## 2 Results and Discussion  
### Chemical Constituents from Olibanum of Boswellia carterii  
Olibanum was sequentially extracted with petroleum ether and methanol to provide non-polar and polar fractions. The polar methanolic fraction was then separated by silica gel column chromatography and further purified by MPLC and semi-preparative HPLC to obtain 12 diterpenoids (Figure 1), including nine undescribed diterpenoids, named boswellianols A-I (**1-9**). Three known compounds were identified as boscartin D (**10**) [27], boscartol O (**11**) [22], and encorubin A (**12**) [28] by comparing their NMR data with those of compounds reported in the literature. Boswellianol A (**1**) was isolated as a yellow oil with a molecular formula C\({}_{20}\)H\({}_{30}\)O\({}_{3}\) determined by HR-ESI-MS ion at \(m/z\) 319.2259 [M + H]\({}^{+}\) (calculated to be 319.2268), yielding six degrees of unsaturation. The \({}^{1}\)H spectrum of **1** (Table 1) showed characteristic resonances for the following: four methyl groups at \(\phi_{\rm H}\) 0.91 (3H, s, Me-12), 1.19 (3H, s,  
Figure 1: Chemical structures of compounds **1–12**. Me-14), 1.30 (3H, s, Me-11), and 1.84 (3H, s, Me-20); three olefinic protons at \(\delta_{\rm H}\) 5.80 (1H, d, \(J\) = 15.0 Hz, H-15), 6.42 (1H, dd, \(J\) = 15.0, 11.6 Hz, H-16), and 6.79 (1H, d, \(J\) = 11.6 Hz, H-17); and an aldehyde proton at \(\delta_{\rm H}\) 9.40 (1H, s, H-19). The \({}^{13}\)C and DEPT 135 spectra showed 20 carbon signals (Table 2) corresponding to four methyls, four methylenes, eight methines [including three alkenyl sp\({}^{2}\) carbons (\(\delta_{\rm C}\) 156.5, 120.3, and 150.0), one aldehyde carbon (\(\delta_{\rm C}\) 195.0), and one oxygenated sp3 carbon (\(\delta_{\rm C}\) 78.3)], and four quaternary carbons [including an alkenyl sp\({}^{2}\) carbon (\(\delta_{\rm C}\) 138.4) and an oxygenated sp\({}^{3}\) carbon (\(\delta_{\rm C}\) 72.1)]. The above spectroscopic data indicated that **1** is a diterpenoid with three rings (one aldehyde and two double bonds occupying three of the six degrees of unsaturation). The planar structure of **1** was elucidated by the analysis of HSQC, HMBC, and \({}^{1}\)H-\({}^{1}\)H COSY spectra (Figure 2). Two spin-coupling systems were first established by \({}^{1}\)H-\({}^{1}\)H COSY correlations of H-1/H\({}_{2}\)-2/H\({}_{2}\)-3 and H-5/H-6/H-7/H\({}_{2}\)-8/H\({}_{2}\)-9, shown as bold lines in Figure 2. HMBC correlations from Me-11 to C-3/C-4 /C-5 revealed the connectivity of C3-C4-C5. A 6/6 ring system was then assigned by HMBC correlations from Me-12 to C-1/C-5/C-9/C-10. A cyclopropyl moiety was indicated by the characteristic relatively shielded signals of C-6, C-7, and C-13 [\(\delta_{\rm C}\) 22.4 (C-6), 24.0 (C-7), 25.7 (C-7); \(\delta_{\rm H}\) 1.20 (m, H-6), 1.07 (m, H-7)] and the HMBC correlations from Me-14 to C-6/C-7/C-13 and H-6 to C-7/C-13, which further formed a 1,4-dihydroxy-maaliane fragment with a 6/6/3 ring system [29]. In addition, a prenylaldehyde moiety was suggested by the HMBC correlations from Me-20 to C-19 (\(\delta_{\rm C}\) 195.0)/C-18 (\(\delta_{\rm C}\) 134.8)/C-17 (\(\delta_{\rm C}\) 150.0) and \({}^{1}\)H-\({}^{1}\)H COSY resonances of H-17/H-16 (Figure 2). The NMR data on the prenylaldehyde moiety in **1** were consistent with those of boscardi F [16], further confirming the presence of the \(\alpha\), \(\beta\), \(\gamma\), \(\delta\)-unsaturated aldehyde in **1**. The 1,4-dihydroxy-maaliane unit and prenylaldehyde moiety were connected by C-15 (\(\delta_{\rm C}\) 156.5) and C-16 (\(\delta_{\rm C}\) 120.3), corresponding to the \({}^{1}\)H-\({}^{1}\)H COSY correlation of olefinic protons H-15/H-16 (Figure 2). The aforementioned data analysis elucidated that compound **1** is a new diterpenoid with an undescribed prenylmaaliane skeleton. The relative configuration was further analyzed by the NOESY spectrum.