Synthesis of lipophilic clovamide derivatives and their antioxidative potential against lipid peroxidation

Some N-(hydroxycinnamoyl)-L-tyrosine and L-DOPA alkyl esters were synthesized and evaluated as a variation of the clovamide (N-caffeoyl-L-3,4-dihydroxyphenylalanine) structure, a known antioxidant found in red clover. The amides were prepared in good yields starting from methyl and dodecylesters of L-tyrosine and L-DOPA by reacting with the N-hydroxysuccinimidyl esters of ferulic, sinapic, and acetyl-protected caffeic acid, respectively. In the DPPH* (2,2-diphenyl-1-picrylhydrazyl) and superoxide radical quencher assays they showed radical scavenging activity equal to or higher than those of the standard antioxidants ascorbic acid and tocopherol. The antioxidative potentials of the clovamide derivatives against bulk lipid oxidation, as determined by the accelerated autoxidation of oils, were equal to or higher than those of the standard antioxidants; some of the compounds were able to protect an emulsion of linoleic acid/beta-carotene against oxidation. N-Caffeoyl L-tyrosine methyl ester and the N-cinnamoyl L-DOPA alkyl esters especially were potent antioxidants in bulk lipids and moderate protectants in emulsions.