1-Hydroxy-6-(3-methyl-but-2-enyloxy)-xanthen-9-one was a derivative compound of 1,6-dihydroxyxanthone. This compound was prepared from the reaction between 1,6-dihydroxyxanthone and prenyl bromide in the presence of potassium carbonate in the organic acetone medium. Potassium carbonate was used as a base and catalyst to accelerate the reaction speed. The total mass of pure 1-hydroxy-6-(3-methyl-but-2-enyloxy)-xanthen-9-one synthesized was 0.5471 g, which was 50.5% of the percentage yield. The overall equation of reaction is shown in Figure 4.8.
O O
HO
OH
Br O
O OH
O
Figure 4.8: Equation of reaction for synthesis of 1-hydroxy-6-(3-methyl-but- 2-enyloxy)-xanthen-9-one
4.3.1 Structure Elucidation of 1-Hydroxy-6-(3-methyl-but-2-enyloxy)- xanthen-9-one
1-Hydroxy-6-(3-methyl-but-2-enyloxy)-xanthen-9-one was isolated in the form of yellowish solid with a melting point range of 123oC to 125oC. When the TLC was developed with solvent system of 70% hexane and 30% acetone, the collected fractions from 2 to 4 containing the compound gave a single spot with Rf value of
+ Acetone
K2CO3
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0.57. The summary of physical properties of 1-hydroxy-6-(3-methyl-but-2- enyloxy)-xanthen-9-one is tabulated in Table 4.4.
Table 4.4: Summary of physical properties of 1-hydroxy-6-(3-methyl-but-2- enyloxy)-xanthen-9-one
Mass obtained, g 0.5471
Percentage yield, % 46.1
Physical appearance Yellowish solid
Melting point 123oC to 125oC
Rf value on TLC 0.57 in hexane: acetone (7:3)
1H-NMR spectral data (Figure 4.9) indicated the compound to have a xanthone skeleton with one prenyl moiety. The presence of six carbon signals at δ 8.10 (1H, d, J=8.5 Hz), 7.65 (1H, t, J=8.5 Hz), 7.04 (1H, s), 7.03 (1H, d, J=8.5 Hz), 6.95 (1H, d, J=8.5 Hz), and 6.74 (1H, d, J=8.5 Hz) were due to the aromatic protons H-8, H-3, H-5, H-7, H-4, and H-2, respectively. A signal at 12.82 ppm (1H, s, 1- OH) indicated the presence of a chelated hydroxyl proton. A doublet signal at 4.76 ppm (2H, d, J=6.7 Hz) was assigned to proton H-11 and the low intensity peak at 5.51 ppm (1H, t, J=6.7 Hz) was assigned to proton H-12. An intense upfield singlet at 1.78 ppm, integrated for six protons, was attributed to the two groups of equivalent vinylic methyl protons H-14 and H-15.
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From the 13C-NMR spectrum (Figure 4.10), a signal at 181.3 ppm was assigned to the highly deshielded carbonyl carbon C-9. Besides, the carbon signals at 165.6 ppm, 162.0 ppm, 158.4 ppm, 156.5 ppm, 138.6 ppm, 113.9 ppm, and 108.4 ppm were assigned to seven quaternary carbons C-6, C-1, C-10a, C-4a, C-13, C-8a, C- 9a, respectively. The protonated aromatic carbons C-3, C-8, C-12, C-7, C-2, C-4, and C-5 gave signals at 136.7 ppm, 127.1 ppm, 119.0 ppm, 114.5 ppm, 110.3 ppm, 106.8 ppm, and 100.9 ppm, respectively. Carbon signal at 65.8 ppm was assigned to methylene carbon C-11, and the other two signals at 25.0 ppm and 17.5 ppm were due to the methyl carbons C-14 and C-15, respectively. These were consistent with the structure of 1-hydroxy-6-(3-methyl-but-2-enyloxy)-xanthen-9- one. The existence of the prenyl group in the compound was revealed by the characteristic carbon signals at 138.6 ppm (C-13), 119.0 ppm (C-12), 65.8 ppm (C-11), 25.0 ppm (C-14), and 17.5 ppm (C-15). Highly deshielded carbon signals at 165.6 ppm and 162.0 ppm were assigned to the oxygenated aromatic carbons C-6 and C-1, respectively.
The assignment of resonance in the 1H-NMR and 13C-NMR spectra of this compound was further confirmed by a 2D- NMR analysis. The verification of the structure of the proposed compound was accomplished through the combination of HMQC and HMBC spectral data.
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HMQC spectrum (Figure 4.11) showed the direct 1J coupling between proton and carbon. From the spectrum, the chemical shifts of the carbon signals which correlated to protons H-2 (6.74 ppm), H-3 (7.65 ppm), H-4 (6.95 ppm), H-5 (7.04 ppm), H-7 (7.03 ppm), H-8 (8.10 ppm), H-11 (4.76 ppm), H-12 (5.51 ppm), H-14 (1.78 ppm), and H-15 (1.78 ppm) were corresponded to 110.3 ppm (C-2), 136.7 ppm (C-3), 106.8 ppm (C-4), 100.9 ppm (C-5), 114.5 ppm (C-7), 127.1 ppm (C-8), 65.8 ppm (C-11), 119.0 ppm (C-12), 25.0 ppm (C-14), and 17.5 ppm (C-15), respectively
Quaternary carbons gave no correlation in HMQC spectrum but the assignment of their carbon positions in the structure can be accomplished through the long range coupling. HMBC analyses revealed the interaction of either 2J or 3J long range coupling between proton and its farther distance carbon. In the HMBC spectrum (Figure 4.12), it was observed that proton H-2 was 3J coupling with carbon C-4.
The assignment of this position was further confirmed by 3J coupling of proton H- 4 with carbon C-2. Besides, proton H-11 was 2J coupling with carbon C-12 and 3J coupling with carbon C-6 and C-13. From this 3J long range heteronuclear connectivity between proton signals H-11 and carbon signal C-6 indicated that the prenyl group was attached to the oxygen atom that bonded to carbon C-6 position.
On the other hand, the methyl protons H-14 and H-15 in prenyl group showed 2J coupling with C-13 and 3J coupling with C-12, indicating the presence of prenyl group in the structure. The aromatic proton H-3 was found to have 3J coupling with C-1 and C-4a while proton H-5 was 2J coupling with C-6 and C-10a, and 3J
64
coupling with C-7. These correlations supported the assignment of xanthone nucleus in the structure proposed.
The combination of 1H-NMR, 13C-NMR and 2D- NMR spectra has proposed the structure of compound as 1-hydroxy-6-(3-methyl-but-2-enyloxy)-xanthen-9-one with molecular formula of C18H16O4.
From the IR spectrum (Figure 4.13), a broad band at 3402 cm-1 was due to the presence of –OH functional groups in the compound. Other bands were observed at about 1617 cm-1 and 1280 cm-1 indicating the presence of C=O and C-O functional groups. Together, these data were found to be in correspondence to the characteristic of the proposed structure 1-hydroxy-6-(3-methyl-but-2-enyloxy)- xanthen-9-one.
From UV-Vis spectrum (Figure 4.14), UV absorption maxima at 238.71 nm, 306.98 nm, and 353.58 nm implied that the compound is a highly conjugated compound which was in agreement with the characteristic of the proposed 1- hydroxy-6-(3-methyl-but-2-enyloxy)-xanthen-9-one.
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O O
O
OH 1
2 3 4 4a 9a
10a 8a
5 6 7
8
11 12 13
14
15
9
1-hydroxy-6-(3-methyl-but-2-enyloxy)-xanthen-9-one Molecular formula: C18H16O4
Molecular weight: 296.168 g mol-1
Table 4.5: Summary of NMR spectral data of 1-hydroxy-6-(3-methyl- but-2-enyloxy)-xanthen-9-one