The thio-Mitsunobu reaction: a useful tool for the preparation of 2,5-anhydro-2-thio- and 3,5-anhydro-3-thiopentofuranosides.

The unprotected methyl L-arabinofuranosides, D-ribofuranosides and D-xylofuranosides are transformed into the corresponding S-acetyl-5-thio derivatives by the thio-Mitsunobu reaction. Mesylation and subsequent reaction with sodium hydrogen carbonate led, depending on the configuration of the intermediate, to 2,5-anhydro-2-thio- or 3,5-anhydro-3-thiopentofuranosides. Due to inversion at C-3 or C-2 during the intramolecular nucleophilic displacement the products exhibit L-lyxo-, D-arabino- or D-lyxo-configuration. Analogously, the methyl 2,3-anhydro-D-ribofuranosides yielded 5-thio-S-acetates with intact 2,3-oxirane groups, which were cyclised with sodium hydrogen carbonate by epoxide ring opening and concomitant ring closure to form exclusively 3,5-anhydro-3-thio-D-xylofuranosides. A related 3,5-anhydro-3-seleno-D-lyxofuranoside was obtained by reaction of a 3,5-di-O-mesyl-D-arabinofuranoside with sodium hydrogen selenide. Several X-ray diffraction analyses proved the structures of the products.