## 2 Results and Discussion
### Chemical Constituents from Olibanum of Boswellia carterii
The molecular formula of boswellianol F (**6**) was confirmed as C\({}_{20}\)H\({}_{34}\)O\({}_{5}\) by 20 signals in the \({}^{13}\)C NMR spectrum and protonated molecules at \(m/z\) 355.2478 [M + H]\({}^{+}\) (calculated as 355.2479 for C\({}_{20}\)H\({}_{35}\)O\({}_{5}\)) in the HR-ESI-MS spectrum, implying four indices of hydrogen deficiency. The \({}^{1}\)H and \({}^{13}\)C NMR data of **6** (Tables 1 and 3) showed four protons at oxygen-bearing methines [\(\delta_{\rm H}\) 3.75 (1H, dd, \(J\) = 3.3, 2.6 Hz, H-3), 4.30 (1H, dd, \(J\) = 10.6, 4.7 Hz, H-7), \(\delta_{\rm H}\) 3.21 (1H, d, \(J\) = 10.0 Hz, H-5), and \(\delta_{\rm H}\) 3.27 (1H, dd, \(J\) = 11.4, 2.6 Hz, H-11)] and three quaternary oxygenated carbons at \(\delta_{\rm C}\) 87.7 (C-1), 76.7 (C-4), and 85.7 (C-12), indicating that **6** was also a highly oxidized chem  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**No. ** & **5** & **6** & **7** & **8** & **9** \\ \hline
1 & & & & & 1.50, m & \\
2 & 1.56, t, (4.8) & 1.54, m & 1.66, overlap & 1.72, overlap & \\
2 & 1.99, m & 2.20, dd, (15.0, 3.9) & 1.69, overlap & 2.12, overlap & 5.55, d, (16.3) \\
3 & 3.81, d, (7.2) & 3.75, dd, (3.3, 2.6) & 3.67, dd, (8.5, 2.5) & 5.12, dd, (8.3, 4.6) & 5.23, d, (16.3) \\
5 & 1.73, overlap & 3.21, d, (10.0) & 2.48, dd, (12.4, 7.2, 1.3) & 2.13, overlap & 1.57, q, (7.7) \\
2.22, overlap & & 2.30, dd, (14.4, 6.5) & 2.25, overlap & 1.93, m \\
1.73, overlap & 1.92, td, (13.6, 10.4) & 5.56, td, (15.7, 6.9) & 2.13, overlap & 1.23, m \\
6 & 2.28, overlap & 2.32, dd, (13.7, 5.1) & 5.36, td, (15.7, 6.9) & 2.25, overlap & 1.29, m \\
7 & 3.90, dd, (10.6, 5.0) & 4.30, dd, (10.6, 4.7) & 5.77, td, (15.9, 1.4) & 5.05, t, (5.34) & 3.35, d, (8.9) \\
9 & 1.63, m & 2.22, m & 1.64, overlap & 2.16, overlap & 1.65, m \\
1.80, m & 2.40, m & 1.74, m & 2.31, t, (7.3) & 1.76, overlap \\
10 & 1.85, overlap & 1.44, m & 1.41, m & 1.47, m & 1.64, m \\
11 & 4.25, dd, (11.4, 2.7) & 3.27, dd, (11.4, 2.6) & 4.88, dd, (11.0, 2.2) & 3.96, t, (7.4) & 4.12, dd, (9.5, 3.0) \\
13 & 1.84, overlap & 1.37, (12.2, 7.9) & 1.66, overlap & 1.65, m & 1.44, m \\
12 & 2.12, overlap & 2.09, td, (12.4, 7.6) & 1.78, m & 1.73, m & 1.85, m \\
14 & 1.44, dd, (11.4, 2.0) & 1.31, (11.9, 7.6) & 1.65, overlap & 2.00, m & 1.77, overlap \\
15 & 2.16, dd, (13.9, 7.1) & 1.77, td, (12.3, 8.0) & 1.97, m & 2.06, m & 1.97, m \\
15 & 2.15, t, (6.2) & 2.02, t, (7.0) & 2.06, overlap & - & 1.79, overlap \\
16 & 0.94, d, (6.8) & 0.94, d, (6.9) & 0.91, d, (6.8) & 1.19, s & 0.88, d, (6.9) \\
17 & 0.90, d, (6.8) & 0.95, d, (6.9) & 0.86, d, (6.8) & 1.26, s & 0.83, d, (6.9) \\
18 & 1.18, s & 1.16, s & 1.28, s & 1.55, s & 1.31, s \\
19 & 1.16, s & 5.05, s & 1.30, s & 1.64, s & 1.08, s \\
20 & 1.12, s & 1.02, s & 1.17, s & 1.18, s & 1.07, s \\ AcO & & & 2.07, s & & \\ \hline \end{at \(\delta_{\rm H}\) 5.05 (1H, s, H-19a) and \(\delta_{\rm H}\) 5.30 (1H, s, H-19b) and two olefinic carbons at \(\delta_{\rm C}\) 116.2 (C-19) and 152.5 (C-8) revealed that a methyl group was reduced to an alkenyl group in the membrane-scaffold. The double bond and the ring of the cembrane skeleton occupied two of four degrees of unsaturation, suggesting two oxygen-bridged rings in **6**. By comparison with **5**, the major difference is that a tetrahydrofuran ring (C4-O-C7) in **5** was replaced with an 8 membered oxane-ring (C5-O-C11) in **6**, deduced by HMBC correlations from H-11 to C-5 and H-5 to C-11. This 8 membered oxane-ring moiety was first reported in cembranoids isolated from the _Boswellia_ genus. Additionally, the planar structure of **6** was determined by HMBC and \({}^{1}\)H-\({}^{1}\)H COSY correlations (Figure 4). The NOESY cross peaks of H-3/Me-18, H-15, H-5/H-7, Me-18, H-11, and H-11/H-5, Me-20 exhibited that all the above protons at asymmetric centers were cofacial in the same \(\beta\)-orientation (Figure 5).