4
Journal of Chemistry
(CH3)2). 13C NMR (101 MHz, DMSO-d6) δ 173.10, 165.39,
147.89, 147.70, 139.85, 138.12, 130.78, 125.13, 124.07, 122.69,
108.40, 105.64, 101.23, 57.29, 29.83, 19.17, 18.10. ESI: m/z
318.1 [M + 1]+.
was a dark orange solid (yield: 12.3%). 1H NMR (400 MHz,
DMSO-d6) δ 9.76 (s, 2H, -OH), 7.91 (s, 1H, CHNHCO),
7.17-7.11 (m, 1H, CH�CH-CO), 6.96 (s, 1H, Ar-CH�), 6.84-
6.74 (m, 4H, Ar-H, Ar-CH�CH-), 6.29-6.26 (d, 1H,
CH�CH-CO), 4.22-4.19 (m, 1H, CONHCH), 2.10 (s, 1H,
CH (CH3)2), 0.88-0.81 (m, 6H, CH (CH3)2). ESI: m/z 320.1
[M + 1]+.
2.2.9. Piperine L-Leucinate Conjugate (4c, C18H21NO5).
-e synthesis process of compound 4c was similar to that of
compound 4a. -e product was a white solid (yield: 85.7%).
1H NMR (400 MHz, DMSO-d6) δ 12.53 (s, 1H, -COOH),
8.31-8.29 (d, 1H, CHNHCO), 7.28 (d, 1H, CH�CH-CO),
7.20-7.14 (m, 1H, Ar-CH�), 7.02-6.86 (m, 4H, Ar-H, Ar-
CH�CH-), 6.19-6.15 (d, 1H, CH�CH-CO), 6.05 (s, 2H,
-OCH2O-), 4.35-4.30 (m, 1H, CONHCH), 1.67-1.61 (m, 1H,
CH (CH3)2), 1.58-1.53 (m, 2H, CH2CH (CH3)2), 0.92-0.86
(dd, 6H, CH (CH3)2). 13C NMR (101 MHz, DMSO-d6) δ
174.17, 165.18, 147.89, 147.71, 139.86, 138.13, 130.78, 125.13,
123.94, 122.68, 108.40, 105.64, 101.22, 50.29, 40.00, 24.37,
22.82, 21.27. ESI: m/z 332.1[M + 1]+.
2.2.13. ((2E,4E)-5-(3,4-Dihydroxyphenyl)penta-2,4-dienoyl)
Leucine (5c, C17H21NO5). -e synthesis process of com-
pound 5c was similar to that of compound 5a. -e product
was a dark orange solid (yield: 29.3%). 1H NMR (400 MHz,
DMSO-d6) δ 9.31 (s, 2H, -OH), 8.23-8.21 (d, 1H,
CHNHCO), 7.19-7.12 (m, 1H, CH�CH-CO), 6.95 (d, 1H,
Ar-CH�), 6.86-6.71 (m, 4H, Ar-H, Ar-CH�CH-), 6.14-6.10
(d, 1H, CH�CH-CO), 4.32-4.30 (m, 1H, CONHCH), 1.65-
1.60 (m, 1H, CH (CH3)2), 1.56-1.52 (m, 2H, CH2CH
(CH3)2), 0.91-0.85 (dd, 6H, CH (CH3)2). ESI: m/z 320.2
[M + 1]+.
2.2.10. Piperine L-Methionine Conjugate (4d, C17H19NO5S).
-e synthesis process of compound 4d was similar to that of
compound 4a. -e product was a pale yellow solid (yield:
2.2.14. ((2E,4E)-5-(3,4-Dihydroxyphenyl)penta-2,4-dienoyl)
Methionine (5d, C16H19NO5S). -e synthesis process of
compound 5d was similar to that of compound 5a. -e
1
71.3%). H NMR (400 MHz, DMSO-d6) δ 12.71 (s, 1H,
1
-COOH), 8.37-8.35 (d, 1H, CHNHCO), 7.29-7.28 (d, 1H,
CH�CH-CO), 7.21-7.15 (m, 1H, Ar-CH), 7.02-6.86 (m, 4H,
Ar-H, Ar-CH�CH-), 6.18-6.15 (d, 1H, CH�CH-CO), 6.05 (s,
2H, -OCH2O-), 4.44-4.43 (m, 1H, CONHCH), 2.06-1.86 (m,
5H, SCH2, SCH3). 13C NMR (101 MHz, DMSO-d6) δ 173.91,
165.82, 148.39, 148.23, 140.49, 138.75, 131.27, 125.61, 124.32,
123.22, 108.91, 106.13, 101.74, 51.54, 31.23, 30.22, 15.03. ESI:
m/z 350.1[M + 1]+.
product was a dark green solid (yield: 20.4%). H NMR
(400 MHz, DMSO-d6) δ 12.69 (s, 1H, -COOH), 9.31 (s, 1H,
-OH), 9.02 (s, 1H, -OH), 8.31-8.26 (m, 1H, CHNHCO), 7.20-
7.14 (m, 1H, CH�CH-CO), 6.95 (m, 1H, Ar-CH�), 7.87-6.72
(m, 4H, Ar-H, Ar-CH�CH-), 6.14-6.10 (d, 1H, CH�CH-
CO), 4.43-4.37 (m, 1H, CONHCH), 2.07-2.02 (m, 5H, SCH2,
SCH3). ESI: m/z 338.2[M + 1]+.
2.3. ABTS Radical-Scavenging Properties. -e ABTS radical-
scavenging activities were assayed according to the method
reported by -aipong et al. [22]. (1) ABTS+ aqueous solution
(7 mmol/L) preparation: 30 mg ABTS+ was weighed, 8 mL of
ultrapure water was added, and it was dissolved by ultra-
sonication. (2) K2S2O8 solution preparation: 10 mg K2S2O8
was weighed, 15 mL of ultrapure water was added, and it was
dissolved by ultrasonication. (3) ABTS working solution
preparation: (1) and (2) were mixed at the ratio of 1 :1 and
kept in the dark for 12 to 16 h. -e above mixture was diluted
four to eight times with 95% ethanol, such that it had an
absorbance of ca. 0.7 Abs at 734 nm, to obtain the ABTS
working solution. After keeping the 300 μL ABTS working
solution and 100 μL ethanol solution of compounds (4a–4d,
5a–5d, and piperine) with a certain concentration gradient
in the dark for 30 min, their absorbances at 734 nm were
measured. -e experiment was repeated three times in
parallel.
2.2.11. ((2E,4E)-5-(3,4-Dihydroxyphenyl)penta-2,4-dienoyl)
Alanine (5a, C14H15NO5). Compound 4a (200 mg,
0.69 mmol) was dispersed in 10 mL of anhydrous DCM. A
DCM (5 mL) solution of boron tribromide (260.9 μL,
2.76 mmol) was added dropwise to the reaction mixture in
an ice-bath condition. After 4 h, the reaction was quenched
by adding 10 mL of ice-cold saturated ammonium chloride
solution in an ice-bath condition. After stirring at room
temperature for 0.5 h, ethyl acetate (3 ×15 mL) was used for
the extraction. -e organic phase was dried with anhydrous
sodium sulfate for 24 h and then filtered to obtain the
concentrated crude product. -e crude product was then
purified by column chromatography on silica gel (devel-
oping agent DCM : MeOH : HAc � 7.5 :1 : 0.1) to afford
36.52 mg of the product (5a) (yield: 19.1%) as an orange
1
solid. H NMR (400 MHz, DMSO-d6) 9.83 (s, 2H, -OH),
8.05-8.03 (d, 1H, CHNHCO), 7.17-7.11 (m, 1H, CH�CH-
CO), 6.95 (s, 1H, Ar-CH�), 6.84-6.6.71 (m, 4H, Ar-H, Ar-
CH�CH-), 6.18-6.14 (d, 1H, CH�CH-CO), 4.30-4.27 (m,
1H, CONHCH), 1.28-1.26 (d, 3H, -CH3). ESI: m/z 278.19
[M + 1]+.
2.4. DPPH Radical-Scavenging Properties. -e DPPH radi-
cal-scavenging activities were assayed according to the
method reported by Nimse et al. [26]. -e DPPH working
solution was prepared by dissolving 20 mg DPPH in 250 mL
of 95% ethanol and diluting it until its absorbance was ca. 0.8
at 517 nm. After keeping 200 μL DPPH· working solution
and 200 μL ethanol solution of compounds (4a–4d, 5a–5d,
2.2.12. ((2E,4E)-5-(3,4-Dihydroxyphenyl)penta-2,4-dienoyl)
Valine (5b, C16H19NO5). -e synthesis process of com-
pound 5b was similar to that of compound 5a. -e product