2-Methyl-4-phenylbutenolide (5) is prepared in a quantitative manner by an elimination of HBr from the corresponding bromolactones 3. This reaction, induced by halide ions at 100° C in polar aprotic solvents (DMF, DMSO), is accompanied by a bimolecular nucleophilic substitution. In these reactions, the solvent seems able to play an active role. A polar solvent induces the necessary activation of the substrate (by a prior strong polarization of the C-Br ...
