The present invention provides a process for preparing 2-isopropenyl-5-methyl-4-hexenoic acid of the following formula (4), comprising steps of:
subjecting a Grignard reagent of the following general formula (1), wherein R 1 represents a linear, branched, or aromatic monovalent hydrocarbon group having 1 to 8 carbon atoms, and X represents a chlorine atom, a bromine atom, or an iodine atom, and 1,1,1,3,3,3-hexamethyldisilazane to a deprotonation reaction to form a 1,1,1,3,3,3-hexamethyldisilazane derivative; and subjecting 2-methyl-3-buten-2-yl 3-methyl-2-butenoate of the following formula (3) to a rearrangement reaction in the presence of the 1,1,1,3,3,3-hexamethyldisilazane derivative to form 2-isopropenyl-5-methyl-4-hexenoic acid (4).