Stereospecific Cheletropic Reactions with Sulfur Dioxide

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Cheletropic reactions are a subclass of cycloadditions where two new bonds are formed to one molecule. In this example, the sulfur dioxide molecule donates two electrons to form the ring whilst being simultaneously attacked. The stereochemistry of the product is determined by the geometry of the starting diene. The reactions can also be reversed by heating.

D. Suárez, E. Iglesias, T. L. Sordo and J. A. Sordo, J. Phys. Org. Chem., 1996, 9, 17–20.