The addition of vinylmagnesium chloride to 2,3-O-isopropylidene-d-glyceraldehyde (I) gave a mixture of the epimeric pentene derivatives II and V, which were separated by preparative gas–liquid chromatography. Ozonolysis and subsequent deketalization of the individual epimers gave d-threose and d-erythrose, respectively, in yields of ca. 40%. This represents a method of lengthening an aldose carbon chain by one carbon atom.