Abstract: An .alpha.-hydrazino acid derivative of general formula (I) is described, wherein R.sub.1, R.sub.2, R.sub.3 are hydrogen or a carbon radical, such that when R.sub.2 and R.sub.3 are different, C* is an asymmetric carbon of L, D or DL configuration, and R.sub.4 and R.sub.5 are a protecting group, characterized in that R.sub.4 is a benzyl ArCH.sub.2 radical of formula (a), wherein Ar is a phenyl radical or phenyl substituted by one or more X groups; X being hydrogen, halogen, a nitro or alkyl radical; and in that R.sub.5 is a Y--O--CO group, where Y is a carbon radical different from R.sub.4.