A convenient synthesis of some cross-linked amino acids and their diastereoisomeric characterization by nuclear magnetic resonance

Cross-linked amino acids may be formed in foods submitted to thermal or alkaline conditions during processing. The present paper presents a convenient procedure for the preparation of some known components of this class: histidinoalanine (HAL) and methyl-lysinoalanine (Me-LAL), and of another cross-linked amino acid never prepared before, methyl-ornithinoalanine (Me-OAL). The addition of ornithine and lysine onto the double bond of N-acetyldehydroaminobutyric acid methyl ester, the unsaturated intermediate for the synthesis of Me-LAL and Me-OAL, was diastereospecific and gave only the (2S,3R) or (2R,3S) alanine derivatives. This procedure is highly reproducible and allows production of pure standards, useful for the quantification of these compounds in foods or for physiological and toxicological studies. In addition, an efficient method (by nuclear magnetic resonance) for the determination of the diastereoisomeric ratio of these cross-linked amino acids is presented.