The reactivity of 3-substituted 4-methyl-4-(3-oxobutyl)-2-cyclohexen-1-ones (1) in the presence of a combination of a Lewis acid and a 1, 2-diol was studied. The results suggest several factors that influence 6-membered ring formation, including two types of intramolecular aldol reaction and intramolecular 1, 4-addition, due to the C3-substituent, Lewis acid, and the presence of diol. In this study, novel methodology to prepare two types of decalin skeleton could be developed.