The synthesis of 8-azaguanine N9-, N8-, and N7-(2'-deoxyribonucleosides) 1-3, related to 2'-deoxyguanosine (4), is described. Glycosylation of the anion of 5-amino-7-methoxy-3H-l,2,3-triazolo[4,5-d]pyrimidine (5) with 2-deoxy-3,5-di-O-(4-toluoyl)-a -D-erythro -pentofuranosyl chloride (6) afforded the regioisorneric glycosylation products 7a/7b, 8a/8b, and 9 (Scheme 1 ) which were detoluoylated to give 10a, lob, l l a , l l b , and 12a. The anomeric configuration as well as the position of glycosylation were determined by combination of UV, I3C-NMR, and 'H-NMR NOE-difference spectroscopy. The 2-amino-8-aza-2'-deoxyadenosine (13), obtained from 7a, was deaminated by adenosine deaminase to yield 8-aza-2'-deoxyguanosine (l), whereas the N7-and N8-regioisomers were no substrates of the enzyme. The N-glycosylic bond of compound 1 ( 0 . 1~ HCI) is ca. 10 times more stable than that of 2-deoxyguanosine (4).

The synthesis of oligonucleotides containing 8-aza-Z'-deoxyguanosine (zxGd; 1) or its Nx-regioisomer zsG$ (2) instead of 2-deoxyguanosine (G,) is described. For this purpose, the NH, group of1 and 2 was protected with a (dimethy1amino)methylidene residue (+ 5, 6), a 4,4'-dimethoxytrityl group was introduced at 5'-OH (4 7, 8), and the phosphonates 3a and 4 as well as the phosphoramidite 3b were prepared. These building blocks were used in solid-phase oligonucleotide synthesis. The oligonucleotides were characterized by enzymatic hydrolysis and melting curves (T, values). The thermodynamic data of the oligomers 12-15 indicate that duplexes were stabilized when 1 was replacing Gd. The aggregation ofd(T-G-G-G-G-T) (18) was studied by RP18 HPLC, gel electrophoresis and CD spectroscopy and compared with that of oligonucleotides containing an increasing number of zxGd residues instead of Gd. Similarly to [d(C-G)I3 (12a), the hexdmer d(C-zxG-C-zxG-C-G) (14) underwent saltdependent B-2 transition.

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Oligonucleotides continuing 3‐deaza‐2′‐deoxyguanosine (I) or its N7‐regioisomer 2 were prepared by solid‐phase synthesis using P111 chemistry. Protection of 1 or 2 with N,N V‐dimethylformamide diethyl acetal followed by 4,4′‐dimethoxytritylation afforded imidazo[4,5‐c]pyridines 10b and 11b, respectively. The latter were converted into the 3′‐phosphonates 10c or lie, respectively; the cyanoethyl N,N‐diisopropylphosphoramidite 10d was also prepared. The oligonucleotide building blocks were employed in automated solid‐phase synthesis. 1 he self‐complementary oligomers 13, 15, and 17 were prepared and characterized by enzymatic hydrolysis with snake‐venom phosphodiesterase followed by alkaline phosphatase. There CD spectra exhibited the general structure of a B‐DNA.

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