Synthesis and antioxidant activities of benzylic bromophenols inclusive of natural products

The synthesis of natural products 2-(2,3-dibromo-4,5-dihydroxyphenyl)acetic acid (1) and 2-(2,6-dibromo-3,5-dihydroxyphenyl)acetic acid (2) and as well as their derivatives 25 and 26 were carried out by substitution, hydrolysis and demethylation reactions of the corresponding four benzyl bromides. The antioxidant potentials of benzylic acid-derived bromophenols were, for the first time, appraised by several outstanding bioanalytical methods. Besides these, we estimated the antioxidant effects which were studied using the methods of DPPH·, ABTS•+ scavenging activities, ferric (Fe3+) and cupric (Cu2+) ions and Fe3+-TPTZ reducing capacities. Benzylic acid-derived bromophenols were found as effective DPPH•, and ABTS•+ scavengers. The potential antioxidant activities of bromophenol derivatives 1, 2 and 17–28 were compared to standard antioxidants including BHA, BHT, α-Tocopherol, and Trolox, which is a water-soluble analog of vitamin E. We expect that this innovative work will direct future studies exploring the antioxidant properties of food, medicinal, and industrial applications.

and efficient reducing ability by using the Fe[Fe(CN-) 6 ] 3 , Cu 2+ and Fe 3+ -TPTZ reduction abilities. For measuring of reduction ability of bromophenol derivatives 1, 2, and 17-28, Fe 3+ -Fe 2+ transformation was realized according to Oyaizu's method [46]. As given in Table 1 and Figure 1A, bromophenol derivatives 1, 2, and 17-28, demonstrated efficient Fe 3+ reducing capability. Since the reducing potencies and radical scavenging capacities of bromophenol derivatives 17-24 are relatively lower due to the structure-activity relationship, they will not be considered much in the discussion part. The other bromophenol derivatives 17-24 demonstrated moderate Fe 3+ reducing ability ranging from 0.253 ± 0.004-0.463 ± 0.010 when compared to standards. Generally, the antioxidant properties of the test sample are compared with standard antioxidants. Many compounds are used as standards for this purpose. In addition, the standard selection and criteria are also related to the stability, price, and solubility of the standard antioxidant in the solvent environment [47]. The increased absorbance shows the complex formation and increased enhanced reducing effect ( Figure 2A). The results clearly show that bromophenol derivatives 1, 2, and 17-28 have strong Fe 3+ reducing effects with edonating properties for neutralizing free radicals and ROS. They can apply in biochemical and biological systems to reduce oxidative stress or damage.
Aside Fe 3+ and Cu 2+ reduction properties of bromophenol derivatives 1, 2, and 17-28, they had powerful reducing potentials in FRAP assay ( Figure 1C and Table 2). Reducing ability of bromophenol derivatives 1, 2, and 17-28 was found to be in descending order of 25 ( 24 showed relatively weaker Fe 3+ -TPTZ reducing ability between in 0.304 ± 0.004-0.667 ± 0.002 when compared to standard reducing compounds. As mentioned in prior reduction assay, high reducing absorbance shows high reducing ability of the complex. The FRAP method is realized in an acidic medium to protect iron ions solubility [49].

Radicals scavenging results
DPPH· and ABTS ·+ scavenging assays are among the most convenient spectrophotometric scavenging methods. Both assays were used to determine the antioxidant abilities and radical scavenging capacities of plants, foods, and beverages [50]. The percent inhibition depends on the concentrations of oxidizers such as antioxidants and radicals, the ratios of solvent and reagent used, incubation time, temperature, and also the presence of hydrogen, metal, and water in the antioxidant test systems [51]. On the other hand, IC 50 is called the effective concentration that causes 50% removal of oxidants such as radicals in antioxidant studies. It is often used to assess the antioxidant activity [47]. For DPPH radical scavenging were found to be in following order: BHT (IC 50 Figure 1D). In another study, DPPH radical scavenging activity of nineteen bromophenols from Rhodomela confervoides was realized. Among these bromophenols, bromophenols 1 was also studied. It was shown that bromophenols 1 had IC 50 value of 19.84 µM for DPPH radical scavenging activity. Also, it was demonstrated that the metabolites with ortho-dihydroxy groups on the aromatic ring generally display higher activity than the compounds having a single free hydroxyl group on the ring [16,52]. The bromophenol derivatives 1, 2, and 17-28 exhibited effective ABTS ·+ removing ability. As given in Table 2 and Figure 1E, bromophenol derivatives 1, 2, and 17-28 effectively scavenged ABTS radicals as concentration-dependently (20-60 µg/mL, p < 0.001). EC 50 values of bromophenol derivatives 1, 2, and 17-28 in ABTS ·+ scavenging assay were found to be in descending order of 25 (IC 50 50 values when compared to standard radical scavengers. As DPPH in radical scavenging, a lower IC 50 value shows higher ABTS ·+ scavenging ability. In a previous study, it was shown that 2.87 mM TEAC ABTS ·+ scavenging ability. In this study, the isolated nineteen bromophenols were found effective radical scavenging potential against ABTS cation radicals [16]. Already, it is well-known that ABTS radical scavenging properties of antioxidants can attribute H-donating effect [53].

Conclusion
As a result, after bromides 11, 12, 15, and 16 were synthesized, the acids including natural products were obtained via the corresponding nitriles 17-20. Chronic diseases, mutagenesis, DNA damage, carcinogenesis, and inhibition of pathogenic bacterial growth are generally associated with the scavenging of ROS and free radical propagation in living systems. Antioxidant activity is used as an effective and common parameter for medicinal bioactive components and newly synthesized biologically important molecules. For this reason, natural products 1 and 2, including their derivatives 25 and 26 were synthesized through the conversion of benzyl bromides 11-16 into the corresponding nitriles 17-20 followed by base catalyzed hydrolysis and demethylation. In addition to these products, benzyl nitriles 27 and 28 were also synthesized through direct demethylation of 18 and 20. Once these bromophenol derivatives were obtained 1, 2, and 17-28 they were assessed for their antioxidant ability and compared with established antioxidants including, BHA, α-tocopherol, BHT, and Trolox.

Synthesis of 2-(3-bromo-4,5-dihydroxyphenyl)acetic acid (25): Standard procedure for the demethylation reaction with BBr 3
A solution of BBr 3 (728 mg, 2.91 mmol) in CH 2 Cl 2 (10 mL) was added to a stirring solution of the compound 21 (400 mg, 1.45 mmol) CH 2 Cl 2 (5 mL) under N 2 (g) at RT, and then the mixture was stirred at the same condition for 16 h. Termination of the reaction mixture and then slow addition of H 2 O (3 mL) over 15 min at 0 °C, removal of the solvent under vacuum, the addition of H 2 O (15 mL) again and extraction of the mixture with EtOAc (2 × 25 mL) were done, respectively. After combination and drying over Na 2 SO 4 of organic phases and evaporation of the solvent under vacuum were performed, the bromophenol 25 (290 mg 85%, brown solid) was obtained. Mp
HRMS spectrum of the compound 2.