Recently we have turned our attention to Lewis acid mediated cyclisations of methylenecyclopropyl ketones and aldehydes. Thus we have found that treatment of methylenecyclopropyl ketones or aldehydes such as 1 with suitable Lewis acids (TiCl₄ or SnCl₄) provides a new route to cycloalkanols. The mechanism for this reaction (first proposed by Hosomi et al., Tetrahedron Lett., 1997, 38, 4587) is presumed to proceed by nucleophilic addition of the alkene to the activated carbonyl, leading to a cyclopropyl cation intermediate 2, which rearranges to an allyl cation 3, which, in turn, is trapped by chloride anion derived from the Lewis acid.

In further work we have found that incorporation of a silyl-substituent on the methylenecyclopropane precursor encourages the initial alkene addition to the carbonyl, since the silyl group can stabilise the intermediate cyclopropyl cation - indeed silyl-substituted methylenecyclopropanes such as 6 are strained allylsilanes - and cyclisations of silyl-substituted methylenecyclopropyl ketones can be carried out under milder conditions. For example, when treated with either BF₃.Et₂O or BF₃.2AcOH, silylated ketone 6 gives the bicyclic ether 9 in good yield. Here, in the absence of a sufficiently nucleophilic anion derived from the Lewis acid, the intermediate allyl cation 7 is trapped intramolecularly by the alkoxide nucleophile.