Process for preparing .alpha.-hydroxyaldehyde

A process for preparing an optically active or racemic .alpha.-hydroxyaldehyde represented by the general formula (2), ##STR8## wherein R.sub.1 represents a C.sub.6 -C.sub.14 aryl group, C.sub.1 -C.sub.10 alkyl group, C.sub.3 -C.sub.10 alkenyl group, C.sub.2 -C.sub.10 alkynyl group, C.sub.7 -C.sub.14 aralkyl group, or a group containing a C.sub.1 -C.sub.4 alkoxy or silyloxy group in the organic portion of said groups and R.sub.2 represents a C.sub.1 -C.sub.10 alkyl group, C.sub.2 -C.sub.10 alkenyl group, C.sub.2 -C.sub.10 alkynyl group, C.sub.7 -C.sub.14 aralkyl group, C.sub.6 -C.sub.14 aryl group, or a group containing a C.sub.1 -C.sub.4 alkoxy or silyloxy group in the organic portion of these groups, which comprises allowing an optically active or racemic compound represented by the general formula (1), ##STR9## (wherein A represents a C.sub.6 -C.sub.14 aryl group or a C.sub.1 -C.sub.4 alkyl or alkoxy group- or halogen-substituted C.sub.6 -C.sub.14 aryl group and R.sub.1 is as defined above) to react with a Grignard reagent represented by the formula, R.sub.2 MgX (wherein R.sub.2 is as defined above and X is a halogen atom), and thereafter hydrolyzing the reaction product.