The novel cationic Ag(I)-catalyzed cycloisomerization, which is associated with alkyl rearrangements, from dimethyl 2-allyl-2-prenylmalonate ( 1 ) to dimethyl 4-isopropylcyclohex-3-ene-1,1-dicarboxylate ( 2 ) has been developed. Derivatization from the diester 2 into the diol 3 and its X-ray crystallographic analysis determined the structure. The mechanisms of the novel reaction were investigated by isotopic experiments, which supported the unusual alkyl shifts. In addition, the product 2 was used for the total syntheses of three natural products, 1,2,5,6-tetrahydrocuminic acid ( 12 ), p -menth-3-en-7-ol ( 13 ), and p -menth-3-en-7-al ( 14 ) in short steps.