7‐Deazaguanosine: Synthesis of an oligorbonucleotide building block and disaggregation of the U‐G‐G‐G‐G‐U G4 structure by the modified base

Building blocks derived from 7‐deazaguanosine (c7G, 1) were prepared for solid‐phase oligoribonucleotide synthesis. Compound 1 was converted into the isobutyurl derivative 2b and the (dimethylamino)methylidene compound 3 (Scheme 1). After tritylation (→ 4a, b), silylation was studied with regard to regioselectivity. It was found that the triisopropylsilyl group in combination with the (dimethylamino)methylidene residue gave the highest 2′ ‐selectivity (→ 5e). The 2′ ‐O ‐silyl derivative 5e was reacted with PCl3 affording the 3′ ‐phosphonate 7 which was used in solid‐phase oligoribonucleotide synthesis. Oligonucleotides derived from U‐G‐G‐G‐G‐U with an increasing number of c7G residues instead of G were synthesized. Aggregation was studied by polyacrylamidegel electrophoresis and CD Spectroscopy. Disaggregation of the G4‐structure of U‐G‐G‐G‐G‐U was observed when c7G replaced G, demonstrating that guanine N(7) participates in the aggregation process.