N7‐DNA: Synthesis and Base Pairing of Oligonucleotides containing 7‐(2‐Deoxy‐β ‐ D ‐erythro‐pentofuranosyl)adenine (N7Ad)

The synthesis of oligonucleotides containing 7‐(2‐deoxy‐β ‐D‐erythro‐pentofuranosyl)adenine (N7Ad; 1) is described. Compound 1 was obtained from the precursor 4‐amino‐1H ‐imidazole‐5‐carbonitrile 2‐deoxyribonucleoside 6 and was found to be much more labile than Ad. The N6‐benzoyl protecting group (see 8) destabilized the N‐glycosylic bond further and was difficult to remove by NH3‐catalyzed hydrolysis. Therefore, a (dimethyl‐amino)methylidene residue was introduced (→9). Amidine 9 was blocked at OHC(5′) with the dimethoxytrityl residue ((MeO)2Tr), and phosphonate 4 as well as phosphoramidite 5 were prepared under standard conditions. Phosphonate 4 was employed in solid‐phase oligonucleotide synthesis. Homooligonucleotides as well as self‐complementary oligonucleotides were prepared. The oligomer d[(N7A)11‐A] (11) formed a duplex with d(T12) (13). Antiparallel chain polarity and reverse Watson‐Crick base pairing was deduced from duplex formation of the self‐complementary d[(N7A)8‐T8] (14).