STEREOCHEMISTRY OF α,α'-DIFLUOROSUCCINIC ACIDS

Article abstract of DOI:10.1016/S0022-1139(00)82575-0

Treatment of both dimethyl (-)-D-tartrate (IVa) and dimethyl (+)-L-tartrate (Va) with sulfur tetrafluoride gave dimethyl meso-α,α'-difluorosuccinate (Ia).The same reagent converted dimethyl meso-tartrate (IIIa) to a racemic mixture of dimethyl D- and L-α,α'-difluorosuccinate (IIa).This outcome resulting from the replacement of hydroxyl by fluorine with inversion of configuration at one and retention of configuration at the other chiral carbon atom can be rationalized by assuming the formation of a cyclic intermediate.This is opened by a subsequent S_N2 reaction with fluoride ion followed by a four-center displacement of sulfuroxy group by fluorine.The respective configuration of the dimethyl α,α'-difluorosuccinates Ia and IIa were established by (1)H and (19)F NMR using an optically active chemical shift reagent and confirmed by converting the esters to the corresponding acids and these in turn to the cis- and trans-α,α'-difluorosuccinic anhydrides, respectively.