Conformationally constrained tricyclic nucleosides. Role of electronic and steric factors.

Wyosine [4,9-dihydro-4,6-dimethyl-9-oxo-3-(beta-D-ribofuranosyl) imidazo [1,2-a] purine], a component of tRNA, is known to be constrained in a solution in anti conformation around the glycosylic bond. Guanosine, when substituted with a bulky electronegative group at C8, position analogous to C2 of wyosine, assumes syn conformation. The conformation arrangements of wyosine 2',3',5'-triacetate 1 and its analogs C2 substituted with electronegative groups methoxy, benzyloxy and oxo were studied by diverse 1H NMR techniques. For C2 substituted congeners, significant increase in the North conformer population of sugar moiety was found. It was accompanied by an increase of the tg rotamer contribution at the expense of the gg one. Only in 3 the preferred segment of the anti region was changed and oriented towards high-syn.