## 2 Results and Discussion
### Chemical Constituents from Olibanum of Boswellia carterii
On the other hand, the oxygenation, 1,5-cyclisation(shown as the red arrow in Figure 6), and double bond rearrangement of encorubin Y could produce the prenylaromadendrane-type diterpenoid, encorubin X [33; 34], which could be further oxidized to form compound **2** (Figure 6). Compound **3** shared the same formula as 2 deduced by its sodium adduct ion at \(m/z\) 343.2250 [M + Na]\({}^{+}\) (calculated for C\({}_{20}\)H\({}_{32}\)NaO\({}_{3}\) as 343.2244) and protonated ion at \(m/z\) 321.2411 [M + H]\({}^{+}\) (calculated for C\({}_{20}\)H\({}_{33}\)O\({}_{3}\) as 321.2424), indicating that **3** could be an isomer of **2** bearing the same prenylaromadendrane skeleton. The \({}^{1}\)H and \({}^{13}\)C NMR spectrum of 3 showed similar proton and carbon chemical shifts of aromadendrane moiety with **2** (Tables 1 and 2). The major difference between the two compounds was the premyl group side chain. The 2,3-dihydroxy-2-methylpent-4-ene side chain in **3** was established by \({}^{1}\)H-\({}^{1}\)H COSY correlation between H-15 (\(\delta_{H}\) 5.36, d, \(J\) = 15.5 Hz), H-16 (\(\delta_{H}\), 5.41, dd, \(J\) = 15.5, 7.6 Hz), and H-17 (\(\delta_{H}\) 3.84 (1H, d, \(J\) = 7.6 Hz), as well as HMBC correlations from H-17 to C-18/C-19/C-20 (Figure 4). The NOESY correlations of Me-14/H-5, H-6/H-7, and Me-11/ H-1, H-6 (Figure 5) suggested that the relative configuration of the aromadendrane moiety in **3** was the same as that of **2**. Because of the free rotation of the premyl moiety, the configuration of C-17 was not determined in this study. Therefore, the structure of **3** was determined as shown and named boswellianol C.  
Boswellianol D (**4**) has the same molecular formula of C\({}_{20}\)H\({}_{30}\)O\({}_{3}\) as **1**. The 1D and 2D NMR data demonstrated that **4** shared the same prenylaromadendrane skeleton as **2** and **3**. The presence of the four methyl singlets, a quaternary oxygenated carbon at C-10 (\(\delta_{\rm C}\) 74.8), and the absence of two terminal olefinic methylene protons in **4** showed that the exocyclic double bonds at C-10 of **2** and **3** were oxidized, which was proved by HMBC correlations from Me-12 (\(\delta_{H}\) 1.24, s) to C-1, C-9, and C-10. The remarkable \(\alpha\), \(\beta\)  
Figure 6: The plausible biosynthesis pathway of compounds **1** and **2**. \(\gamma\), \(\delta\)-unsaturated aldehyde fragment was established by the \({}^{1}\)H-\({}^{1}\)H COSY correlation of H-15/H-16/H-17, and key HMBC correlation from H-19 to C-17/C-18/C-20 and H-20 to C-17/C-18/C-19 (Figure 4). The 15\(E\) double bond was confirmed by the big coupling constants (\(J_{15,16}\) = 15.1 ). The 17\(E\) was suggested by the NOE correlation of H-17/H-19 and the NMR data comparison with those of **1** and boscartl F [16]. The relative configuration of compound **4** was defined by NOESY interactions of H-1/H-11, H-12 and H-6/H-1, H-7 suggesting the same \(\alpha\)-orientation as **2** and **3** (Figure 5). The molecular formula of boswellianol E (**5**) was deduced as C\({}_{20}\)H\({}_{34}\)O\({}_{5}\) by HR-ESI-MS [M + Na]\({}^{+}\) ion peak at \(m/z\) 379. 2452 (calculated to be 379.2455), yielding three degrees of unsaturation. The \({}^{1}\)H NMR spectrum (Table 3) revealed three methyl singlets [\(\delta_{\rm H}\) 1.12 (3H, s, Me-20), 1.16 (3H, s, Me-19), and 1.18 (3H, s, Me-18)], two methyl doublets [\(\delta_{\rm H}\) 0.94 (3H, d, \(J\) = 6.8 Hz, H-17) and 0.90 (3H, d, \(J\) = 6.8 Hz, Me-16)], and three protons at oxygen-bearing methines [\(\delta_{\rm H}\) 3.81 (1H, d, \(J\) = 7.2 Hz, H-3), 3.90 (1H, dd, \(J\) = 10.6, 5.0 Hz, H-7), and 4.25 (1H, dd, \(J\) = 11.4, 2.7 Hz, H-11)]. The \({}^{13}\)C, HSQC, and DEPT 135 spectra gave five methyls, four methanes, seven methylenes, and four quaternary carbons. By comparison with the MS and NMR data of boscartin R (a cembrane-type diterpenoid from olibanum) [19], it was believed that **5** was the 11-deacetylation product of boscartin R. The 2D NMR data analysis (Figures 4 and 5) further established the structure of **5** as shown.