Ester Enolate Claisen Rearrangements of Allyl α-Fluoroacetates and α-Fluoropropanoates

The ester enolate Claisen rearrangement of allyl α-fluoroacetates 1 forms 2-fluoroalkenoic acids 2 in good to excellent yield with good internal asymmetric induction. This selectivity was unexpected as stereoselective deprotonation of fluoroacetates is not normally possible. The selective formation of the required α-fluoro silyl ketene acetal 3 was found to result from the stereoselective rearrangement of the allyl α-fluoro-α-silylacetate isomer. Although silyl ketene acetals derived from α-fluoropropanoates 7 also rearranged, control of internal asymmetric induction was not possible.