of 4,6-di-O-acetyl-3-deoxy-3-(ethoxycarbonylamino)-D-glucal and 6-chloropurine in nitrometliane solution gave 6-ehloro-9-(4′,6′-di-O-acetyl-2′,3′-dideoxy-3′-ethoxy-carbonylamino-α- and β-D-arafemohexopyranosyl)purine. These were converted to the corresponding deblocked 6-dimetliylaminopurine nucleosides by treatment with ethanolic dimethylamine; acetylation of these gave the respective 4′,6′-di-O-acetyl derivatives. The anomeric assignments for the nucleosides were based on their nmr spectral data.     

9-deazapurine nucleosides. The synthesis of certain N-5-2′-deoxy-β-D-erythropentofuranosyl and N-5-β-D-arabinofuranosylpyrrolo[3,2-d]pyrimidines

A number of 2,4-disubstituted pyrrolo[3,2-d]pyrimidine N-5 nucleosides were prepared by the direct glycosylation of the sodium salt of 2,4-dichloro-5H-pyrrolo[3,2-d]pyrimidine (3) using 1-chloro-2-deoxy-3,5-di-O-(p-toluoyl)-α-D -erythropentofuranose (1) and 1-chloro-2,3,5-tri-O-benzyl-α-D-arabinofuranose (11) . The resulting N-5 glycosides, 2,4-dichloro-5-(2-deoxy-3,5-di-O-(p-toluoyl) -β-D-erythropentofuranosyl)-5H-pyrrolo-[3,2-d]pyrimidine (4) and 2,4-dichloro-5-(2,3,5-tri-O-benzyl-β-D-arabinofuranosyl-5H -pyrrolo [3,2-d)pyrimidine (12) , served as versatile key intermediates from which the N-7 glycosyl analogs of the naturally occurring purine nucleosides adenosine, inosine and guanosine were synthesized. Thus, treatment of 4 with methanolic ammonia followed by dehalogenation provided the adenosine analog, 4-amino-5-(2-deoxyerythropentofuranosyl) -5H-pyrrolo[3,2-d]pyrimidine (6) . Reaction of 4 with sodium hydroxide followed by dehalogenation afforded the inosine analog, 5-(2-deoxy-β-D-erythropentofuranosyl) -5H-pyrrolo[3,2-d]pyrimidin-4(3H)-one (9) . Treatment of 4 with sodium hydroxide followed by methanolic ammonia gave the guanosine analog, 2-amino-5-(2-deoxy-β-D-erythropentofuranosyl) -5H-pyrrolo[3,2-d]pyrimidin-4(3H)-one (10) . The preparation of the same analogs in the β-D-arabinonucleoside series was achieved by the same general procedures as those employed for the corresponding 2′-deoxy-β-D-ribonucleoside analogs except that, in all but one case, debenzylation of the sugar protecting groups was accomplished with cyclohexene-palladium hydroxide on carbon, providing 4-amino-5-β-D-arabinofuranosyl-5H-pyrrolo [3,2-d]pyrimidin-4(3H)-one (18) . Structural characterization of the 2′-deoxyribonucleoside analogs was based on uv and proton nmr while that of the arabinonucleosides was confirmed by single-crystal X-ray analysis of 15a . The stereospecific attachment of the 2-deoxy-β-D-ribofuranosyl and β-D-arabinofuranosyl moieties appears to be due to a Walden inversion at the C₁ carbon by the anionic heterocyclic nitrogen (S_N2 mechanism).     

Synthesis and conformational properties of 2′-deoxy-2′-methylthio-pyrimidine and -purine nucleosides: Potential antisense applications

A convenient and shorter synthesis of 2′-deoxy-2′-methylthiouridine analogs 5 , ?5-methyluridine 6 , -cyti-dine 15 , ?5-methylcytidine 16 , -adenosine 27 and -guanosine 34 was accomplished. Successful conversion of ribonucleosides (5-methyl U, U, A, G) into the corresponding 2′-substituted nucleosides involves nucleophilic displacement (S_N2) of an appropriate leaving group at the 2′-position by methanethiol, a soft nucleophile. Reaction between 2,2′-anhydrouridine and methanethiol in the presence of N¹,N¹,N³,N³-tetramethylguani-dine in N,N-dimethylformamide gave 5 , in 75% yield. Preparation of 6 by a similar route was described. Acylated 5 and 6 were transformed into their triazole derivatives, which on ammonolysis furnished 15 and 16 , respectively in good yield. Similarly, tetraisopropyldisiloxanyl (TIPS) protected 2′-O-aratriflates- of-adenosine and -guanosine reacted with methanethiol in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene at - 25°, followed by deblocking of the TIPS protecting group furnished 27 and 34 , respectively. The confor-mational flexibility (N/S equilibrium) of the sugar moiety in nucleosides 5 , 15 , 27 and 34 was studied utilizing nmr spectroscopy, suggesting that the 2′-methylthio group influenced the sugar conformation to adopt a rigid S-pucker in all cases. The extra stiffness of the sugar moiety in these analogs is believed to be due to the electronegativity of the substituent and the steric bulk. The usefulness of these nucleosides to prepare uniformly modified 2′-deoxy-2′-methylthio oligonucleotides for antisense therapeutics is proposed.     

Synthesis of certain 6-alkylthio-2,2′-anhydro-5-azauridines

β-D-Arabinofurano[1′,2′:4,5]oxazolo-s-triazin-4-one-6-thione ( 7b ) and its t-butyldimethylsilyl protected counterpart 7a were synthesized by treating the appropriate 2-amino-β-D-arabinofurano[1′,2′:4,5]-2-oxazoline with ethoxycarbonyl isothiocyanate. These 2,2′-anhydro-s-triazine nucleosides were then subjected to alkylation under similar reaction conditions. Alkylation of 3′,5′-bis(O-t-butyldimethylsilyl)-β-D-arabinofurano[1′,2′:-4,5]oxazolo-s-triazin-4-one-6-thione ( 7a ) provided the targeted S-alkylated nucleosides, i.e., the C6-SCH₃ ( 9a ), C6-SCH₂-CH = CH₂ ( 10a ), and C6-S-CH₂-C = CH ( 11a ), in reasonable yields. Attempted deprotection of these nucleosides failed. In order to circumvent this problem, 7b was alkylated with the same reagents. In each case, instead of the expected S-alkylated anhydronucleosides, a mixture of the 5-N-alkylanhydro-s-triazine-4,6-dione and 5-N-alkylanhydro-s-triazin-4-one-6-thione derivatives were obtained. The 2,2′-anhydro linkage of 7a was also found to be more stable than the s-triazine ring to mild base. Basic conditions displaced the C6-sulfur substituent and eventually caused ring opening of the s-triazine aglycone.     

Stereospezifische Synthese des Cancerostatikums 5′-Desoxy-5-fluor-uridin (5-DFUR) und seiner 5′ -deuterierten Derivate

Stereospecific Synthesis of the Anticancer Agent 5′-Deoxy-5-fluorouridine and its 5′-deuteriated Derivatives 5′-Deoxy-5-fluorouridine (5′-DFUR) has been obtained in high yield and purity by Stereospecific condensation of the anomeric 5¨deoxy-1,2,3-tri- O-acetyl-D-ribofu-ranose with bis(trimethylsilylated)-5-fluorouracil in the presence of trimethylsilyl trifluoromethanesulfonate, and by subsequent cleavage of the acetate protecting groups. A minor by-product of the synthesis, the α-anomeric nucleoside is produced by a (β-α)-epimerization, a procedure which is catalyzed by trimethylsilyl trifluoromethanesulfonate. The corresponding 5′-deuteriated, and 5′,5′-dideuteriated nucleosides have also been synthesized using an analogous way. The synthesis of the sugar components of the latter nucleosides - starting from D-ribose, D-xylose and D-glucose -is also described.     

Single‐Stranded DNA: Replacement of Canonical by Base‐Modified Nucleosides in the Minihairpin 5′‐d(GCGAAGC)‐3′ and Constructs with the Aptamer 5′‐d(GGTTGGTGTGGTTGG)‐3′

The minihairpin 5′‐d(GCGAAGC)‐3′ ( 1 ) was modified either in the loop region, in the base‐paired stem, or at the 5′‐terminus by incorporation of base‐modified nucleosides. The thermal melting was correlated to the structural changes induced by the various donor‐acceptor properties of the nucleosides. Overhanging nonpaired nucleosides at the 5′‐terminus stabilized the hairpin, while a reverse of the dG³?dA⁵ sheared base pair to dA³?dG⁵ severely affected the stability. The combination of the minihairpin 5′‐d(GCGAAGC)‐3′ ( 1 ) and the thrombin‐binding aptamer 5′‐d(GGTTGGTGTGGTTGG)‐3′ ( 2 (= 46 )) resulted in the new construct 5′‐d(GGTTGGGCGAAGC GGTTGG)‐3′ ( 43 ) arising by replacement of the 5′‐d(TGT)‐3′ loop of 2 by the minihairpin. The fused oligonucleotide 43 exhibits a two‐phase thermal transition indicating the presence of the two unaltered moieties. According to slight changes of the T_m values of the construct 43 as compared to the separate units 1 and 2 , cooperative distorsions are discussed.