Primary allylic bromides A–Br and 1,3-dithian react to give sulphonium bromides; these form ylides which undergo.[2,3]-sigmatropic rearrangement to 2-substituted 1,3-dithians; hydrolysis then gives the βγ-unsaturated aldehydes A′–CHO. (A′= rearranged allylic system corresponding to A).