DAST-Mediated Fluorination of 1-[4-Thio-beta-D-arabinofuranosyl] uracil: Investigation of Thiolane vs Thietane Formation and Stereoselective Synthesis of 4'-ThioFAC

The unprecedented DAST-mediated (DAST = diethylamino-sulfur trifluoride) deoxygenative fluorination of benzoyl-, TBDPS-, and Bn-protected 1-(beta-D-4-thioarabinofuranosyl)uracil at the sugar portion was examined. Three kinds of nucleoside (Ns) products were formed: target thiolane Ns, ring-contracted thietane Ns, and anhydro Ns products. The reaction pathway was determined by the electronic effect of the protecting groups at the sugar and base moieties. The benzoylated uracil starting material gave the 2,2'-anhydronucleoside (anhydro Ns) as a major product, whereas the silylated and benzylated starting materials furnished the corresponding fluorinated products, in which the ring-contracted thietanes predominantly formed. The desired thiolane Ns could be obtained as major product by the addition of a pyridine derivative as an additive. Upon reacting N-3-benzoylated 1-(beta-D-4-thioarabinofuranosyl)uracil with DAST in the presence of 2,4,6-collidine, the target 2'-deoxy-2'-beta-fluoro-4'-thiouracil nucleoside could be obtained in 72% isolated yield along with the corresponding thietane Ns (7%) and anhydro Ns (3%) (thiolane Ns/thietane Ns/anhydro Ns = 10.3:1.00:0.43), with recovery of the starting material (12%). In this study, the first stereoselective synthesis of the beta-anomer of 1-(2-deoxy2-fluoro-4-thio-beta-D-arabino-pentofuranosyl)cytosine (4'-thioFAC) has been developed.