Homolytic reaction of 1-propane- and 1-pentanethiols to 2,4-dimethyl-2-vinyl-1,3-dioxa-2-silacyclohexane, initiated by di-tert-butyl peroxide at 130 degreesC, yields a mixture of isomeric sulfides (95%) as a result of addition at the alpha- and beta -positions of the vinyl group. Conjugate addition of alkanethiols in the presence of the corresponding sodium thiolate at 100 degreesC gives 10% of the same alpha- and beta -regioisomeric sulfides at a ratio of I : 20.