NATURAL PRODUCT RESEARCH
3
2. Results and discussion
Compound 1 was isolated and purified as white amorphous power. Its positive HRESIMS
spectrum exhibited a sodium adduct molecular ion peak [M + Na]+ at m/z 525.2321 (Calcd
525.2306), corresponding to the molecular formula C24H38O11 with 6° of unsaturation. The
IR spectrum indicated the presence of alkyne (vmax 2209 cm−1) and alkene (vmax 1623 cm−1)
1
groups. The H NMR spectrum of 1 (Table S1, Supplementary material) displayed proton
signals corresponding to two oxygenated methine groups at δH 3.72 (1H, m, H-3) and 4.78
(1H, q, J = 6.5 Hz, H-9), two methylene moieties at δH 1.26 (1H, t, J = 12.1 Hz, H-2ax), 1.69 (1H,
ddd, J = 12.1, 3.7, 2.1 Hz, H-2 eq), 1.90 (1H, dd, J = 17.5, 9.7 Hz, H-4ax) and 2.25 (1H, dd,
J = 17.5, 5.6 Hz, H-4 eq), four methyl residues at δH 1.05 (3H, s, H-11), 1.09 (3H, s, H-12), 1.40
(3H, d, J = 6.5 Hz, H-10) and 1.80 (3H, s, H-13). Combining with HSQC spectrum of 1, the 13
C
NMR spectrum showed 13 signals of aglycone, assigned to four methyl groups at δC 22.2
(C-13), 22.5 (C-10), 28.4 (C-11) and 30.3 (C-12), two methylene moieties at δC 41.1 (C-4) and
46.4 (C-2), two oxygenated methine groups at δC 62.5 (C-3) and 62.6 (C-9) and five quaternary
carbons at δC 35.9 (C-1), 83.1 (C-7), 92.7 (C-8), 122.5 (C-6) and 138.3 (C-5). The above data,
together with 6° of unsaturation, implied that compound 1 should be a C13-norisoprenoid
(Cutillo et al. 2005; Picerno et al. 2008). Acid hydrolysis of 1 resulted in liberating d-glucose
and d-arabinose, which were analysed by TLC, ORD and GC, and the configuration of the
pentose was determined by Hara’s method (Hara et al. 1987; Seo et al. 2002; Sun et al. 2016).
The d-glucose and d-arabinose were confirmed to have β- and α-linkages by the coupling
constants of anomeric protons at δH 4.43 (1H, d, J = 7.8 Hz, H-1′) and 4.17 (1H, d, J = 6.2 Hz,
H-1″), respectively (Pei et al. 2011). In the HMBC spectrum of 1 (Figure S16, Supplementary
material), key correlations from H-10 (δH 1.40) to C-9 (δC 62.6) and C-8 (δC 92.7), from H-9 (δH
4.78) to C-8 and C-7 (δC 83.1), from H-2 (δH 1.26, 1.69) and H-4 (δH 1.90, 2.25) to C-3 (δC 62.5),
and from H-13 (δH 1.80) to C-5 (δC 138.3) and C-6 (δC 122.5) indicated that two oxygenated
groups, an alkynyl moiety, and an alkenyl residue were substituted at C-9, C-3, C-7 and C-5
positions, respectively. Also, the correlations of H-1′ (δH 4.43) with C-9 (δC 62.6) and H-1″ (δH
4.17) with C-6′ (δC 64.8) indicated that the linkage points of the glucose and arabinose units
were at C-9 and C-6′, respectively. Based on the above data and comprehensive 2D NMR
experiments (1H-1H COSY, HSQC-TOCSY and HMBC), the structure of 1 was unambiguously
assigned as shown in Figure 1. According to the literature, in the 13C NMR spectra of 1 in D2O
(Table S1, Supplementary material), the chemical shift (δC 68.2) of C-9 indicated that the
absolute configuration of C-9 was R (Yamano et al. 2002). The relative stereochemistry at C-3
in the aglycone group of 1 was identified by a large coupling constant of H-4ax (J = 9.7 Hz).
This implied that H-3 must be in the axial position (Yue et al. 2012). C13-norisoprenoids are
biosynthetically formed in nature from the hydrolytic breakdown of complex secondary
metabolites derived from carotenoids which have R-configuration for C-3 (Yamano et al.
2000; Baumes et al. 2002;Yamano et al. 2002). Thus, compound 1 was determined as (3R,9R)-
3-hydroxy-7,8-didehydro-β-ionyl 9-O-α-d-arabinopyranosyl-(1→6)-β-d-glucopyranoside.
Compound 2, white amorphous powder, had the molecular formula C17H26O10 as deduced
by analysis of positive HRESIMS (m/z 413.1442 [M + Na]+, Calcd 413.1418). Hydrolysis of 2
with β-glucosidase produced an aglycone and β-d-glucose, which was identified by the
positive optical rotation[ꢀ]2D3 = +48.6 (c 0.2, H2O) (Jiang et al. 2015). The 1H NMR spectrum
of 2 (Table S2, Supplementary material) indicated the presence of a 1,3,4,5-tetrasubstituted
benzene ring residue [δH 6.52 (2H, s, H-2′, 6′)], two methoxyl groups [δH 3.74 (6H, s, -OCH3-3′,