Synthesis,Base Pairing,and Structural Transitions of Oligodeoxyribonucleotides Containing 8-Aza-2′-deoxyguanosine

The synthesis of oligonucleotides containing 8-aza-2′-deoxyguanosine (z⁸G_d; 1 ) or its N⁸-regioisomer z⁸G_d^* ( 2 ) instead of 2′-deoxyguanosine (G_d) is described. For this purpose, the NH₂ group of 1 and 2 was protected with a (dimethylamino)methylidene residue (→ 5, 6 ), a 4,4′-dimethoxytrityl group was introduced at 5′-OH (→ 7, 8 ), and the phosphonates 3a and 4 as well as the phosphoramidite 3b were prepared. These building blocks were used in solid-phase oligonucleotide synthesis. The oligonucleotides were characterized by enzymatic hydrolysis and melting curves (T_m values). The thermodynamic data of the oligomers 12–15 indicate that duplexes were stabilized when 1 was replacing G_d. The aggregation of d(T-G-G-G-G-T) ( 18 ) was studied by RP 18 HPLC, gel electrophoresis and CD spectroscopy and compared with that of oligonucleotides containing an increasing number of z⁸G_d residues instead of G_d. Similarly to [d(C-G)]₃ ( 12a ), the hexamer d(C-z⁸G-C-z⁸G-C-G) ( 14 ) underwent salt-dependent B-Z transition.     

Synthesis of oligoribonucleotides containing isoguanosine

Oligoribonucleotides containing isoguanosine ( ? 1,2-dihydro-2-oxoadenosine; isoG; 1 ) were prepared. The building block 2 was synthesized using the (dimethylamino)methylidene residue as NH₂ protecting group. The monomethoxytrityl as well as dimethoxytrityl group were introduced at OH–C(5′) (→ 5 and 6 ). Silylation of 5 with triisopropylsilyl chloride formed the 2′-O-blocked derivative 7 almost exclusively. Reaction with PCl₃/1,2,4-1H-triazole furnished the phosphonate 2 which was used in solid-phase synthesis of the oligoribonucleotides 10 and 11 . RNAse T₁ hydrolyzed U-A-G-U-U-isoG-U-U-A-G ( 10 ) at the 3′-site of G but not of isoG. The self-complementary oligomer (A-U-isoG-U)₃ ( 11 ) formed a duplex which was less stable than that of (A-U)₆.     

Stereoselective synthesis of pyrrolo[2,3-d]pyrimidine α- and β-D-ribonucleosides from anomerically pure D-ribofuranosyl chlorides: Solid-Liquid Phase-Transfer Glycosylation and 15N-NMR Spectra

Solid-liquid phase-transfer glycosylation (KOH, tris[2-(2-methoxyethoxy)ethye]amine ( = TDA-1), MeCN) of pyrrolo[2,3-d]pyrimidines such as 3a and 3b with an equimolar amount of 5-O-[(1,1 -dimethylethyl)dimethylsilyl]-2,3-O-(1-methylethylidene)-α-D -ribofuranosyl chloride (1) [6] gave the protected β-D -nucleosides 4a and 4b , respectively, stereoselectively (Scheme). The β-D -anomer 2 [6] yielded the corresponding α-D -nucleosides 5a and 5b with traces of the β-D -compounds. The 6-substituted 7-deazapurine nucleosides 6a , 7a , and 8 were converted into tubercidin (10) or its α-D -anomer (11) . Spin-lattice relaxation measurements of anomeric ribonucleosides revealed that T₁ values of H? C(8) in the α-D -series are significantly increased compared to H? C(8) in the β-D -series while the opposite is true for T₁ of H? C(1′). ¹⁵N-NMR data of 6-substituted 7-deazapurine D -ribofuranosides were assigned and compared with those of 2′-deoxy compounds. Furthermore, it was shown that 7-deaza-2′deoxyadenosine ( = 2′-deoxytubercidin; 12 ) is protonated at N(1), whereas the protonation site of 7-deaza-2′-deoxyguanosine ( 20 ) is N(3).     

Synthesis of oligonucleotides containing 7-(2-deoxy-β-d-erythro-pentofuranosyl)guanine and 8-amino-2′-deoxyguanosine

The synthesis of oligonucleotides containing 7-(2-deoxy-β-D-erythro-pentofuranosyl)guanine and 8-amino-2′-deoxyguanosine was accomplished. The viable intermediate N²-isobutyryl-7-(2-deoxy-β-D-erythro-pentofuranosyl)guanine ( 6 ) was prepared via a four step deoxygenation procedure from 7-β-D-ribofuranosylguanine ( 1 ). The 5′-hydroxyl group of 6 was protected as 4,4′-dimethoxytrityl ether and then converted to the target phosphoramidite ( 8 ) via conventional phosphitylation procedure. The amino groups of 8-amino-2′-deoxyguanosine ( 9 ) were protected in the form of N-(dimethylainino)methylene functions to give the protected nucleoside 10 , which was subsequently converted to the target phosphoramidite 12 via dimethoxytritylation followed by phosphitylation. The phosphoramidites 8 and 12 were incorporated into a 26-mer and a 31-mer G-rich oligonucleotide using solid-support, phosphoramidite methodology. Analysis of antiparallel triplex formation by the oligonucleotides containing 7-(2-deoxy-β-D-erythro-pentofura-nosyl)guanine in place of 2′-deoxyguanosine showed no enhancement in triple helix formation.     

Facile Synthesis of 2′-Deoxyisoguanosine and Related 2′,3′-Dideoxyribonucleosides

The 2′-deoxyisoguanosine ( 1 ) was synthesized by a two-step procedure from 2′-deoxyguanosine ( 5 ). Amination of silylated 2′-deoxyguanosine yielded 2-amino-2′-deoxyadenosine ( 6 ) which was subjected to selective deamination of the 2-NH₂ group resulting in compound 1 . Also 2′,3′-dideoxyisoguanosine ( 2 ) was prepared employing the photo-substitution of the 2-substituent of 2-chloro-2′,3′-dideoxyadenosine ( 4 ). The latter was synthesized by Barton deoxygenation from 2-chloro-2′-deoxyadenosine ( 3 ) or via glycosylation of 2,6-dichloropurine ( 12 ) with the lactol 13 . Compound 1 was less stable at the N-glycosylic bond than 2′-deoxyguanosine ( 5 ). The dideoxynucleoside 2 was deaminated by adenosine deaminase affording 2′,3′-dideoxyxanthosine ( 17 ).     

N6-(Carbamoylmethyl)-2′-deoxyadenosine,a rare DNA Constituent: Phosphoramidite Synthesis and Properties of Palindromic Dodecanucleotides

N⁶-(Carbamoylmethyl)-2′-deoxyadenosine ( 1 ), a modified nucleoside occurring in bacteriophage Mu, was synthesized by two different routes. Glycinamide was introdued by nucleophilic displacement of(2,4,6,-triisopro-pylphenyl)sulfonyloxy or ethylsulfinyl groups at C(6) of the purine moiety. Compound 1 was converted into the protected phosphoramidite 6b and employed in solid-phase synthesis of the self-complementary oligonucleotides 7–14 . Replacement of 2′-deoxyadenosine by 1 led to a strong decrease of the T_m values of the oligomers d(A-T)₆ ( 7 ) and d(A-T-G-A-A-G-C-T-T-C-A-T)( 10 ), respectively. As the oligemer 10 contains the recognition site d(A-A-G-C-T-T) of the endodeoxyribonuclease Hind III, it was subjected to sequence-specific hydrolysis experiments. Replacement of the first or second A_d by 1 prevented enzymatic phosphodiester hydrolysis (results with 11 and 12 ). In contrast, slow hydrolysis was observed if the less bulky N⁶-methyl-2′-deoxyadenosine replaced the second A _d residue (results with 14 ).     

A convenient synthesis of 2′-deoxy-6-thioguanosine,ara-guanine,ara-6-thioguanine and certain related purine nucleosides by the stereospecific sodium salt glycosylation procedure

A simple and high-yield synthesis of biologically significant 2′-deoxy-6-thioguanosine ( 11 ), ara-6-thioguanine ( 16 ) and araG ( 17 ) has been accomplished employing the Stereospecific sodium salt glycosylation method. Glycosylation of the sodium salt of 6-chloro- and 2-amino-6-chloropurine ( 1 and 2 , respectively) with 1-chloro-2-deoxy-3,5-di-O-(p-toluoyl)-α-D-erythro-pentofuranose ( 3 ) gave the corresponding N-9 substituted nucleosides as major products with the β-anomeric configuration ( 4 and 5 , respectively) along with a minor amount of the N-7 positional isomers ( 6 and 7 ). Treatment of 4 with hydrogen sulfide in methanol containing sodium methoxide gave 2′-deoxy-6-thioinosine ( 10 ) in 93% yield. Similarly, 5 was transformed into 2′-deoxy-6-thioguanosine (β-TGdR, 11 ) in 71 % yield. Reaction of the sodium salt of 2 with 1-chloro-2,3,5-tri-O-benzyl-α-D-arabinofuranose ( 8 ) gave N-7 and N-9 glycosylated products 13 and 9 , respectively. Debenzylation of 9 with boron trichloride at ?78° gave the versatile intermediate 2-amino-6-chloro-9-β-D-arabinofuranosyl-purine ( 14 ) in 62% yield. Direct treatment of 14 with sodium hydrosulfide furnished ara-6-thioguanine ( 16 ). Alkaline hydrolysis of 14 readily gave 9-β-D-arabinofuranosylguanine (araG, 17 ), which on subsequent phosphorylation with phosphorus oxychloride in trimethyl phosphate afforded araG 5′-monophosphate ( 18 ).     

8-Aza-2′-deoxyguanosine and Related 1,2,3-Triazolo[4,5-d]pyrimidine 2′-Deoxyribofuranosides

The synthesis of 8-azaguanine N⁹-, N⁸-, and N⁷-(2′-deoxyribonucleosides) 1–3 , related to 2′-deoxyguanosine ( 4 ), is described. Glycosylation of the anion of 5-amino-7-methoxy-3H-1,2,3-triazolo[4,5-d]pyrimidine ( 5 ) with 2-deoxy-3,5-di-O-(4-toluoyl)-α-D -erythro-pentofuranosyl chloride ( 6 ) afforded the regioisomeric glycosylation products 7a/7b, 8a/8b , and 9 (Scheme 1) which were detoluoylated to give 10a, 10b, 11a, 11b , and 12a . The anomeric configuration as well as the position of glycosylation were determined by combination of UV, ¹³C-NMR, and ¹H-NMR NOE-difference spectroscopy. The 2-amino-8-aza-2′-deoxyadenosine ( 13 ), obtained from 7a , was deaminated by adenosine deaminase to yield 8-aza-2′-deoxyguanosine ( 1 ), whereas the N⁷- and N⁸-regioisomers were no substrates of the enzyme. The N-glycosylic bond of compound 1 (0.1 N HCl) is ca. 10 times more stable than that of 2′-deoxyguanosine ( 4 ).     

Thermotropic liquid crystals based on ferrocenylbiphenyl and ferrocenylterphenyl

4′‐Ferrocenyl‐1,1′‐biphenyl‐4‐yl 4‐alkoxybenzoates Fc–(C₆H₄)₂–OC(O)–C₆H₄–O–C _n H_2n+1 (n = 8, 10, 12) (3a–c), representing a new class of ferrocene‐containing thermotropic mesogens with nematogenic properties, were prepared. Two approaches were used for the construction of these mesogens: (i) reaction of 4′‐ferrocenyl‐1,1′‐biphenyl‐4‐ol with 4‐alkoxybenzoylchlorides, and (ii) crosscoupling of tris(4‐ferrocenylphenyl)boroxine with the corresponding halobenzenes. Crosscoupling was also applied for the synthesis of terphenyl‐containing mesogens Fc–(C₆H₄)₃–OC(O)–C₆H₄–O–C _n H_2n+1 (n = 10, 12) (6a,b) and (RC₅H₄)Fe‐[C₅H₄–(C₆H₄)₃–OC(O)–C₆H₄–O–C₁₀H₂₁] (11a, R = Et; 11b, R = n?Bu). The latter compounds also form nematic phases. Mesogens 6a,b form mesophases with wider temperature ranges than their biphenyl‐containing counterparts 3b,c. The most pronounced mesomorphism was displayed by compounds 11a and 11b, which have mesophases in the ranges 141–253°C and 120–238°C, respectively. The purity of compounds was established by ¹H NMR spectra and elemental analysis. Mesophases were identified by polarizing optical microscopy and differential scanning calorimetry.     

Oligonucleotide Analogues with a Nucleobase‐Including Backbone,Part 5, 2′‐Deoxy‐8‐(hydroxymethyl)adenosine‐ and 2′‐Deoxy‐6‐(hydroxymethyl)uridine‐Derived Phosphoramidites: Synthesis and Incorporation into 14‐Mer DNA Strands

The pairing propensity of new DNA analogues with a phosphinato group between O−C(3′) and a newly introduced OCH₂ group at C(8) and C(6) of 2′‐deoxyadenosine and 2′‐deoxyuridine, respectively, was evaluated by force‐field calculations and Maruzen model studies. These studies suggest that these analogues may form autonomous pairing systems, and that the incorporation of single modified units into DNA 14mers is compatible with duplex formation. To evaluate the incorporation, we prepared the required phosphoramidites 3 and 4 from 2′‐deoxyadenosine and 2′‐deoxyuridine, respectively. The phosphoramidite 5 was similarly prepared to estimate the influence of a CH₂OH group at C(8) on the duplex stability. The modified 14‐mers 6 – 9 were prepared by solid‐phase synthesis. Pairing studies show a decrease of the melting temperature by 2.5° for the duplex 13 ⋅ 9 , and of 6 – 8° for the duplexes 10 ⋅ 6 , 11 ⋅ 6 , 13 ⋅ 7 , and 14 ⋅ 8 , as compared to the unmodified duplexes.     

Nucleosides and Oligonucleotides with Diynyl Side Chains: Base Pairing and Functionalization of 2′‐Deoxyuridine Derivatives by the Copper(I)‐Catalyzed Alkyne?Azide ‘Click’ Cycloaddition

Oligonucleotides containing the 5‐substituted 2′‐deoxyuridines 1b or 1d bearing side chains with terminal C?C bonds are described, and their duplex stability is compared with oligonucleotides containing the 5‐alkynyl compounds 1a or 1c with only one nonterminal C?C bond in the side chain. For this, 5‐iodo‐2′‐deoxyuridine ( 3 ) and diynes or alkynes were employed as starting materials in the Sonogashira cross‐coupling reaction (Scheme 1). Phosphoramidites 2b – d were prepared (Scheme 3) and used as building blocks in solid‐phase synthesis. T_m Measurements demonstrated that DNA duplexes containing the octa‐1,7‐diynyl side chain or a diprop‐2‐ynyl ether residue, i.e., containing 1b or 1d , are more stable than those containing only one triple bond, i.e., 1a or 1c (Table 3). The diyne‐modified nucleosides were employed in further functionalization reactions by using the protocol of the Cu^I‐catalyzed Huisgen–Meldal–Sharpless [2+3] cycloaddition (‘click chemistry’) (Scheme 2). An aliphatic azide, i. e., 3′‐azido‐3′‐deoxythymidine (AZT; 4 ), as well as the aromatic azido compound 5 were linked to the terminal alkyne group resulting in 1H‐1,2,3‐triazole‐modified derivatives 6 and 7 , respectively (Scheme 2), of which 6 forms a stable duplex DNA (Table 3). The Husigen–Meldal–Sharpless cycloaddition was also performed with oligonucleotides (Schemes 4 and 5).     

Purine nucleosides XXVIII. The synthesis of 7-(2′-deoxy-α- and β-d-ribofuranosyl)purines from 2′-deoxyribofuranosylimidazoles. Preparation of the 2′-deoxy derivative of the nucleoside from pseudovitamin B12 factor G

The synthesis of 1-(2′-deoxyribofuranosyl)imidazoles have been achieved for the first time via the fusion method of glycosidation. 4-Amino-5-carboxamido-1-(2′-deoxy-α-D-ribofuranosyl)-imidazole ( 8 ) and 4-amino-5-carboxamido-1-(2′-deoxy-β-D-ribofuranosyl)imidazole ( 10 ) have been obtained and their structures established by spectroscopic methods. The first examples of 7-(2′-deoxyglycosyl)purines [7-(2′-deoxy-α-D-ribofuranosyl)hypoxanthine ( 6 ) and 7-(2′-deoxy-β-D-ribofuranosyl)hypoxanthine ( 11 )] have been obtained from the requisite 2′-deoxyribofuranosylimidazoles. The preparation of 6 has furnished the 2′-deoxy derivative (α-configuration) of the nucleoside from pseudovitamin B₁₂ Factor G, which constitutes the first 2′-deoxy derivative of any nucleoside isolated from the various naturally occurring pseudovitamin B₁₂ factors.     

Synthesis of 2′-Deoxyribofuranosides of 8-Aza-7-deazaguanine and Related Pyrazolo[3,4-d]pyrimidines

The N(1)- and N(2)-(2′-deoxyribofuranosides) 1 and 2 , respectively, of 8-aza-7-deazaguanine were prepared via phase-transfer glycosylation in the presence or absence of Bu₄NHSO₄ as catalyst of 6-amino-4-methoxy-lH-pyrazolo[3,4-d]pyrimidine ( 7c ) with 2-deoxy-3,5-di-O-(p-toluoyl)-α-D -erythro-pentofuranosyl chloride ( 10 ). On a similar route, but without catalyst and employing THF as organic phase, the 6-amino-4-chloronucleosides 11b and 12b were synthesized from 7a and converted into the N(1)-and N(2)-substituted 4-thioxo analogues 17a and 18a , respectively. The ratio of N(1)- to N(2)-glycosylation was 2:1 for 7c and 1:1 for 7a , viz. depending on the nucleobase structure. The rate of the H⁺-catalyzed N-glycosyl hydrolysis was strongly decreased for the N(2)-(β-D -2′-deoxyribofuranosides) as compared to the N(1)-compounds. However, the N(1)-nucleoside 1 , which is an isostere of 2′-deoxyguanosine, is sufficiently stable to be employed later in solid-phase oligonucleotide synthesis.     

The Radical Cationic Repair Pathway of Cyclobutane Pyrimidine Dimer: The Effect of Sugar‐Phosphate Backbone

Radical cationic repair process of cis–syn thymine dimer has been investigated when (1) sugar‐phosphate backbones were substituted by hydrogen atoms, (2) phosphate group was substituted by two hydrogen atoms each on a sugar ring and (3) sugar‐phosphate backbone was taken into account. The effect of the interactions between N1 and N1′ lone pairs and the C6‐C6′ antibonding orbital are the most important evidences for the cleavage of the C6‐C6′ bond in the first step of radical cationic repair mechanism in the absence of the sugar‐phosphate backbone. The impact of the N1 and N1′ lone pairs on the C6‐C6′ bond cleavage decreases and the energy barrier of the cleavage of that bond significantly increases in the presence of the deoxynucleoside sugars and the sugar‐phosphate backbone.     

8-Aza-7-deazaadenine N8- and N8-(β-D-2′-Deoxyribofuranosides): Building blocks for automated DNA synthesis and properties of oligodeoxyribonucleotides

Oligonucleotides with alternating 8-aza-7-deaza-2′-deoxyadenosine (= c⁷z⁸A_d2) and dT residues (see 11, 14 and 16 ) or 4-aminopyrazolo [3,4-d] pyrimidine N²-(β-D -2′-deoxyribofuranoside) (= c⁷z⁸A′_d¹); ( 3 ) and dT residues (see 12 ) have been prepared by solid-phase synthesis using P(III) chemistry, Additionally, palindromic oligomers derived from d(C-T-G-G-A-T-C-C-A-G) but containing 2 or 3 instead of dA (see 18 – 22 ) have been synthesized. Benzoylation of 2 or 3 , followed by 4,4′-dimethoxytritylation and subsequent phosphitylation yielded the methyl or the cyanoethyl phosphoramidites 8a,b and 9 . They were employed in automated. DNA synthesis. Alternating oligomers containing 2 or 3 showed increase dT_m values compared to those with dA, in particular 12 with an unusual N²-glycosylic bond. The palindromic oligomers 18 - 22 containing 2 or 3 instead of dA outside of the enzymic recognition side reduced the hydrolysis rate, replacement within d(G-A-T-C) abolished phosphodiester hydrolysis.     

1-(2′-Deoxy-β-D-xylofuranosyl)thymine Building Blocks for Solid-Phase Synthesis and Properties of Oligo(2′-Deoxyxylonucleotides)

1-(2′-Deoxy-β-D -threo-pentofuranosyl)thymine (= 1-(2′-deoxy-β-D -xylofuranosyl)thymine; xT_d; 2 ) was converted into its phosphonate 3b as well as its 2-cyanoethyl phosphoramidite 3c . Both compounds were used for solid-phase synthesis of d[(xT)₁₂-T] ( 5 ), representing the first DNA fragment build up from 3′–5′-linked 2′-deoxy--β-D -xylonucleosides. Moreover, xT_d was introduced into the innermost part of the self-complementary dodecamer d(G-T-A-G-A-A-xT-xT-C-T-A-C)₂ (9). The CD spectrum of d[(xT)₁₂–T] ( 5 ) exhibits reversed Cotton effects compared to d(T₁₂) ( 6 ; see Fig. 1), implying a left-handed single strand. With d(A₁₂) ( 7 ) it could be hybridized to form a propably Left-handed double strand d(A₁₂) · d[(xT)₁₂–T] ( 7 · 5 ) which was confirmed by melting experiments in combination with temperature-dependent CD spectroscopy. While 5 was hydrolyzed by snake-venom phosphodiesterase, it was resistant towards calf-spleen phosphodiesterase. The modified, self-complementary duplex 9 was hydrolyzed completely by snake-venom phosphodiesterase, at a twelvefold slower rate compared to unmodified 8 ; calf-spleen phosphodiesterase hydrolyzed 9 only partially.     

Synthese von diastereo- und enantio-selektiv deuterierten β,ε-, β,β-, β,γ- und γ,γ-Carotinen

Synthesis of Diastereo- and Enantioselectively Deuterated β,ε-, β,β-, β,γ- and γ,γ-Carotenes We describe the synthesis of (1′R, 6′S)-[16′, 16′, 16′-²H₃]-β, εcarotene, (1R, 1′R)-[16, 16, 16, 16′, 16′, 16′-²H₆]-β, β-carotene, (1′R, 6′S)-[16′, 16′, 16′-²H₃]-γ, γ-carotene and (1R, 1′R, 6S, 6′S)-[16, 16, 16, 16′, 16′, 16′-²H₆]-γ, γ-carotene by a multistep degradation of (4R, 5S, 10S)-[18, 18, 18-²H₃]-didehydroabietane to optically active deuterated β-, ε- and γ-C₁₁-endgroups and subsequent building up according to schemes \documentclass{article}\pagestyle{empty}\begin{document}${\rm C}_{11} \to {\rm C}_{14}^{C_{\mathop {26}\limits_ \to }} \to {\rm C}_{40} $\end{document} and C₁₁ → C₁₄; C₁₄+C₁₂+C₁₄→C₄₀. NMR.- and chiroptical data allow the identification of the geminal methyl groups in all these compounds. The optical activity of all-(E)-[²H₆]-β,β-carotene, which is solely due to the isotopically different substituent not directly attached to the chiral centres, is demonstrated by a significant CD.-effect at low temperature. Therefore, if an enzymatic cyclization of [17, 17, 17, 17′, 17′, 17′-²H₆]lycopine can be achieved, the steric course of the cyclization step would be derivable from NMR.- and CD.-spectra with very small samples of the isolated cyclic carotenes. A general scheme for the possible course of the cyclization steps is presented.     

Enantioselective Synthesis of Distorted π-Extended Chiral Triptycenes Consisting of Three Distinct Aromatic Rings by Rhodium-Catalyzed [2+2+2] Cycloaddition

The enantioselective synthesis of distorted π-extended chiral triptycenes, consisting of three distinct aromatic rings, has been achieved with high ee value of 87 % by the cationic rhodium(I)/segphos complex-catalyzed enantioselective [2+2+2] cycloaddition of 2,2′-di(prop-1-yn-1-yl)-5,5′-bis(trifluoromethyl)-1,1′-biphenyl with 6-methoxy-1,2-dihydronaphthalene followed by the diastereoselective Diels–Alder reaction and aromatization. Demethoxy derivatives were also synthesized by the C−O bond cleavage. In this synthesis, the use of the electron-deficient diyne and the electron-rich alkene is crucial to suppress the undesired strain-relieving carbocation rearrangement and stabilize the distorted triptycene structure.     

3′-Amino-Modified Nucleotides Useful as Potent Chain Terminators for current DNA sequencing methods

We describe the synthesis of modified nucleoside triphosphates of the four DNA bases containing a 3′-amino group which were prepared from the corresponding 3′-azido derivatives. Introduction of the triphosphate and subsequent reduction of the N₃ to the NH₂ group led directly to the target molecules 6a–d . Furthermore, 3′-amino-2′,3′-dideoxynucleoside 5′-triphosphates proved to be potent inhibitors of the enzymatic synthesis of DNA catalyzed by the standard sequencing enzymes T7 DNA polymerase, sequenase version 2.0, Thermus aquaticus DNA polymerase, and Thermus thermophilus DNA polymerase. Both radioactive and fluorescent sequencing methods were applied successfully to the 3′-amino-modified terminators. Investigations in view of using these chain terminators according to Sanger's sequencing method for fluorescence labeling were done.