The reaction of azetidinone (I) with sodium methyl mercaptan in acetonitrile gives the methylsulfanyl derivative (II), which is alkylated with para-methoxybenzyl 2-iodoacetate (III) by means of BuLi in THF to yield the azetidine-acetic acid derivative (IV). The acylation of (IV) with 5-azido-2,2-dimethylpentanoyl chloride (V) by means of LiHMDS affords the beta-oxopentanoate (VI), which is desilylated by means of TBAF and AcOH in THF to provide the 1-hydroxyethyl-azetidine (VII). The reaction of (VII) with chlorine or methyl sulphenyl chloride furnishes the chloroazetidine (VIII), which, without isolation, is cyclized by means of TEA in THF to give the precursor (IX). Finally, this compound is submitted to hydrogenation with H2 over Pd/C to provide the target aminoacid.