Benzyloxycarbonyl (Cbz)-amino acid esters were reduced to the corresponding α-aminoaldehydes with diisobutylaluminum hydride. Comparison of specific rotation of the Cbz-aminoalcohols, which were derived from the aldehydes by sodium borohydride reduction, with that of the optically pure material showed that chromatography on a silica gel caused marked racemization in the Cbz-α-aminoaldehyde through keto-enol tautomerism. Cbz-S-Bzl-cysteinal was liable to racemize to a great extent. On the other hand, little racemization occurred in Cbz-NG-nitroargininal. This fact might be accounted for the characteristic cyclic carbinolamine structure of the argininal derivative. The semicarbazones prepared from the crude Cbz-α-aminoaldehydes could be reproduced to the initial aldehydes without racemization. These semicarbazones might be used as good starting materials in peptide aldehyde synthesis.
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Itô et al. (1975) studied this question.