N-Boc-L-Leucine (I) is activated as the corresponding succinimidyl ester (II) upon treatment with N-hydroxysuccinimide and EDC. Coupling of active ester (II) with (S)-alpha-amino-gamma-butyrolactone (III) affords amide (IV). After N-Boc group deprotection of (IV) under acidic conditions, the resultant amine (V) is condensed with phenyl isothiocyanate to furnish thiourea (VI). Finally, reduction of the lactone function of (VI) with DIBAL gives rise to the target cyclic hemiacetal compound.