A 1H NMR study of the syn-anti dynamic equilibrium in adenine nucleosides and nucleotides with the aid of some synthetic model analogues with fixed conformations.

The syn-anti equilibrium about the glycosidic bond in adenosine and some related analogues was studied by means of 1H NMR spectroscopy, with the aid of several model analogues fixed in given conformations either by intramolecular bonding, or by introduction of a bulky substituent. A model unambiguously and exclusively in the syn conformation is 8- (alpha-hydroxyisopropyl) adenosine; while one fixed in the anti conformation is 8,5'-anhydro-8-oxoadenosine. A new analogue, fixed in the high anti conformation, is 8,2-'O-isopropylidenearabinofuranosyladenine. Several additional new model compounds were synthesized and their properties are described. With the aid of these models, the syn-anti dynamic equilibrium was examined for adenosine and some related compounds in different solvent systems, and the conformer populations evaluated quantitatively. The validity of the procedure applied, and the accuracy of the results are critically examined, and compared with findings obtained by other procedures. Available literature data on the syn-anti equilibrium in other 8-substituted adenosines are re-analyzed in the light of the present results. An analysis is also presented of the interdependence of the various conformational parameter, i. e. conformation about the glycosidic bond and those of the sugar ring and exocyclic carbinol group, in adenosine and 2',3'-O-isopropylidenadenosine.