PURPOSE: To obtain a malic acid useful for synthesis of agricultural chemicals and medicines from L- or D-tartaric acids by stereoselectively converting diol to alcohol and more particularly non-natural D-malic acids from natural L- tartaric acids at a high yield and high optical purity by using inexpensive reagents and a simple method.

CONSTITUTION: The compd. of formula I [(n) is 0, 1, R1 to R4 are H, aliphat. group, arom. group, COY, etc.; Y is H, halogen, W(R5)m, etc.; W is N, O, S; R5 is the respective groups in R1 to R4; (m) is 1, 2), more particularly its optically active substances, more preferably the deriv. of tartaric acid not having a free acid group or thioacid group and more particularly the diester or diamide of the L- or D-tartaric acid are brought into reaction with thionyl halide to obtain a cyclic sulfite. This cyclic sulfite is brought into reaction with a halide salt, by which the halo-substd. acyclic sulfite is obtd. Further, this sulfite is brought into reaction with a suitable reducing gent, by which the compd. of formula II, particularly its optically active substance, more preferably the deriv. of tartaric acid not having the free acid group and more particularly its L- or D-isomer are stereoselectively obtd.