Additive Pummerer Reaction of 3,5-O-(Di-tert-butyl)silylene-4-thiofuranoid Glycal: A High-Yield and β-Selective Entry to 4′-Thioribonucleosides

Upon reacting 3,5-O-(di-tert-butyl)silylene-4-thiofuranoid glycal S-oxide (6) with Ac2O/TMSOAc/BF3·OEt2 in CH2Cl2, the additive Pummerer reaction proceeded to furnish the corresponding 1,2-di-O-acetyl-4-thioribofuranose 7. Compound 7 serves as a highly β-selective glycosyl donor in the Vorbrüggen condensation carried out in the presence of TMSOTf. Thus, the 4-thio-β-d-ribofuranosyl derivatives of uracil, thymine, N-acetylcytosine, 6-chloropurine, and 2-amino-6-chloropurine were synthesized. The use of 7 can be extended to the β-selective synthesis of 4′-thio-C-ribonucleosides.