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 T m 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 .