Model synthesis of 2,3:5,6-di-O-isopropylidene- S-α-D-mannofuranosyl- N,N-diisopropylphosphoramidofluoridothioate and 2,3:5,6-di-O-isopropylidene-Se-α-D-mannofuranosyl-N,N-di-isopropylphosphoramidofluoridoselenoate via P(III)-OAr intermediates and a thiono-thiolo (selenono-selenolo) rearrangement

Phosphitylation of the glycosidic center of sugar derivative 2 by phosphoroamidite 1 leads to the intermediate P(III) ester 3. Conversion of 3 into fluorophosphoroamidite 4 and subsequently oxidation by elemental sulfur or selenium affords the corresponding thiono- and selenono- esters 5 and 6. The latter compounds are readily rearranged into their thiolo- or selenolo- isomers in the presence of Bu4NI as catalyst (TBAI). The total yield of 7 and 8 exceeds 90%.