Compound 3 was isolated as white amorphous powder. A molecular formula C
29H
36O
11 was established from the HRESIMS quasi-molecular ion peak at
m/
z 561.2339 [M + H]
+. The IR, UV, and NMR spectral data of 3 were in agreement with those of 4 except for the appearance of new signals at
H 2.08 /
C 20.8 (CO
CH3) and 171.7 (
COCH
3) characteristic for an acetyl group in 3. Its attachment at C-17 was confirmed by the
3J HMBC cross peak of H-16 to the carbonyl group of acetyl moiety at
C 171.7 and further secured by the ESIMS ion peak at 357.2 [(M + H)-(Glu+Acetyl)]
+. In addition, 3 was 42 mass units and one degree of unsaturation more than 4, confirming the presence of the acetyl moiety. The UV absorption maxima at 219, 251, and 293 nm indicated a diarylheptanoid structure of 3 (
27). Its IR spectrum displayed bands ascribable to hydroxyl (3465 cm
-1), ester carbonyl (1725 cm
-1), ketone carbonyl (1715 cm
-1), and benzene (1598 cm
-1) functionalities (
28). The
1H NMR spectrum showed two singlet methoxy groups at
H 3.95 (4-OCH
3) and 3.79 (3-OCH
3) (
Table 2). They correlated with the carbons resonating at
C 61.9 (4-OCH
3) and 61.7 (3-OCH
3), respectively in HMQC spectrum. Their connectivity at C-4 and C-3 was proven by the HMBC cross peaks of 4-OCH
3 to C-4 (
C 145.0) and 3-OCH
3 to C-3 (
C 146.9). Four aromatic proton signals at
H 7.06 (dd,
J = 6.6, 1.5 Hz, H-15), 6.87 (d,
J = 6.6 Hz, H-16), 6.69 (d,
J = 1.5 Hz, H-18), and 6.67 (s, H-19), which correlated with carbons at
C 129.6 (C-15), 122.8 (C-16), 132.4 (C-18), and 129.1 (C-19) in HMQC spectrum, indicating the presence of 1,2,4-
tri-substituted and 1,2,3,4,5-
penta-substituted benzene moieties in 3 (20,21). They were established by the COSY cross peaks of H-15 to H-16 and H-18 and further secured by the HMBC correlations of H-15 to C-17 and C-18, H-16 to C-1 and C-14, H-18 to C-14 and C-17, and H-19 to C-3 and C-5 (
Figure 2). The connectivity of two phenyl moieties at C
1-C
2 was secured based on the HMBC cross peaks of H-19 to C-1 and H-18 to C-2 (
Figure 2). Moreover, the doublet proton signal at
H 4.80 (
J = 7.6 Hz, H-1`) showed cross peak to the signal at
C 105.0 (C-1`), indicating the presence of
β-glucopyranose moiety (
23,
26). This was established by the observed ESIMS fragment peaks at
m/
z 398.9 [(M + H)-Glu]
+ and 357.2 [(M + H)-(Glu+Acetyl)]
+. In the HMBC, the cross peak of H-1` to C-5 (
C 148.7) established the placement of glucose at C-5. Furthermore, signals for six methylene groups at
H 1.80-3.07 and ketone carbonyl at
C 213.6 (C-11), characteristic for heptanoid moiety in 3 were observed. In the COSY spectrum, the spin system started from H-7 to H-10 and cross peak of H-12 to H-13 established this moiety. It was secured by the observed HMBC cross peaks of H-9/C-7 and C-11, H-10/C-8, H-12/C-13, and H-13/C-11. The attachment of heptanoid moiety at C6-C14 of the biphenyl moiety was secured by the HMBC cross peaks of H-8/C-6, H-19/C-7, and H-18/C-13. Consequently, 3 was concluded to be 17-
O-acetyl myricanone 5-
O-
β-glucopyranoside and named alnuheptanoid B.
Compounds 1-5 were identified to be myricanone (1) (
29,
30), (+)-
S-myricanol (
2) (
30,
31), myricanone 5-
O- -D-glucopyranoside (
4) (
32), and (+)-
S-myricanol 5-
O- -D-glucopyranoside (
5) (
27) by the interpretation of the spectroscopic data and comparison with literature.
The antioxidant activity of the isolated cyclic diarylheptanoids (
1-
5) was determined using DPPH free radical scavenging system at concentration 50
µM. The results showed that, 1 and 2 had significant antioxidant activity. While, 3-5 showed moderate activity in comparison with propyl gallate (a known antioxidant) (
Table 3). Their antioxidant effect was related to the number of free hydroxyl groups in their structures. Compounds 1 and 2 showed significant activities compared to propyl gallate at the same concentration. However, blocking of the hydroxyl group by an acetyl or glucose moiety leads to a decrease in the activity as in 3-5 (
33).
Compounds 1-5 were evaluated for their anti-inflammatory effects using carrageenin induced paw edema model. Compound 2 showed the highest activity comparable to indomethacin (10 mg/kg) (
Table 4). Also, 1, 3, 4, and 5 showed potent activity at dose 10 mg/kg after 4 h. The phenolic compounds are known to inhibit prostaglandins synthesis enzymes, more specifically the endoperoxide (
26). It was reported that, prostaglandin like substances are released during the second phase of carrageenin induced edema (
34,
35). So, the anti-inflammatory effects of the tested compounds may be due to inhibition of prostaglandin like substances.
The observed activity of these compounds might be through the inhibition of the inflammatory prostanoids (
36,
37). In this work, we can make a conclusion on the SAR of the tested cyclic diarylheptanoids. It was observed that, the phenolic hydroxyl groups are responsible for anti-inflammatory activity as in 1 and 2 (
36,
37). Glucosidation of phenolic OH group leads to reduce the activity as in 3-5. Secondary alcoholic hydroxyl group in aliphatic chain might increase the activity as in 1 and 5 in comparison to the other compounds.