Substituent Reactivity and Tautomerism of Isoguanosine and Related Nucleosides

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Abstract
The substituent reactivity and tautomerism of isoguanine nucleosides is studied. Benzoylation or tosylation of isoguanine nucleosides (pyridine, room temperature) yields the 2‐benzoyl derivatives 7c, 11, and 12 or the 2‐tosyl compounds 13 and 14. The isobutyrylation of the 6‐amino group which did not occur under these conditions was induced in the presence of Me3SiCl. In the absence of Me3SiCl, the reactivity of isoguanine substituents decreases in the order from 2‐oxo → 5′‐OH → 3′‐OH → 6‐NH2. From isoguanine nucleosides, the N1‐(2b), N3‐(17), N6‐(15a,b), and 2‐O‐alkylated (3b) derivatives were prepared. Their pKa values were determined and the UV and 13C‐NMR spectra compared with regard to the alkylation position. Also the tautomeric forms of isoguanine nucleosides were determined UV‐spectrophotometrically in aqueous and nonaqueous solution. Isoguanosine (1a), its 2′‐deoxy analogue 1b as well as the N6‐methyl‐ and 8‐substituted derivatives form lactam tautomers in aqueous solution, whereas the lactim form is present in dioxane.