Seela et al.36 have synthesized fluorescent nucleosides (Figure 2.4) by click conjugation between 7-deazapurine and 8-aza-7-deazapurine nucleosides related to dA and dG bearing 7-octadiynyl or 7-tripropargylamine side chains with 1- azidomethyl pyrene and studied the fluorescence property. Octadiynyl derivative showed only monomer fluorescence, while the tripropargylamine derivatives caused excimer emission due to the proximal alignment of pyrene residues. 8-Aza-7- deazapurine pyrene “click” conjugates exhibited fluorescence emission much higher than that of 7-deazapurine derivatives because in the later case the fluorescence was quenched by intramolecular charge transfer between the nucleobase and the dye.
Figure 2.4: Fluorescent pyrene “click” conjugate nucleosides of different linker length.
A novel three step approach for the synthesis of N6-substituted-2-(1,2,3-triazol-1- yl)-adenine nucleosides have been described by Maris Turks et al.37a The second step of their synthesis involved the Cu(I) catalysed cycloaddition reaction of 2,6- diazidopurine nucleosides resulting in the formation of 2,6-bis-(1,2,3-triazol-1-yl) purine nucleosides which was undergone regioselective nucleophilic aromatic substitution with amines at C(6) position (Scheme 2.13). The resulting compounds exhibited interesting levels of fluorescence with quantum yields of up to 53%. Later on, they have synthesized 2,6-bis-(triazolyl)purine analogues by double azide–alkyne 1,3-dipolar cycloaddition (CuAAC) reactions of 2,6-diazidopurine derivatives which underwent selective nucleophilic aromatic substitution with various thiols at C(6) position to afford thiopurine nucleosides (Scheme 2.13).37b These 2,6-bis-(triazolyl) nucleosides were prepared for synthetic interest so that it can be used as substrate for further reactions and/or application in DNA.
N N N
N3 N
N3
R2 Cu(I)
N N N N N
N
N N N N
R2 R2
DMF K2CO3
N N N N N
N N
R2 SR3
O AcO
OAc OAc
O AcO
OAc OAc
O AcO
OAc OAc
HNR2R3
N N N N N
N N
R2 NR2R3
O AcO
OAc OAc
2.63 2.64 2.65
2.66
R3SH
Scheme 2.13: Cu(I) catalyzed cycloaddition reaction of 2,6-diazidopurine ribonucleosides.
Triazolyl-N-methylpyrene decorated 7-deazapurines have been synthesized by Seela et al. to test the photophysical properties and duplex stability.38 The pyrene was connected to C-7 of 7-deaza-2′-deoxyguanosine or to the 2′-deoxyribofuranose moiety through triazolyl linkers of different length (Figure 2.5). These modified bases were tested for their ability to stabilize the DNA duplex. Thus, it was observed that the nucleoside 2.68 with rigid triazolyl N-methyl pyrene moiety destabilized DNA while the nucleoside 2.67 containing the same fluorophore via a flexible acetylenic linker stabilized the DNA.
HN N O
H2N N HO O
OH
N N N
HN N O
H2N N HO O
OH N N N
2.67 2.68
Figure 2.5: Fluorescent nucleosides decorated with pyrene using click chemistrty.
2.2.5. Modification of the Nucleobases
Recently click chemistry is being adopted to replace the natural DNA bases and thus, to generate a set of new nucleoside analogues usable in biological and biophysical applications. As for an example, Nakahara et al.,39 reported the synthesis of triazolyl DNA using click chemistry. Thus, the oligonucleotides containing 1- ethynyl-2- deoxy-β-D-ribofuranose were reacted with a series of azides using the Cu(I)-catalyzed click reaction to produce ODNs containing artificial triazole nucleobases (Scheme 2.14). The thermal denaturation study of the post synthetically modified DNAs was done and the Tm values were compared with ethynyl modified ODN and the natural ones. It was observed that the oligonucleotide bearing a (phenylthio)methyl group was more favourable for duplex formation as compared to its benzyl counterpart because of the hydrophobicity induced by an additional sulfur atom to the nucleobase. The oligonucleotide 2.71 formed equally stable duplexes with all the ssDNA containing the natural bases which makes it a non-discriminatory nucleobase, namely a universal base.
Scheme 2.14: Synthetic scheme of triazolyl DNA by postsynthetic modification of DNA via click reaction.
Benhida et al. reported a highly efficient microwave-assisted solvent-free synthesis of α- and ß-2′deoxy-1,2,3-triazolyl nucleosides.40 This methodology comprising of microwave activation coupled with Cu(I) catalysis dramatically increased the rate of the 1,3-dipolar cycloaddition reaction between the azido-2′- deoxyribose and terminal alkynes (Scheme 2.15). In the same manner, Ermolat’ev et al.41(Scheme 2.16) and Broggi et al.42(Scheme 2.17) reported that the 1,3- cycloaddition can be accomplished in high yield within few minutes using microwave irradiation. These nucleosides have been generated out of synthetic interest mainly with the aim of conducting the reactions under neat conditions within a short time.
Scheme 2.15: Synthesis of 1,2,3-triazolo nucleoside analogues by microwave irradiation.
Scheme 2.16: Synthesis of 1,2,3-triazolo nucleoside analogues using microwave irradiation.
Scheme 2.17:Synthesis of 1,2,3-triazolo carbanucleoside analogues using microwave irradiation.
Driowya et al.43 have developed a clean and efficient one-pot procedure involving a cooperative effect of iron-copper catalysis and ultrasound activation to generate triazolyl nucleosides. This one-pot procedure is simple to operate, gives high yield, is a safe and environment friendly protocol. The anticancer activity of the resulting substituted nucleosides were evaluated against K562 chronic myelogenous leukemia (CML) cell line and some of the synthesized compounds were found to be more active than 5-amino-1-β-D-ribofuranosyl-imidazole-4-carboxamide(AICAR) to kill CML cancer in K562 cell line. Pradere et al. reported the preparation of ribavirin analogues by copper- and ruthenium-catalyzed azide-alkyne 1,3-dipolar cycloaddition as a part
of the drug discovery program.44 The synthesis of 1,4- and 1,5-disubstituted-1,2,3- triazolo-nucleosides from various alkynes with 1′-azido-2′,3′,5′-tri-O-acetylribose using either copper-catalyzed azide-alkyne cycloaddition (CuAAC) or ruthenium- catalyzed azide-alkyne cycloaddition (RuAAC), respectively was described (Scheme 2.18). The commercially available [Cp*RuCl(PPh3)2] was used for the ruthenium- catalysed reaction under microwave assisted conditions which led to significant enhancement of the reaction rate.
Scheme 2.18: Synthesis of 1,2,3-Triazolo Nucleoside Analogues by (CuAAC) and