Stereoselective Synthesis of 2'-Deoxy-β-D-threo-pentofuranosyl Nucleosides by the NBS-Promoted Coupling Reaction of Thioglycosides with Silylated Heterocyclic Bases

Article abstract of DOI:10.1021/jo00104a020

The NBS-promoted coupling reaction of phenyl 3,5-O-isopropylidene-2-deoxy-1-thio-α-D-threo-pentofuranoside (5e) with silylated pyrimidine bases was found to proceed in a highly stereoselective manner (α:β = 1:24 - 0:1) to afford 2'-deoxy-β-D-threo-pentofuranosyl pyrimidine nucleosides in satisfactory yields.The highly stereoselective outcome is thought to result from an in situ anomerization-type mechanism, in which intimate ionic intermediates would be in equilibrium and anomerize to the sterically preferable α form.A subsequent S_N2 type attack to the intermediate will lead to the β-nucleosides.By using this method, the synthesis of L-nucleosides, 1-(2-deoxy-β-L-threo-pentofuranosyl)thymine and cytosine derivatives, was also demonstrated by starting from the L-enantiomer of the thioglycoside.On the other hand, the reaction with purine bases was accompanied by the production of undesirable N-7 regioisomers besides the desired N-9 products.The product distribution of the regioisomers was, however, proved to change with reaction time.For instance, a long reaction period allowed the thermodynamically stable N-9 isomers to be exclusively produced with moderate selectivity (α:β = 1:2 - 1:4.8).The isolated yields to the 9-β isomers after purification were acceptable for practical use.