Oligodeoxyribonucleotides containing 1-deaza-2′-deoxyadenosine ( = 7-amino-3-(2-deoxy-β-D-erythro-pentofuranosyl)-3H-imidazo[4, 5-b]pyridine; 1b) form Hoogsteen duplexes. Watson-Crick base pairs cannot be built up due to the absence of N(1). For these studies, oligonucleotide building blocks – the phosphonate 3a and the phosphoramidite 3b – were prepared from 1bvia4a and 5, as well as the Fractosil-linked 6b, and used in solid-phase synthesis. The applicability of various N-protecting groups (see 4ac) was also studied. The Hoogsteen duplex d[(c1A)20] · d(T20) (11 · 13; Tm 15°) is less stable than d(A20) · d(T20) (12 · 13; Tm 60°). The block oligomers d([c1A)10–;T10] (14) and d[T10–(c1A)10] (15) containing purine and pyrimidine bases in the same strand are also able to form duplexes with each other. The chain polarity was found to be parallel.