Synthesis of mesoionic xanthine nucleosides

The synthesis of the first examples of Class II mesoionic xanthine nucleosides is described. Tri-O-acetylribofuranosylaminothiazole is cyclized by condensation with bis (2,4,6-tri-chlorophenyl) malonate and the resultant product is de-protected to yield $\text{anhydro}$ (8-β-$\text{D}$-ribofuranosyl-5-hydroxy-7-oxothiazolo [3,2-a] pyrimidinium hydroxide); the glucosyl derivative is prepared in a similar manner.