7-Deaza-2'-deoxyadenosine (1, c7Ad) and 3-deaza-2'-deoxyadenosine (2, c3Ad) have been incorporated into d(AAAAAA) tracts replacing dA at various positions within oligonucleotides. For this purpose suitably protected phosphonates have been prepared and oligonucleotides were synthesized on solid-phase. The oligomers were hybridized with their cognate strands. The duplexes were phosphorylated at OH-5' by polynucleotide kinase and self-ligated to multimers employing T4 DNA ligase. Oligomerized DNA-fragments were analyzed by polyacrylamide gel electrophoresis and the bending was determined from anomalies of electrophoretic mobility. Replacement of dA by c3Ad decreased the bending more than replacement by c7Ad. Reduction of bending was much stronger when the modified nucleosides replaced one or several dA residues at the 3'-site of an d(AAAAAA)-tract whereas replacement at the 5'-site showed no significant influence [1, 2].

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The synthesis of 4-(rnethylthio)-lH-imidazo[4,5-c]pyridine 2'-deoxy-/3-D-ribonucleosides 2 and 9 and the conversion of the N'-isomer 2 into the 2',3'-didehydro-2',3'-dideoxyribonucleoside 3a or (oiu 7) 3-deaza-2'-deoxyadenosine (1) is described. Phosphonate building blocks of 1 were employed in solid-phase synthesis of self-complementary base-modified oligonucleotides. Their properties were studied with regard to duplex stability and hydrolysis by the restriction enzyme EcoRI.Introduction. -The 3-deaza-2'-deoxyadenosine [I] (1) is a structural analogue of the naturally occurring DNA constituent 2'-deoxyadenosine (dA) in which N(3) is replaced by CH. As N(3) is located within the minor groove of double-stranded DNA, it acts as acceptor for DNA ligands. . Apart from 1, the 2',3'-dideoxyribonucleoside and the 2',3'-didehydro-2',3'-dideoxynucleoside 3b of 3-deazaadenine were prepared as potential antiviral agents [8].

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