Regioselectivity of ring closure of 2-thia- and 2-sulfonyl-5-methyl-5-hexenyl radicals.

Ring closure of the alpha-substituted radicals 4 (X = S, SO(2)) is observed to be irreversible and to lead to significant amounts of the product of 6-endo cyclization. Indeed, reduction of the sulfonyl-based radical 4 (X = SO(2)), in which regioselectivity is believed to be controlled by a combination of both steric and FMO interactions, is found to provide an excellent route to the cyclic sulfone 7 (X = SO(2)) in high yield. [reaction: see text]