PURPOSE: To provide a new α,β-dihydroxy-γ-δ-unsaturated carboxylic acid thiol ester derivative useful as an intermediate useful for the synthesis of various optically active monosaccharides.

CONSTITUTION: An α,β-dihydroxy-γ,δ-unsaturated carboxylic acid thiol ester derivative of formula I (R1-R3 are H, lower alkyl; R4 is arylmethyl; R5 is lower alkyl), e.g. S-ethyl-2-benzyloxy-3-hydroxy-4-hexenethioate. The compound of formula I is obtained by reacting an α,β-unsaturated aldehyde of formula II with a ketene monothioacetal of formula III (R6-R8 are lower alkyl, aryl) in the presence of an optically active diamine and a tin compound.