PURPOSE: To easily obtain the titled substance by silylating a pyruvic acid ester and reacting the resultant 2-(trialkylsiloxy)acrylic acid ester with an acetal or a ketal in the presence of a Lewis acid.

CONSTITUTION: 2-(Trialkylsiloxy)acrylic acid ester derived from a pyruvic acid ester and a chlorotrialkylsilane (preferably chlorotrimethylsilane) is made to react with an acetal or a ketal at -78W0°C (usually 0°C) for several minWseveral hr in the presence of a Lewis acid to obtain the titled substance. The Lewis acid is preferably boron trifluoride etherate and the reaction is carried out preferably by using dichloromethane as a solvent. The titled substance is useful as a synthetic intermediate for a constituent component of a physiologically active substance.