International audienc

The self‐complementary guanosine‐ and cytidine‐derived aminomethylene‐linked C*[n]G dinucleoside 9 was synthesized by reductive amination of aldehyde 3 with an iminophosphorane derived from azide 7. Deacylation of 9 gave the isopropylidene‐protected dinucleoside 10. The sequence‐isomeric G*[n]C dinucleoside 11 was similarly prepared from aldehyde 8 and azide 5, and deacylated to 12. The association of 10 and 12 in CHCl3 or in CHCl3/DMSO mixtures, and the structure of the associates were studied by 1H‐NMR, ESI‐MS, CD, and vapor pressure osmometry (VPO). Broad 1H‐NMR signals of dinucleosides 10 and 12 evidence an equilibrium between duplexes and quadruplexes (Hoogsteen base pairing between the WatsonCrick base‐paired duplexes). The quadruplex dominates for the G*[n]C dinucleoside 12 between −50° and room temperature. The sequence‐isomeric C*[n]G 10 forms mostly only a cyclic duplex in CDCl3 and in CDCl3/(D6)DMSO 9 : 1.