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From the analysis of the HRNMR spectra of two tricyclic analogues of adenosine and guanosi­ne, 4,5-diamino-9-(β-D-ribofuranosyl) pyrimido[5,4-f]pyrrolo[2,3-d]pyrimidine (adenosine-adenosine, AA) and 4,7-diamino-9-(β-ᴅ-ribofuranosyl)pyrimido[5,4-f]pyrrolo-[2,3-d]pyrimidin-5-one (adenosine-guanosine, AG), dissolved in liquid ND 3  the preferred conformations of the ribose moiety are derived in the temperature range between + 40 and -60 °C. The analysis is based on the two state N ↔ S model of the furanoside ring proposed by Altona and Sundaralingam. Both compounds show a pronounced stabilization of the S-conformer of the sugar ring ([S] ~ 0.8). The van't Hoff enthalpy for the S ↔ N equilibrium is - 3 kJ mol -1 . The syn ↔ anti equilibrium is even at -60 °C fast compared to the HRNMR time scale.

Further Evidence for a S-Syn Correlation in the Purine(β)ribosides: The Solution Conformation of Two Tricyclic Analogs of Adenosine and Guanosine