Various 2-substituted 2'-deoxyadenosines and 7-deazaadenosines have been synthesized. The phosphonate building block 9 of 2-chloro-7-deaza-2'-deoxyadenosine (7-deazacladribine; 2) was prepared by 4,4'-dimethoxytritylation of the parent nucleoside (-->7), followed by protection of the amino function with a formamidine residue (-->8). The latter was reacted with PCl3/N-methylmorpholine/1,2,4-triazole to give compound 9. Moreover, 2-methoxy-2'-deoxyadenosine (2'-deoxyspongosine; 1b) was converted into the fully protected derivative 12, which was then transformed into the 2-cyanoethyl phosphoramidite 14. Also the 2-(trifluoromethyl)-substituted 2'-deoxyadenosines 19-21 were prepared by glycosylation of the chromophore 16 with the halogenose 17, followed by one-pot deprotection and nucleophilic displacement of the 6-Cl substituent. The new DNA building blocks 9 and 14 were used--together with formerly prepared cladribine derivative 4--for solid-phase synthesis of a series of oligodeoxyribonucleotides. These were studied with respect to their thermal stability as well as of the base pairing mode (Watson-Crick vs. Hoogsteen) of modified bases.