AbstractThe 2′‐deoxyisoguanosine phosphonates3aand4aand the phosphoramidites3band4bwere prepared as building blocks for solid‐phase oligonucleotide synthesis. The diphenylcarbamoyl (dpc) residue was introduced as 2‐oxo protecting group which stabilizes theN‐glycosylic bond against hydrolysis and prevents the molecule from side reactions. The dpc‐protected building blocks4a, bwere employed in solid‐phase synthesis and were found to be much more efficient than the unprotected compounds3a, b.Oligonucleotides with alternating (11) or consecutive isoguanine residues (13–15) were synthesized. They form duplexes with parallel chain orientation. The aggregate d(T4‐iG4‐T4) (15) containing four consecutive 2′‐deoxyisoguanosine is shown to be a tetramer similar