Abstract: Over the past years chiral cyanohydrins (a-hydroxynitriles) have become a versatile source for a variety of chiral building blocks. Several classes have already been synthesized, including b-hydroxy-a-amino acids [1], a-hydroxy-b-amino acids [2], a-hydroxy ketones [3] and b-hydroxy nitrones [4]. Although chiral a-hydroxy aldehydes are interesting compounds, their synthesis from chiral cyanohydrins has always been difficult. Some examples of the synthesis of chiral a-hydroxy aldehydes have been published but the results, both in yield and optical purity, seem to be contradictory [5]. This poster describes the first results of the synthesis of O-protected chiral a-hydroxy aldehydes and their subsequent use in the Horner-Wittig reaction.