Reactions of quadricyclane with sulphur dioxide: formation of a stable β-sultine and catalysed lsomerization to norbornadiene

Sulphur dioxide, as solvent or in chloroform, catalyses the isomerization of quadricyclane (1) to norbornadiene and cycloadds to (1) to afford exo-3-oxa-4-thiatricyclo [4.2.1.02,5]non-7-ene-4-oxide (3); in the presence of a deficiency of sulphur dioxide,(3) is remarkably stable in solution, but decomposes rapidly as pure product.