With the optimal conditions in hand, we next examined the substrate generality of oxetane formation (Table ). With regard to the scope of homomethallyl alcohol derived from various cyclic ketones, we could prepare various spiro-oxetanes, which are attractive building blocks for drug discovery. For example, substrates bearing piperidine (1b-1d) or the acid-sensitive ketal (1e) afforded the desired products 2b-2e in excellent yields. Notably, we observed a small quantity of the corresponding ketone as a byproduct in some cases (10% from 1c and 1d and 18% from 1e), probably via demethallylation (retrocarbonyl-ene reaction) from cationic alkylcobalt complexes. This undesired pathway was substrate dependent; the rigid spiro-oxetane bearing adamantane (2f) and cyclobutene (2g, 2h), a proline derivative (2i), and an estrone derivative (2j) were synthesized in excellent yields without the formation of the corresponding ketones. Compound 2j was synthesized from 1j on the 1 g scale in almost the same yield. In contrast, a significant amount of ketone was formed from the nortropane derivative 1k (>50% yield) during the formation of the oxetane product 2k in 33% yield. The structure of 2k was determined by X-ray analysis of its derivative 2k′. Compound 2l bearing an azetidine ring afforded a yield of only 31% in a complex product mixture, which did not include the ketone. We also briefly investigated the generality of the alkene moiety by replacing the methyl group of 1a with an aryl group (1m-1o) or a hydrogen atom (1p). For the aryl group, we found that electron-withdrawing chorine (1o) and neutral hydrogen (1n) on the aromatic ring produced a higher yield than the electron-donating methoxy group (1m). When the methyl group was replaced with a hydrogen atom (1p), under the optimal conditions (Table ), a trace amount of the desired product 2p was obtained. Although the yield improved slightly when the catalyst, oxidant, and solvent were replaced, there was still room for improvement (Table ). Remarkably, monosubstituted alkenes were not suitable despite the MHAT/RPC-mediated epoxide synthesis from a monosubstituted alkene reported by Tertiary benzyl alcohols 1q and 1r were not applicable under the optimal conditions. The substrates derived from linear ketone afforded the desired oxetanes 2s-2u with unavoidable demethallylation. Various homomethallyl alcohols derived from aromatic or aliphatic aldehydes were suitable substrates that afforded the corresponding products in acceptable yields. The presence of an electron-withdrawing group at the 4-position was preferable for oxetane formation. For example, the 4-methoxycarbonyl-substituted product 2z was obtained in 58% yield. In contrast, the 4-methoxylphenyl product 2v was not obtained. We obtained demethallylated benzaldehydes in 9%-12% yield from 1w-1z. However, this demethallylation does not fully explain the origin of the electronic effect. The substrates bearing a nucleophile (1aa-1ad) potentially undergo a bifurcated competitive cyclization pathway, oxetane formation, or six-membered ring formation. Despite the challenging 4-exo cyclization process, the desired oxetanes 2aa-2ad were selectively synthesized without six-membered-ring formation. Cinnamyl alcohol 1ae was not applicable. Overall, this method is suitable for the synthesis of polysubstituted oxetane.