## 2 Results and Discussion
### Chemical Constituents from Olibanum of Boswellia carterii

Table 2: \({}^{13}\)C NMR (150 MHz) spectroscopic data of compounds **1–9** (\(\delta\) in ppm, CDCl\({}_{3}\)). Figure 2: Key \({}^{1}\)H–\({}^{1}\)H COSY, HMBC, and NOESY correlations of compound **1**. Compound **2** was obtained as a white amorphous solid with the molecular formula C\({}_{20}\)H\({}_{32}\)O\({}_{3}\) determined by HR-ESI-MS ion at \(m/z\) 343.2232 [M + Na]\({}^{+}\) (calculated for C\({}_{20}\)H\({}_{32}\)NaO\({}_{3}\) as 343.2244) and 321.2410 [M + H]\({}^{+}\) (calculated for C\({}_{20}\)H\({}_{33}\)O\({}_{3}\) as 321.2424). Its \({}^{1}\)H NMR spectrum (Table 1) displayed three methyl singlets at \(\delta_{\rm H}\) 1.10 (3H, s, Me-14), 1.28 (3H, s, Me-11), and 1.82 (3H, s, Me-20), two vinyl singlets at \(\delta_{\rm H}\) 4.70 (1H, s, H-12a) and 4.72 (1H, s, H-12b), an olefinic doublet at \(\delta_{\rm H}\) 5.34 (1H, d, \(J\) = 8.0 Hz, H-17), and a proton at oxygen-bearing meth at \(\delta_{\rm H}\) 4.69 (1H, overlap, H-16). \({}^{13}\)C NMR and DEPT 135 spectra showed 20 carbon resonances (Table 2) including two double bonds at \(\delta_{\rm C}\) 106.6, 131.1, 138.6, and 153.5, a secondary alcohol at \(\delta_{\rm C}\) 62.1, and a quaternary oxygenated carbon at \(\delta_{\rm C}\) 80.7. The \({}^{1}\)H and \({}^{13}\)C NMR data of **2** (Tables 1 and 2) were similar to those of encrobin A (**12**), a known prenylaromadendrane-type [28] diterpenoid from dibanum, except that a hydroxymethyl group [\(\delta_{\rm C}\) 62.1 (C-19), \(\delta_{\rm H}\) 4.21 (d, \(J\) = 12.2 Hz, H-19a), 4.08 (d, \(J\) = 12.2 Hz, H-19b)] in **2** replaced the methyl group (C-19) in **12**. HMDEC correlation (Figure 4) from H-19 to C-17/C-18/C-20 confirmed the hydroxymethyl group at C-19 in **2**. Detailed 2D NMR data analysis further determined the planar structure of **2**. The NOE enhancement of Me-14/H-5 indicated that Me-14 and H-5 were \(\beta\)-oriented, while H-1, H-6, H-7, and Me-11 were \(\alpha\)-oriented according the NOESY correlations of H-1/H-6, H-6/H-7, and H-6/Me-11 (Figure 5). It was observed that the NMR data of C-16 to C-19 in **2** and the large coupling constant (\(J\) = 8.0 Hz) between H-16 and H-17 were identical with those of boscartol E, a similar compound with the same prenyl moiety isolated from this plant [16]. Considering the same skeleton and biogenetic pathway, the orientation of 16-OH was deduced as 16-\(\beta\)-OH, which is the same as that of boscartol E [16]. The 17-\(Z\) configuration was assigned by the NOESY correlations of H\({}_{2}\)-19/H-16 and H-17/Me-20 (Figure 5) as well as the similar NMR data of C-16 to C-19 between **2** and boscartol E [16]. Thus, the structure of **2** was established as shown and named boswellianol B.  
Figure 3: Experimental and calculated ECD spectra of **1**. Figure 4: Key \({}^{1}\)H-\({}^{1}\)H COSY and HMBC correlations of compounds **2–9**. Figure 5: Key NOESY correlations of compounds **2–9**. Compound **1** is the first example of a preynlmaalanine-type diterpenoid. The biosynthesis pathway of prenylaromadendrane-type diterpenoids is unclear. It was proposed (Figure 5) that compounds **1** and **2** could originate from the same intermediate encorubin Y, a bicyclogermacrene-type diterpenoid, which should be formed from geranylgeranyl pyrophosphate (GGPP) by enzyme-catalyzed cyclization and deprotonation [31; 32]. The 1,3-deprotonation and cyclization of intermediate **i** could be essential steps to construct the primary scaffold with a cyclopropane ring in these diterpenoids. The key step to form the 6,6-bicyclic system of the prenylmaalanine-type intermediate **iii** from encorubin Y by oxygenation and 1,6-cyclisation (shown as the blue arrow in Figure 6) would be similar to the reported enzyme-catalyzed cyclization to generate the maalanine-type sesquiterpene skeleton from bicyclogermacrene-type sesquiterpenoids [33; 34]. Subsequently, the intermediate iii may undergo further oxygenation and deprotonation to form compound **1**.