PURPOSE: To readily obtain the subject compound as a raw material for manufacturing vitamin A acetate by a new method using a pentenyn-ol as a starting material.

CONSTITUTION: At first, 2-hydroxy-2-methyl-3-butynal of formula II and a 1- alkoxyalkylhydroperoxide of formula III as a by-product are obtained by decomposing pentenyn-3-ol of formula I (R is H or methyl) with ozone in an alkanol of the formula: R1OH (R1 is a 1-4C alkyl) (preferably methanol) at a temp. from -20°C to 0°C. Next, the compound of formula II and the compound of formula III are subjected to catalytic hydrogenation preferably by a method of dropping an ozone decomposing solution onto a suspension of Pd catalyst while introducing hydrogen to obtain 2-hydroxy-2-methyl-3-butenal, then the compound is converted to the objective compound of formula IV by conventional method. The catalytic hydrogenation may be performed under a normal pressure to 20 bar at a room temperature.