The hetero-Diels-Alder addition of sulfur dioxide to (E)- and (Z)-4-(fluoromethylidene)-3-methylene-2,3-dihydro-naphthalene follows the endo Alder rule. The first example of a crystalline sultine has been obtained. In agreement with high level quantum calculations, (1SR,3SR)-1-fluoro-1,4,5,6-tetrahydronaphtho[2,1-d][1,2]oxathiine 3-oxide adopts a sofa conformation in its ground state.