Oligonucleotides Containing Consecutive 2′-Deoxyisoguanosine Residues: Synthesis, duplexes with parallel chain orientation, and aggregation

The 2′‐deoxyisoguanosine phosphonates 3a and 4a and the phosphoramidites 3b and 4b were prepared as building blocks for solid‐phase oligonucleotide synthesis. The diphenylcarbamoyl (dpc) residue was introduced as 2‐oxo protecting group which stabilizes the N‐glycosylic bond against hydrolysis and prevents the molecule from side reactions. The dpc‐protected building blocks 4a, b were employed in solid‐phase synthesis and were found to be much more efficient than the unprotected compounds 3a, 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 to that of d(T4‐G4‐T4).