2.1. Structure Analysis of Compound 1
Compound
1 (
Figure 1,
Table 1) was obtained as yellow powder and showed a quasi-molecular ion [M + H]
+ at
m/z 307 in the LC-MS, which was consistent with the molecular formula C
20H
18O
3 (calcd for 306.3551), and required 12 degrees of unsaturation. Its
1H NMR spectrum exhibited an aliphatic methyl singlet at δ
H 1.46 (3H, s), one methoxy group at δ
H 4.09 (3H, s), two methylene signals at δ
H 3.35 (2H, dd, 15.0, 20.0 Hz) and δ 2.97 (2H, dd, 10, 15 Hz) and 7 aromatic proton signals at δ
H 10.10 (1H, s), 8.78 (1H, s), 8.26 (1H, d, 8.6 Hz), 7.65 (1H, d, 8.25 Hz), 7.45 (1H, t, 8.25, 7.45 Hz), 7.37 (1H, d, 8.6 Hz), 6.89 (1H, d, 7.55 Hz).
The
13C NMR, DEPT135 and HMBC spectra of compound
1 displayed two methyl resonances at δ
C 28.47 (C-13, 1-Me), 55.10 (C-14), two methylenes at δ
C 45.41 (C-4), 54.28 (C-2), methines at δ
C 135.19 (C-6), 127.37 (C-5), 102.5 (C-9), 125.17 (C-12), 121.52 (C-7), 121.13 (C-11), 126.35 (C-10), quaternary carbons at δ
C 155.38 (C-8), 145.61 (C-4a), 134.54 (C-11a), 130.69 (C-6a), 128.74 (C-12a), 125.46 (C-12b), 124.29 (C-7a,), 70.55 (C-3), in addition to one carbonyl at δ
C 198.76 (C-1). By detailed comparison of the
1H and
13C NMR data of compound
1 with 1,4,8,10-tetramethoxyanthracene-2-carbaldehyde [
13], the similar NMR data of the aromatic moieties confirmed that compound
1 has the anthracene skeleton. Because compound
1 required 12 degrees of unsaturation, there is a six-carbon ring next to the anthracene skeleton. In the NOESY spectrum, the proton at δ
H 3.35 (H-4) showed a correlation with the proton at δ
H 7.37 (H-5) and the proton at δ
H 8.26 (H-6) showed a correlation with the proton at δ
H 8.78 (H-7). It confirmed the combination of the B, C and D rings, also implied carbonyl group was at position 1 not position 4. The methoxyl group at δ
H 4.09 (H-14) showed a NOE correlation with the proton at δ
H 6.89 (H-9). It indicated the combination of the A and B rings.
In the COSY spectra, δ
H 10.10 (1H, s) and δ
H 8.78 (1H, s) there is no correlation with other protons, they connected with C-12 (δ
C 125.17), C-7 (δ
C 121.52), respectively. The full assignment of compound
1 was supported by the HMBC correlations. The observed long range correlations from δ
H 1.46 (13-Me) to δ
C 145.61 (C-4a) and 198.76 (C-1) in the HMBC spectrum showed that a methyl group was attached to C-3, the HMBC correlation from δ
H 4.06 (OH, s) to δ
C 28.47 (C-13, 1-Me), 45.41 (C-4), 54.28 (C-2) and 70.55 (C-3) indicated that the hydroxyl group was attached to C-3. HMBC correlation from δ
H 4.09 (–O–CH
3) to δ
C 155.38 (C-8) indicated that the methoxy group was attached to C-8. The HMBC correlation from δ
H 3.35 (H-4), 7.37 (H-5) and 2.97 (H-2) to δ
C 198.76 (C-1) indicated that a carbonyl unit was in C-1. As shown in
Figure 1B, the linkage of the Six-carbon ring with the anthracene skeleton was established by the correlations from δ
H 3.35 (H-4) and 2.97 (H-2) to δ
C 145.61 (C-4a), δ
H 2.97 (H-2) to δ
C 125.46 (C-12b).
Consequently, the structure of compound
1 was established to be 3-hydroxy-1-keto-3-methyl-8- methoxy-1,2,3,4-tetrahydro-benz[α]anthracene (
Table 1).