PURPOSE: To prepare the titled compound (some of which is novel) useful as a synthetic intermediate of α-cyanoacrylic acid ester, in high yield, by hydrocyanizing pyruvic acid or its ester under acidic condition at a pH of lower than a specific level.
CONSTITUTION: The cyanhydrin of formula II (novel compound when R is alkyl, alkenyl or alkoxyalkyl) is prepared by hydrocyanizing pyruvic acid of formula I (R is H, alyl, alkenyl or alkoxyalkyl) or its ester under acidic condition at ≤5pH. The hydrocyanizing reaction is carried out by using a cyanizing agent (e.g. hydrocyanic acid, sodium cyanide, etc.). By keeping the pH of the reaction liquid constantly to an acidic condition of ≤5, the decomposition of the pyruvic acid used as the raw material can be suppressed, the produced cyanhydrin can be maintained stably, and the hydrocyanidation reaction can be carried out in high yield.