Competition between Novel 8-endo-dig and 6-trig Cyclizations of Samarium Ketyls Leading either to Benzannulated Cyclooctene or to Hexahydronaphthalene Derivatives

Ketoesters 3a–3h with an alkynylaryl substituent were prepared by standard methods starting from siloxycyclopropanes 6a and 6b. Their reactions with 2.2 equiv. of samarium diiodide in the presence of hexamethyl phosphoramide either produced benzannulated cyclooctenol derivatives 4b (or lactone 9), 4f, 4g, and 4h or hexahydronaphthalene derivatives 5a, 5c, and 5d. The competition between 8-endo-dig and 6-trig cyclizations strongly depends on the substitution pattern of 3. Plausible explanations for this competition and of the diastereoselectivity observed are presented in this paper.