8‐Aza‐7‐deaza‐2′‐deoxyguanosine: Phosphoramidite synthesis and properties of octanucleotides

Base‐modified octanucleotides derived from d(G1–G2–A–A–T–T–C–C–) (11) but containing 8‐aza‐7‐deaza‐2′‐deoxyguanosine (2) instead of 2′‐deoxyguanosine (1) have been prepared by solid‐phase synthesis employing P(III) chemistry. Isobutyrylation of 2, followed by 4, 4′‐dimethoxytritylation and subsequent phosphitylation yielded the methyl or the cyanoethyl phosphoramidites 6a or 6b, respectively. They were used as building blocks in automated DNA synthesis. The resulting octanucleotides 12–14 containing 2 showed increased Tm values compared to the parent oligomer 11. The oligomers 11–14 were employed as sequence‐specific probes in endo‐deoxyribonuclease Eco RI oligonucleotide recognition. Whereas displacement of dG‐2 (enzymic cleavage site of 11) abolished phosphodiester hydrolysis, replacement of dG‐1 enhanced the cleavage rate compared to 11.