Synthesis of Novel Iso-4‘-thionucleosides Using the Mitsunobu Reaction

Abstract
A novel class of isomeric 4‘-thionucleosides with the base moiety at the 2‘-position was synthesized from d-glucose. The coupling of 1,4-anhydro-4-thioarabitol (13) with various nucleobases using the Mitsunobu reaction was investigated. With both purines and N3-benzoyluracils, the reaction predominantly gave β-isomers, suggesting that these were produced via an episulfonium intermediate. The β-anomers produced by the reaction of N3-benzoyluracils included both N- and O-alkylated derivatives. Interestingly, only the reaction of N3-benzoyluracil gave a mixture of N-alkylated adduct (20d) and O-alkylated bipyrimidinyl adduct (22), the structure of which was unambiguously determined by NMR spectroscopic data including HMBC and NOE. Deprotection of the Mitsunobu reaction products gave the desired iso-4‘-thionucleosides.