Isolation of Jasminumosides F–K (1–6)

The Oleaceae plant J. sambac, popularly known as jasmine, originates from southern and southeastern Asia. The flowers of this plant are commonly used in the preparation of an essential oil and to make the beverage jasmine tea, as well as in folk medicine for the treatment of diarrhea, abdominal pain, conjunctivitis, asthma, cancer, toothache, dermatitis, and wound healing [22]. We previously reported the isolation of 14 secoiridoids (7–20), including five new oligomeric secoiridoid glycosides, jasminumosides A–E (11–15), and a monoterpene (21), from the methanol extract of the flowers of J. sambac. In addition, the extract and the principal oligomeric secoiridoid glycosides sambacoside A (7) and molihuasides C (16) and D (17) were found to exhibit hepatoprotective activity against d-galactosamine/lipopolysaccharide-induced liver injury in a mouse model [21]. In the present study, we isolated six new oligomeric secoiridoid glycosides, including jasminumosides F (1, 0.0106%, from plant material), G (2, 0.0249%), H (3, 0.0209%), I (4, 0.0039%), J (5, 0.0083%), and K (6, 0.0039%), using normal-phase silica gel and reversed-phase ODS column chromatography, followed by preparative HPLC, as shown in Fig. 1.

Fig. 1The alternative text for this image may have been generated using AI.

Structures of iridoid constituents including jasminamosides F–K (1–6) obtained from the flowers of J. sambac

Structure determinationJasminumoside F (1)

Jasminumoside F (1) was obtained as a white powder with a negative optical rotation ([α]D25 − 156.0 in MeOH). In the UV spectrum of 1, an absorption maximum was observed at 235 nm (log ε 4.46), while the IR spectrum showed absorption bands at 1717 and 1636 cm–1, indicating the presence of the chromophore –OC(= O)–C = CH–O– moiety [23,24,25], as well as broad bands at 3393 and 1076 cm–1 indicative of a glycoside structure. According to positive-ion ESIMS, a quasi-molecular ion peak was observed at m/z 1371 [M + Na]+, while HRESIMS analysis indicated a molecular formula of C60H84O34. Further treatment of 1 with 1 M HCl liberated d-glucose, which was identified by HPLC using an optical rotation detector [21, 26, 27]. The 1H and 13C NMR spectra (CD3OD, Table 1), which were assigned with the aid of distortionless enhancement by polarization transfer (DEPT), 1H-1H homonuclear correlation (COSY), heteronuclear multiple quantum coherence (HMQC), and heteronuclear multiple-bond correlation (HMBC) experiments (Fig. 2), exhibited triple signals for the protons of the above-mentioned chromophore [δ 7.49, 7.516, 7.521 (1H each, all s, H-3, 3’’’, 3’’’’’)], methylenes [δ 2.48 (1H, dd, J = 8.9, 14.4 Hz), 2.49 (2H, dd, J = 8.9, 14.4 Hz), 2.72 (1H, dd, J = 4.6, 14.4 Hz), 2.73 (1H, dd, J = 4.6, 14.4 Hz), 2.76 (1H, dd, J = 4.6, 14.4 Hz), H2-6, 6’’’, 6’’’’’], methines [δ 3.99, 3.99, 3.99 (1H each, all m, H-5, 5’’’, 5’’’’’)], ethylidene groups [δ 1.74, 1.74, 1.74 (3H each, all d, J = 6.9 Hz, H3-10, 10’’’, 10’’’’’), 6.10, 6.10, 6.10 (1H each, all q, J = 7.1 Hz, H-8, 8’’’, 8’’’’’)], acetals [δ 5.92, 5.92, 5.93 (1H each, all br s, H-1, 1’’’, 1’’’’’)], and β-glucopyranosyl parts [δ 4.81, 4.81, 4.81 (1H each, all d, J = 7.8 Hz, H-1’, 1’’’’, 1’’’’’’)] along with signals due to a methyl [δ 1.04 (3H, d, J = 6.6 Hz, H3-6’’)], methylene, and three methylene groups bearing an oxygen function , four methine groups and a methine bearing an oxygen function [δ 1.65, 1.67, 2.07, 2.09 (1H each, all m, H-1’’, 2’’, 8’’, 3’’), 3.69 (1H, m, H-5’’)]. Additionally, two carbomethoxy signals [δ 3.71, 3.71 (3H each, both s) and δC 52.0, 52.0 (COOCH3), and δC 168.6, 168.6 (COOCH3)] and a free carboxyl group [δC 170.1 (COOH)] were observed in the 1H and 13C NMR spectra of 1. These signals were superimposable with those of sambacoside E (8) [21, 25], except for the absence of two methyl ester moieties. Methylation of 1 with trimethylsilyldiazomethane (TMSCHN2) produced 8. Accordingly, the connectivity of the three oleoside ester units to the 5’’,7’’,9’’,10’’-tetraol monoterpene (21) moiety in 1 were clarified to be at the C-7’’, C-9’’, and C-10’’ positions.

Table 1 1H and 13C NMR spectroscopic data (CD3OD) of Jasminumoside F (1) and sambacoside E (8)Fig. 2The alternative text for this image may have been generated using AI.

1H–1H COSY, HMBC, and pROESY correlations of 1

After treatment of 8 and 1 with diethylamine (Et2NH)–MeOH, several partial hydrolysates with common cleaved positions were obtained. As shown in Fig. 3, the 9’’- and 10’’-monodesacyl derivatives, molihuasides D (17, tR: 22.76 min) [21, 28] and E (18, tR: 22.26 min) [21, 28], as well as the 9’’,10’’-bisdeacyl derivative (20, tR: 16.40 min) [21, 29] were identified from 8 together with oleoside 7,11-dimethyl ester (22a, tR: 15.17 min) [21, 30]. The common partial hydrolysates 17, 20, and 22a were also identified upon a similar alkaline hydrolysis of 1 along with the mono-demethyl ester derivative of molihuaside E [1a, tR: 20.57 min, m/z: 985.3507 [M + Na]– (calcd for C43H62O24Na, 985.3523)] and oleoside 7-methyl ester (22b) [21, 28]. Subsequent methylation of 1a afforded 18 (Fig. 4). Consequently, the structure of jasminumoside F was determined to be the 11’’’-monodemethyl ester derivative of sambacoside E (1) and the absolute configurations were assigned by comparison with previously reported sambacosides and monoterpene (21) [21, 28, 29].

Fig. 3The alternative text for this image may have been generated using AI.

Partial alkaline hydrolysis of 8. LC-MS conditions: Instruments: Thermo Fisher Scientific Ultimate + EvactivePlus, Column: Cosmosil 5C18-MS-II (2.0 mm i.d. × 150 mm), HPLC detection: UV (230 nm), Mobile phase: 0–5 min hold MeOH–H2O (25:75, v/v) → 40–50 min hold MeOH, Flow rate: 0.2 mL/min, Injection: 2 µL, Ionization mode: ESI, positive

Fig. 4The alternative text for this image may have been generated using AI.

Methylation and partial alkaline hydrolysis of 1. LC-MS conditions: Instruments: Thermo Fisher Scientific Ultimate + EvactivePlus, Column: Cosmosil 5C18-MS-II (2.0 mm i.d. × 150 mm), HPLC detection: UV (230 nm), Mobile phase: 0–5 min hold MeOH–H2O (25:75, v/v) → 40–50 min hold MeOH, Flow rate: 0.2 mL/min, Injection: 2 µL, Ionization mode: ESI, positive

Jasminumoside G (2)

Jasminumoside G (2) was obtained as a white powder with a negative optical rotation ([α]D25 − 173.7 in MeOH). A quasi-molecular ion peak was observed at m/z 1357 [M + Na]+ by positive-ion ESI-MS analysis, while HRESIMS measurements revealed a molecular formula of C59H82O34. Acid hydrolysis of 2 with 1 M HCl liberated d-glucose. The 1H and 13C NMR (Table 2) spectroscopic properties of 2, which were assigned with the aid of the various 2D NMR experiments (Fig. 5), were superimposable with those of sambacoside F (9) [25], except for the absence of two carbomethoxy signals, triple signals for the secoiridoid glycosidic ester part together with the 5’’,7’’,9’’,10’’-tetraol monoterpene (21) moiety , four methine and a methine bearing an oxygen function [δ 1.77, 1.89, 1.91, 2.05 (1H each, all m, H-2’’, 8’’, 1’’, 3’’), 4.66 (1H, m, H-5’’)]}}. Additionally, a carbomethoxy signal [δ 3.71 (3 H, s) and δC 52.1 (COOCH3), and δC 168.7 (COOCH3)] and two free carboxyl groups [δC 170.1, 170.1 (COOH)] were observed in the 1H and 13C NMR spectra of 2. Because the methylation of 2 provided 9, the connectivity of the three ester bonds between the 7-ester carbonyl groups in the three secoiridoid glycoside units and the common monoterpene (21) moiety in 2 were determined to be the same as those in 9. Next, since one of the three secoiridoid glycoside units in 2 exhibited two free carboxyl groups at the 11-position, chemical conversion of 2 into molihuaside A [28] via 2a was performed. As shown in Fig. 6, partial hydrolysis of 2 with Et2NH–MeOH provided the 5’’,7’’-diacyl ester derivative of 21 bearing the secoiridoid glycoside unit with a free carboxyl group (2a), which was identified by ESI-MS [m/z: 971.3338 [M + Na]– (calcd for C42H60O24Na, 971.3367)] together with 22a. Subsequently, the TMSCHN2 methylation of 2a provided molihuaside A, which was also obtained upon treatment of 7 with 0.1% NaOMe–MeOH. Based on this evidence, the structure of jasminumoside G was determined to be the 11,11’’’-bisdemethyl ester derivative of sambacoside F (2).

Table 2 1H and 13C NMR spectroscopic data (CD3OD) of Jasminumoside G (2) and sambacoside F (9)Fig. 5The alternative text for this image may have been generated using AI.

1H–1H COSY, HMBC, and pROESY correlations of 2–6

Fig. 6The alternative text for this image may have been generated using AI.

Chemical conversions of 2, 3, 5, and 7

Jasminumosides H (3) and I (4)

Jasminumosides H (3) and I (4) were also obtained as white powders with negative optical rotations (3: [α]D25 − 161.5, 4: [α]D25 − 162.2 both in MeOH). In the positive- and negative-mode ESI-MS spectra of 3 and 4, similar quasi-molecular ion peaks were observed at m/z 1357 [M + Na]+ and m/z 1333 [M – H]–, while the molecular formula C59H82O34 was determined by HRESIMS measurements. Acid hydrolysis of 3 and 4 liberated d-glucose. The 1H and 13C NMR spectra (Table 3) of 3 showed triple protons signals for the characteristic iridoid chromophore (vide ante) [δ 7.47 (1H, s, H-3’’’), 7.511, 7.517 (1H each, both s, H-3, 3’’’’’)], methylenes , methines [δ 3.33 (1H, m, H-5’’’), 3.98, 3.98 (1H each, both m, H-5, 5’’’’’)], two ethylidene and a mono-substituted olefin groups , acetals [δ 5.47 (1H, d. J = 4.1 Hz, H-1’’’), 5.92, 5.92 (1H each, both br s, H-1, 1’’’’’)], and β-glucopyranosyl parts [δ 4.66 (1H, d, J = 8.0 Hz, H-1’’’’), 4.81, 4.82 (1H each, both d, J = 7.8 Hz, H-1’, 1’’’’’)] along with signals due to the common monoterpene (21) moiety }. Additionally, a carbomethoxy signals [δ 3.67 (3H, s) and δC 52.1 (COOCH3), and δC 168.8 (COOCH3)] and two free carboxyl group [δC 170.1, 170.1 (COOH)] were observed in the 1H and 13C NMR spectra of 3. As shown in Fig. 5, the 1H-1H COSY spectrum of 3 indicated the presence of partial structures, as denoted by the thick lines. In the HMBC spectrum of 3, long-range correlations were observed between the H-5’’ and H2-9’’ protons and the ester carbonyl carbons assignable to two oleoside units [δC 170.1, 170.1 (C-7, 7’’’’’)], and between the H2-7’’ protons and the ester carbonyl carbons assignable to the secologanoside 11-methyl ester unit, which were quite similar to those of molihuaside B (10) [δ 3.68 (3H, s) and δC 51.8 (COOCH3), and δC 168.8 (C-7’’’) [28]. Finally, the methylation of 3 provided 10), resulting in the connectivities of the ester bonds between the 7-ester carbonyl groups in the two oleoside units and the 5’’- and 10’’-positions and between the 7-ester carbonyl part in a secologanoside 11-methyl ester unit and the 7’’-position in 21. Therefore, the structure of jasminumoside J was determined to be the 11,11’’’’’-bisdemethyl ester derivative of molihuaside B (3). The proton and carbon signals in the 1H and 13C NMR spectra (Table 4) of 4, which were assigned with the aid of various 2D NMR experiments (Fig. 5), were indicative of the same units as those of 21. Furthermore, by comparing the 13C NMR data for 4 with those of 21, the signals due to C-5’’ (δC 83.1), C-7’’ (δC 67.9), and C-9’’ (δC 65.6) in 4 were observed at lower fields compared to those of 21 [δC C-5’’ (δC 79.6), C-7’’ (δC 66.1), and C-9’’ (δC 63.1)]. In contrast, the signals due to C-1’’ (δC 44.2), C-2’’ (δC 49.1), C-4’’ (δC 35.0), and C-8’’ (δC 44.4) were observed at higher fields compared with those of 21 [C-1’’ (δC 46.2), C-2’’ (δC 52.0), C-4’’ (δC 37.7), and C-8’’ (δC 48.4)] [21, 29]. Based on these acylation shifts, the connectivities of the secoiridoid ester moieties at the C-5’’, C-7’’, and C-9’’ positions were determined, as shown in Table 4. The stereostructure was characterized by rotating-frame nuclear Overhauser enhancement spectroscopy (ROESY), which revealed rotating-frame nuclear Overhauser effect (ROE) correlations between the following proton pairs: H-1’’ [δ 1.91 (1H, m)] and H-3’’ [δ 2.05 (1H, m)]; H-2’’ [δ 1.77 (1H, m)] and H3-6’’ [δ 1.05 (3H, d, J = 7.0 Hz)], H-8’’ [δ 1.89 (1H, m)], H2-9’’ [δ 4.13 (2H, d, J = 6.2 Hz)]; H-3’’ and H-5’’ [δ 4.66 (1H, m)], H2-7’’ [δ 4.05 (1H, dd, J = 7.0, 11.2 Hz), 4.15 (1H, dd, J = 5.5, 11.2 Hz)]; H-5’’ and H3-6’’ (Fig. 5). Thus, the structure of jasminumoside I (4) was elucidated.

Table 3 1H and 13C NMR spectroscopic data (CD3OD) of Jasminumoside H (3) and Molihuaside B (10)Table 4 1H and 13C NMR spectroscopic data (CD3OD) of Jasminumoside I (4) and 21Jasminumosides J (5) and K (6)

Jasminumoside J (5) was obtained as a white powder with a negative optical rotation ([α]D25 − 171.8 in MeOH). The molecular formula of C59H82O33 was confirmed by HRESIMS measurements [m/z 1341.4614 [M + Na]+ (calcd for C59H82O33Na, 1341.4631)]. Acid hydrolysis of 5 liberated d-glucose, as identified by HPLC analysis. The 1H and 13C NMR (Table 5) spectra of 5 contained signals assignable to triplet signals for the oleoside ester part together with the monoterpene triol moiety , four methine and a methine bearing an oxygen function [δ 1.68, 1.94, 1.85, 1.85, 4.65 (1H each, all m, H-2’’, 1’’, 8’’, 3’’, 5’’)]}}. Additionally, carbomethoxy signals [δ 3.71 (3 H, s) and δC 52.0 (COOCH3), and δC 168.6 (COOCH3)] were observed in the 1H and 13C NMR spectra of 5 along with signals from two free carboxyl groups [δC 170.2, 170.2 (COOH)]. The 1H and 13C NMR spectroscopic properties of 5 were very similar to those of sambacoside A (7) [21, 25], except for the signals due to the 9’’-hydroxymethyl group in the monoterpene moiety of 7 being replaced by a methyl group and absence of two carbomethoxy signals, which were assigned according to 1H–1H COSY, HMBC, and ROESY analyses (Fig. 5). The molecular formula of jasminumoside K (6) was determined to be C61H86O33 using positive- and negative-mode ESI-MS and HR-ESI-MS measurements. The signals in the 1H and 13C NMR (Table 5) spectra of 6 were compared with those of 5 were observed quite similar except for the number of carbomethoxy signals [δ 3.71, 3.71, 3.72 (3 H each, all s) and δC 52.0, 52.0, 52.0 (COOCH3), and δC 168.6, 168.6, 168.6 (COOCH3)]. Finally, the methylation of 5 provided 6, enabling to identify the structure of 6 as that of the 9’’-dehydroxy analog of sambacoside A.

Table 5 1H and 13C NMR spectroscopic data (CD3OD) of Jasminumosides J (5) and K (6)