Synthesis of 4‑Aminophthalimide and 2,4-Diaminopyrimidine C‑Nucleosides as Isosteric Fluorescent DNA Base Substitutes

The 4-aminophthalimide C-nucleoside 1 was designed as an isosteric DNA base surrogate, and a synthetic route to nucleoside 1 together with the 2,4-diaminopyrimidine-C-nucleoside 2 as a potential counterbase was worked out. The key steps in both synthetic routes represent a stereoselective Heck-type palladium-catalyzed cross-coupling with 2′-deoxyribofuranoside glycal followed by stereoselective reduction with NaBH(OAc)3. The nucleoside 1 shows a solvatofluorochromic behavior and significantly red-shifted fluorescence in solvents of high polarity and with hydrogen bonding capabilities. Both nucleosides 1 and 2 can be further processed to the corresponding phosphoramidite as DNA building blocks that allow incorporation of these chromophores as artificial DNA bases by automated DNA synthesis. The combination of the poor stacking properties of 1 and the hydrogen bonding interface at the phthalimide functionality that does not fit to any of natural DNA bases in the counterstrand yields destabilization of the duplex by 4–11 °C. The fluorescence of 1 in a representative double stranded DNA is characterized by a large Stokes’ shift and a quantum yield of approximately 12%. These are remarkable optical properties considering the very small size of the chromophore and indicate a high potential of these nucleoside analogues for fluorescent DNA analytics and imaging.