The reactions of triethyl, diethyl phenyl, or tributyl orthoformate with 1,3-dialkyl-aluminacyclopentanes results in regioselective cleavage of the sterically less hindered Al-C(5) bond; hydrolysis of the reaction mixture gives 4-methyl aldehyde acetals. In the case of more inert orthoformates, the process is activated with catalytic amounts of ZrCl4.