Reaction of 2',3'-O-isopropylidene inosine with benzyl bromide (1 h, rt) led to the 1,5'-O-dibenzylderivative 4, but by increasing the reaction time or the temperature, compound 4 is further transformed into the 1,7,5'-O-tribenzylinosine derivative 5. Similarly, the 7-methyl-1,5'-O-dibenzylderivative 6 has been synthesized from 4. The H-1-NMR spectra of 5 and 6 showed peculiar chemical shifts for geminal protons (H5' and H5 '' of the ribose, and the CH2 of the benzyl groups). Preliminary NAIR studies have been performed, including NOESY experiments that point toward the predominant existence of conformers that are stabilized by an electrostatic interaction between the positively charged imidazole of the base moiety and the high electron density of the 5'-benzyl substituent.
5‘-<i>O</i>-Tritylinosine and Analogues as Allosteric Inhibitors of Human Thymidine Phosphorylase
On the basis of our previous findings that 5'-O-tritylinosine (KIN59) behaves as an allosteric inhibitor of the angiogenic enzyme thymidine phosphorylase (TPase), we have undertaken the synthesis and enzymatic evaluation of a novel series of nucleoside analogues modified at positions 1, 2, or 6 of the purine ring and at the 5'-position of the ribose moiety of the lead compound KIN59. SAR studies indicate
Despite the well-established importance of intermolecular cation-pi interactions in molecular recognition, intramolecular cation-pi interactions have been less studied. Here we describe how the simultaneous presence of an aromatic ring at the 5'-position of an inosine derivative and a positively charged imidazolium ring in the purine base drive the conformation of the nucleoside toward a very major conformer in solution that is stabilized by an intramolecular cation-pi interaction. Therefore, the cation-pi interaction between imidazolium ions and aromatic rings can also be proposed in the design of small molecules where this type of interaction is desirable. The imidazolium ion can be obtained by a simple acidification of the pH of the media. So a simple change in pH can shift the conformational equilibrium from a random to a restricted conformation stabilized by an intramolecular cation-pi interaction. Thus the here described nucleosides can be considered as a new class of pH-dependent conformationally switchable molecules.