arylidenecyclopropanes 1 with acetals 2 afforded the corresponding indenederivatives3 in good to high yields. For example, in the presence of 10 mol % of Yb(OTf)3 the reaction of 1-phenylbenzylidenecyclopropane 1a with the dimethyl acetals of benzaldehyde 2a, p-tolualdehyde 2b, and p-anisaldehyde 2c gave 1,3-diphenyl-2-(2-methoxyethyl)indene3a, 2-(2-methoxyethyl)-3-phenyl-1-(p-tolyl)indene 3b, and 1-(p-