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The Huisgen-Meldal-Sharpless ‘click’ reaction has emerged as a convenient and effective approach for the preparation of a diversity of new molecules with various functionalities performed under simple reaction conditions. The reaction is orthogonal to most of the common biomolecule substituents and chemoselective under Cu(I) catalysis.1 Thus, the protocol is particularly attractive to be applied in drug design,2 molecular diagnostics3,4 polymer synthesis, material science,5–10 and other areas of chemistry and chemical biology.11–13 Recently, we reported on the synthesis of oligonucleotides incorporating 7-deaza-7-(octa-1,7-diynyl)-2'-deoxyguanosine (1) (purine numbering is used in the results and discussion section). The side chains bearing terminal triple bonds were conjugated with various azides including the dye 3-azido-7-hydroxycoumarin by the Huisgen-Meldal-Sharpless ‘click’ reaction.11c,14 Spectro- scopic studies showed that the fluorescence is strongly quenched when the DNA nucleobase belongs to the class of pyrrolo[2,3- d]pyrimdines. An electron transfer from the nucleobase to the dye moiety was suggested.14 Thus, it was anticipated that other nucleobases having different redox potentials may not show such drawback. Pyrazolo[3,4-d]pyrimidine nucleosides offer properties which make them applicable as alternative systems.15 They show similar base pairing characteristics and can be functionalized at the 7-position. 8-Aza-7-deazapurine (pyrazolo[3,4-d]pyrimidine) nucleosides have already been incorporated into DNA including 7-propynyl and 7-halogeno derivatives.16–18 It was shown that the 8-aza-7-deaza-2'-deoxyguanosine residues 2a–c form stable base pairs with dC giving steric freedom to 7-substituents to be well accommodated in the major groove of duplex DNA.18 Usually, 7-substituted nucleosides form more stable duplexes than the non-functionalized ones.18–22 Thus, pyrazolo[3,4-d]pyrimidine nucleosides can be considered as purine nucleoside shape mimics which will not disturb the DNA duplex structure. This manuscript reports on the synthesis of 8-aza-7-deaza- 7-(octa-1,7-diynyl)-2 '-deoxyguanosine (2d), its conversion into the phosphoramidite building blocks (3a,b) and their ap- plication in solid-phase oligonucleotide synthesis (Fig. 1). Studies regarding base pairing and fluorescence properties were carried out on oligonucleotides incorporating nucleo- side 2d. The azide-alkyne ‘click’ reaction for the dye con- jugation with 3-azido-7-hydroxycoumarin on nucleosides and oligonucleotides with side chains having terminal triple bonds was studied according to Scheme 1. Fluorescence enhance- ment of the oligonucleotide coumarin dye conjugates is ex- pected for oligonucleotides containing pyrazolo[3,4-d]pyrimidine dye conjugates compared to pyrrolo[2,3-d]pyrimidine deriva- tives. |
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