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Regiochemistry of Intramolecular Photocycloaddition of 1,3-Dioxin-4-ones Tethered Through the Ketal Carbon.-Photolysis of the 1,3-dioxin-4-ones (I) containing a two carbon tethered olefin results in head to head cyclization to (II). The three to four carbon tethered substrates (III) give predominantly head to tail products (IV). -(MULLER, C. L.; BEVER, J. R.; DORDEL, M. S.; KITABWALLA, M. M.; REINEKE, T. M.; SAUSKER, J. B.; SEEHAFER, T. R.; LI, Y.; JASINSKI, J. P.; Tetrahedron Lett. 38 (1997) 50, 8663-8666; Dep. Chem., Univ. Wis., Eau Claire, WI 54702, USA; EN)