Synthesis of Novel Isothiazole and Isothiazolo[4,5-d] Pyrimidine Analogues of the Natural C-Nucleosides Pyrazofurin and the Formycins

Abstract The cycloaddition of tri-O-benzyl-2,5-anhydro-D-allononitrile-N-sulfide with dimethyl acetylenedicarboxylate or with dimethyl fumarate followed by DDQ oxidation was found to give the benzoyl protected dimethyl 3-β-D-ribofuranosyl-isothiazoledicarboxylate 10. This compound was converted to C-nucleosides 7a, 8 and 9, analogues of pyrazofurin, oxoformycin B and formycin respectively. Despite their structural similarities they did show neither antiviral nor antitumor activity.