X-ray crystallographic and NMR study of the tautomerism in kinetin, kinetin riboside and their derivatives: A comparison between the solid state and solution

N6-furfuryladenine (kinetin, 1), 2-chloro-N6-furfuryladenine (2), N6-furfuryladenosine (kinetin riboside, 3) and 2-chloro-N6-(5-methylfurfuryl)adenosine (4) have been prepared and their structural properties have been studied using a single crystal X-ray analysis (2, 4) and multinuclear 1D and 2D NMR spectroscopy (1–4). The molecular structure of 2 revealed the presence of the 3H-amino and 7H-amino tautomeric forms with the occupancy of 82(3)%, and 18(3)%, respectively, for the corresponding hydrogen. The NMR study revealed the presence of tautomeric equilibria in 1, 2 and 4 in dimethyl sulfoxide or N,N′-dimethylformamide solutions at 300 K and 340 K. It has been found that the 9H-amino/7H-amino, 1H,7H-imino/1H,9H-imino and 9H-amino/7H-amino, and amino/1H-imino equilibria exist in 1, 2, and 4, respectively. The presence of a tautomeric equilibrium has not been observed in the case of 3.