Lewisacid-promoted cyclizations of methylenecyclohexane siloxy acetals 14, 15, and 21 afford spiro[4.5]decanones 22, 25, and 29 in good yield. In all cases, exclusive pinacol rearrangement of C-1 of the original three-carbon acetal side chain is observed suggesting that pinacol rearrangement of the intermediate 9-decalyl cation occurs more rapidly than conformational equilibration. This selectivity