The condensation of L-valine (I) with 4-fluorobenzenesulfonyl chloride (II) by means of NaOH in THF/water gives N-(4-fluorobenzenesulfonyl)-L-valine (III), which is esterified with N-hydroxysuccinimide (IV) by means of EDC in dichloromethane to yield the activated ester (V). The condensation of (V) with L-leucinol (VI) by means of TEA in dichloromethane affords the N-(4-fluorobenzenesulfonyl)-L-valyl-L-leucinol (VII), which is finally oxidized with the SO3/Pyridine complex in DMSO/dichloromethane to provide the target leucinal derivative.