Substituent reactivity and tautomerism of isoguanosine and related nucleosides

Article abstract of DOI:10.1002/hlca.19950780718

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 pK(a) 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.