STEREOSELECTIVE SYNTHESIS OF 2'-DEOXY-BETA-D-THREO-PENTOFURANOSYL NUCLEOSIDES BY THE NBS-PROMOTED COUPLING REACTION OF THIOGLYCOSIDES WITH SILYLATED HETEROCYCLIC BASES

The NBS-promoted coupling reaction of phenyl 3,5-O-isopropylidene-2-deoxy-1-thio-alpha-D-threo-pentofuranoside (5e) with silylated pyrimidine bases was found to proceed in a highly stereoselective manner (alpha:beta = 1:24-0:1) to afford 2'-deoxy-beta-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 a form. A subsequent S(N)2 type attack to the intermediate will lead to the beta-nucleosides. By using this method, the synthesis of L-nucleosides, 1-(2-deoxy-beta-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 (alpha:beta = 1:2-1:4.8). The isolated yields of the 9-beta isomers after purification were acceptable for practical use.