Further evidence for a S-syn correlation in the purine(beta)ribosides: the solution conformation of two tricyclic analogs of adenosine and guanosine.

From the analysis of the HRNMR spectra of two tricyclic analogues of adenosine and guanosine, 4,5-diamino-9-(beta-D-ribofuranosyl) pyrimido [5,4-f]pyrrolo[2,3-d]pyrimidine (adenosine-adenosine, AA) and 4,7-diamino-9-(beta-D-ribofuranosyl)pyrimido[5,4-f]pyrrolo-[2,3-d]pyrimidin-5-one (adenosine-guanosine, AG), dissolved in liquid ND3 the preferred conformations of the ribose moiety are derived in the temperature range between + 40 and -60 degrees C. The analysis is based on the two state N in equilibrium or formed from 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] approximately 0.8). The van't Hoff enthalpy for the S in equilibrium or formed from N equilibrium is -3 kJ mol-1. The syn in equilibrium or formed from anti equilibrium is even at -60 degrees C fast compared to the HRNMR time scale.