An Efficient Route to Pyrimidine Nucleoside Analogues by [4 + 2] Cycloaddition Reaction

We report here an efficient synthesis for pyrimidine nucleoside analogues by [4 + 2] cycloaddition reaction. These compounds were obtained by convergent chemistry from glycosyl isothiocyanates 3a−f (pyranoses, furanoses, and dissaccharides) and diazadienium salt 5. In fact, diazapentadienium iodide 5 prepared from vinylthioamide 4 is an efficient intermediate in heterocyclic synthesis and reacts with isothiocyanates 3a−f affording β-d-uracil analogues 7a−f in good yields and with total regiocontrol. All compounds were fully characterized by IR, HRMS, and 13C and 1H NMR (COSY and HMQC).