Synthesis And Antioxidation Activity Of L-ascorbic Acid Acetals (Ketals) And D-isoascorbic Acid Acetals (Ketals)

With n-alkyl aldehyde(n-alkyl = n-hexyl,n-heptyl,n-octyl,n-nonyl,n-decyl, n-undecyl,n-dodecyl),cyclohexanone,benzaldehyde and cinnamic aldehyde as examples,nineteen acetal or ketal derivatives of L-ascorbic acid and D-isoascorbic acid were synthesized by the reaction of the carbonyl group in aldehydes or ketons and the C5,C6 ortho-dihydroxyl groups in L-ascorbic acid or in D-isoascorbic acid. These derivatives would possess some similar properties as those of L-ascorbic acid or D-isoascorbic acid,and have the potential applications in oil-based food,cosmetics and pharmaceutical industry as antioxidants,surfactants or emulsifying agents.All nineteen target compounds were characterized by ¹H NMR,FTIR and EI-MS.With n-dodecyl aldehyde D-isoascorbic acid acetal,cyclohexanone L-ascorbic acid ketal,cyclohexanone D-isoascorbic acid ketal,benzaldehyde D-isoascorbic acid acetal,cinnamic aldehyde L-ascorbic acid acetal as examples,the optimal reaction conditions were determined by orthogonal experiments.The yield of benzaldehyde L-ascorbic acid acetal and benzaldehyde D-isoascorbic acid acetal was 24.6%and 38.8%,respectively,and the yield of other L-ascorbic or D-isoascorbic acid acetals and ketals was between 67.0%and 85.8%under the optimal conditions.Based on the solubility test,the procedures for the separation and purification of the target compounds were established.With benzaldehyde L-ascorbic acid acetal and benzaldehyde D-isoascorbic acid acetal,cinnamic aldehyde L-ascorbic acid acetal as examples,the molar ratio of two diastereomers obtained from the synthetic reaction was explained based on the reaction mechanism and conformational stability principle.The antioxidant activity of n-octyl aldehyde L-ascorbic acid acetal,n-octyl aldehyde D-isoascorbic acid acetal,n-dodecyl aldehyde L-ascorbic acid acetal, n-dodecyl aldehyde D-isoascorbic acid acetal,cyclohexanone L-ascorbic acid ketal and cyclohexanone D-isoascorbic acid ketal on fresh tea seed oil was evaluated by peroxide value(POV) determination,and compared with that of L-ascorbic acid, D-isoascorbic acid as well as commonly used lipophilic antioxidants TBHQ,PG.The POV data showed that these acetal and ketal derivatives had definite antioxidant effects on tea seed oil.And the longer the carbon chain was,the better the antioxidation activity of the acetal or ketal became.The antioxidation activity of D-isoascorbic acid acetal and D-isoascorbic acid ketal was respectively stronger than that of its corresponding L-ascorbic acid acetal and L-ascorbic acid ketal.With cinnamic aldehyde L-ascorbic acid acetal,cyclohexanone L-ascorbic acid ketal and cyclohexanone D-isoascorbic acid ketal as examples,their scavenging activity on hydroxyl free radical,superoxide anion free radical and DPPH free radical was tested and compared with that of L-ascorbic acid,D-isoascorbic acid and TBHQ. The results were as follows:(1) The scavenging activity of L-ascorbic acid and D-isoascorbic acid on HO', O₂^-' and DPPH' wasn't affected by ketonization,but improved in oil-soluble media.(2) When the molar concentration of the tested samples was the same,the scavenging activity of cyclohexanone L-ascorbic acid ketal and cyclohexanone D-isoascorbic acid ketal on hydroxyl free radical,superoxide anion free radical and DPPH free radical was almost the same as that of commonly used lipophilic antioxidant TBHQ on the whole.So both cyclohexanone L-ascorbic acid ketal and cyclohexanone D-isoascorbic acid ketal could have the potential application value as lipophilic antioxidants.(3) Cinnamic aldehyde L-ascorbic acid acetal had good scavenging activity on hydroxyl free radical,superoxide anion free radical and DPPH free radical.When the molar concentration of the tested samples was the same,its scavenging activity on all the above mentioned free radicals was basically the same as that of L-ascorbic acid, and little stronger than that of lipophilic antioxidant TBHQ.