2-Methylprop-2-ene-1-sulfonyl fluorides can be easily prepared via the ene reaction of methallylsilanes and SO2. In the presence of a base, aldehydes and 2-methylprop-2-ene-1-sulfonyl fluorides give 1.3-(E) and (Z)-dienes. Their (Z) --> (E) isomerization by classical means fails or leads to their polymerization. It is shown that SO, can isomerize 1-aryl-3-methyl-1.3-dienes at low temperature, without formation of sulfolenes (cheletropic addition/elimination). Preliminary mechanistic studies suggest that SO2 adds to 1.3-dienes forming 1,4-diradical intermediates that are responsible for the (Z)-->(E) isomerizations. (C) 2004 Elsevier Ltd. All rights reserved.