## 2 Results and Discussion
### Chemical Constituents from Olibanum of Boswellia carterii
Thus, the structure of **6** was elucidated as shown in Figure 1.  
Compound **7** was obtained as a white amorphous solid with a molecular formula of C\({}_{22}\)H\({}_{38}\)O\({}_{6}\) established by HR-ESI-MS sodium adduct ion at \(m/z\) 421.2540 (calculated as 421.2561 for C\({}_{22}\)H\({}_{38}\)NaO\({}_{6}\)). The \({}^{1}\)H and \({}^{13}\)C NMR data of **7** (Tables 2 and 3) were similar to those of boscartin \(Z\)[19], except for the absence of the ketone (C-5) and the C3-C4 double bond of boscartin \(Z\) and the presence of two hydroxy groups at C-3 (\(\delta_{\rm C}\) 71.0) and C-4 (\(\delta_{\rm C}\) 75.1) in **7**, which was supported by the HMBC correlations from Me-18 to C-3 and C-5 and from H-3 to C-1 and C-4 (Figure 4), as well as the molecular formula of **7**. The double bond [\(\delta_{\rm H}\) 5.56 (1H, td, \(J\) = 15.8, 7.0 Hz, H-6) and 5.77 (1H, td, \(J\) = 15.9, 1.4 Hz, H-7)] was assigned at the C-6 and C-7 positions based on \({}^{1}\)H-\({}^{1}\)H COSY correlations of H-5/H-6/H-7 and the HMBC correlation from Me-19 to C-7, C-8, and C-9 (Figure 4). It has been reported thatembrane-type diterpneids with 3-a-OH and quaternary C-4 have the smaller coupling between H-3 and H-2 (\(\eta_{\rm H-2,\,H-3}\) = -7.6-8.8 Hz) than those with 3-\(\beta\)-OH (\(J_{\rm H-2,\,H-3}\) = 10.0-10.8 Hz) isolated from the _Boswellia_ genus [15,19]. Considering the similar coupling constant between H-3 and H-2 (\(J_{\rm H-2,\,H-3}\) = 8.5 Hz in **7**) and biogenesis, H-3\(\beta\) of **7** was deduced. The obvious NOE correlations of H-11/H-13a, Me-20/Me-16, and H-3/Me-18 (Figure 5), and the absence of the NOE correlation of H-11/Me-20 in the NOESY spectrum of **7**, as well as the similar C-11-C-13 NMR data with those of boscartin \(Z\)[19], suggested that the Me-20, Me-18, and 11-OAc group were in \(\beta\)-orientation in **7**. The orientation of Me-19 was not determined because of a lack of convincing evidence. Therefore, the structure of **7** was established as shown and named boswellianol G.  
Boswellianol H (**8**) was isolated as a yellow oil, whose chemical formula was deduced as C\({}_{20}\)H\({}_{35}\)O\({}_{2}\) from HR-ESI-MS ion at \(m/z\) 307.2637 ([M + H]\({}^{+}\), calculated as 307.2632), yielding 4 degrees of unsaturation. The \({}^{1}\)H and \({}^{13}\)C NMR data of **8** (Tables 2 and 3) were close to those of decaryiol [35,36], a cembranoid bearing a C12-O-C15 tetrahydroproyran ring isolated from soft coral. The recognizable difference from decaryiol was the hydroxy group position. The \({}^{1}\)H-\({}^{1}\)H COSY correlations of H-11/H-10/H-9 and HMBC correlations from Me-20 to C-11, C-12, and C-13 indicated that the hydroxy group was located at C-11 in **8** (Figure 4). The NOESY interactions of H-11/Me-20, H-11/H-1, and Me-20/H-1 (Figure 5) suggested the \(\beta\)-orientation of H-1, H-11, and Me-20, which is similar to those of decaryiol [35,36]. Boswellianol I (**9**), a yellow amorphous solid, has the molecular formula of C\({}_{20}\)H\({}_{32}\)O\({}_{3}\) determined by HR-ESI-MS ion at \(m/z\) 321.2427 ([M + H]\({}^{+}\), calculated as 321.2424) with 5 degrees of unsaturation. Compound **9** has two less oxygens and two more degrees of unsaturation than **5**. The \({}^{1}\)H and \({}^{13}\)C NMR data of **9** (Tables 2 and 3) were similar to those of **5**, except that a double bond [\(\delta_{\rm H}\) 5.23 (1H, d, \(J\) = 16.3 Hz, H-3) and 5.55 (1H, d, \(J\) = 16.3 Hz, H-2)] was observed in **9**. The HMBC correlations from Me-18 to C-3/C-4/C-5 and from H-2 to C-1/C-14/C-15 fixed the location of the double bond at C-2/C-3. The remaining one degree of unsaturation in **9** results from the additional tetrahydrofuran ring (C11-O-C8), which was verified by HMBC correlation from H-11 to C-8 (Figure 4). The \(\beta\)-orientations of H-7, H-11, Me-18, Me-19, Me-20, and the isopropyl group were assigned by the cross peaks of H-15/Me-20, Me-20/H-11, Me-19/H-11, Me-19/H-7, and Me-17/Me-18 in the NOESY spectrum (Figure 5). Thus, the structure of **9** was established as shown (Figure 1).