PURPOSE: To provide a new phenoxy derivative which is a synthetic intermediate for a pyrimidone derivative useful as a treating agent for peptic ulcer having gastric acid secretion suppressing action as well as digestive tract mucosa protecting action.

CONSTITUTION: A compound of formula I [Y is group of formula II (R1 and R2 are 1-6C lower alkyl) or formula III (n is 1-3)], e.g. 4-[3-(1,3-dioxolan-2-yl) phenoxy-cis-2-butenylamine. The compound can be produced by reacting a phthalimide derivative of formula IV with hydrazine hydrate in a solvent (e.g. ethanol) at room temperature. A treating agent for peptic ulcer can be prepared in high efficiency by reacting the compound of formula I with a pyrimidone compound and reacting the product with piperidine e.g. under reducing condition.