A method is described for the synthesis of oxetan-2-ones comprising protection of the hydroxy group of an hydroxy ester with an acid-labile acetal protecting group, reduction of this O-protected hydroxy ester to an O-protected hydroxy aldehyde, condensation of this aldehyde with a metal enolate of an activated carboxylic acid derivative and spontaneous deprotection of the hydroxy group during the acidic workup procedure. Using the new O-protected hydroxy aldehydes as intermediates the oxetan-2-ones can be obtained after separation in diastereomerically and enantiomerically pure form, in a remarkably reduced number of steps, and in a significantly improved overall yield.