Hypersampsones S–W, new polycyclic polyprenylated acylphloroglucinols from Hypericum sampsonii

Abstract

Five new polycyclic polyprenylated acylphloroglucinols (1–5) with a bicyclo[3.3.1]nonane skeleton, along with five known analogues (6–10), were isolated from the aerial parts of Hypericum sampsonii. Their structures, including the absolute configurations of 1–5, were elucidated by spectroscopic analysis, modified Mosher's method, ECD calculations and in situ dimolybdenum CD method. Besides, all the compounds were evaluated for RXRα transcriptional-inhibitory activities and cytotoxicity against HeLa cells.

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Introduction

Plants of the Clusiaceae (Guttiferae) family are a rich source of polycyclic polyprenylated acylphloroglucinols (PPAPs).^1,2 As a structurally intriguing class of natural products, PPAPs feature a characteristic bicyclo[3.3.1]nonane-2,4,9-tritone core structure, decorated with prenyl or geranyl side chains and an additional acyl group.^1,2 Apart from their fascinating chemical structures, a bewildering array of activities, including antitumor, antimicrobial, anti-HIV, antioxidant and antidepressant, have been established for PPAPs.² Thus, PPAPs have aroused intensive attention among natural product researchers in recent years.^3–13

Hypericum sampsonii, as a member of the Guttiferae family, is also rich in PPAPs.^14–22 In China, H. sampsonii has been used for the treatment of numerous disorders such as backache, burns, diarrhoea, snakebites and swellings.²³ Our previous study on H. sampsonii resulted in the characterization of five new decarbonyl PPAPs and seven new homoadamantanyl-type PPAPs.^20–22 Further investigation of this plant led to the isolation of five new PPAPs (1–5) with a bicyclo[3.3.1]nonane skeleton, along with five known analogues sampsonione P (6),¹⁷ sampsonione O (7),¹⁷ sampsonione N (8),¹⁷ sampsonione L (9),¹⁶ and hypersampsone H (10).¹⁸ In this paper, we report the isolation, structure elucidation, and biological activity evaluation of these new compounds (Fig. 1).

Fig. 1 Chemical structures of 1–5.

Results and discussion

Hypersampsone S (1) was obtained as colorless oil. Its molecular formula was determined as C₃₈H₅₀O₅ by HR-ESI MS at m/z 587.3734 [M + H]⁺ (calcd 587.3736), suggesting the existence of 14 degrees of unsaturation. ¹³C NMR and DEPT spectra showed 38 carbon signals including two carbonyl groups [δ_C 206.4, 192.9], a group of α,β-unsaturated olefinic carbon signals [δ_C 167.9, 123.5], a benzoyl group [δ_C 194.0, 137.2, 132.0, 128.4 × 2, 128.0 × 2], two isopentenyl groups [(δ_C 29.2, 125.0, 137.2, 18.2, 26.0), (δ_C 22.6, 120.6, 132.3, 18.1, 26.1)], five methyls, four methylenes, three methines, and five quaternary carbons. All the features mentioned above indicated that 1 was a derivative of PPAPs. NMR data of 1 were similar to those of 13-benzoyl-6,6,8,14,14-pentamethyl-11,15-di(3-methyl-2-butenyl)-9-oxatetracyclo[11.3.1.0^1,10.0^3,8]heptadec-10-ene-12,17-dione²⁴ with the same core structure of pyran fused bicyclo[3.3.1]nonane skeleton. HMBC correlations from H₃-37, H₃-38 to C-30/C-35/C-36, H-32 to C-31/C-33, H-33 to C-30/C-31/C-34/C-35, and those from H-34 to C-31 suggested that the lateral prenyl group was located at C-33 instead of C-30, and the six-membered ring was cyclized with a carbon–carbon bond formed between C-30 and C-36 (Fig. 2). Compared with the formula C₃₈H₅₀O₅, an additional hydroxyl group was left. The obvious downfield shift of C-35 (δ_C 77.6) indicated that the hydroxy group was located at C-35. Thus, the planar structure was deduced.

Fig. 2 Key ¹H–¹H COSY and HMBC correlations of 1–5.

The chemical shifts of Δδ_H (H₂-6), δ_C (C-7), δ_C (C-17) and δ_C (C-18) were useful in establishing the relative stereochemistry at C-7.^25,26 In compound 1, H₂-6 (Δδ_H 0.28) together with C-7 (δ_C 47.9), C-17 (δ_C 27.0) and C-18 (δ_C 22.4) indicated the axial orientation of the isopentenyl group at C-7. In the NOESY spectrum, the correlations of H₃-32/H₃-37 and H₃-37/H-34β suggested the cyclohexane moiety adopted a chair conformation, and H₃-32, H₃-37 as well as H-34β were located at the axial position on the same orientation. In addition, the NOESY correlations of H-30/H-33α, H-33α/H-35α and H-35α/H-30α indicated that these three protons were also situated at the axial position on the other side (Fig. 3). It was further supported by the large coupling constant observed between H-35 and H-34β (11.1 Hz). The absolute configuration at C-35 was deduced by the modified Mosher's method.²⁷ The Δδ values of the (S)- and (R)-MTPA esters (1a and 1b) indicated the S configuration for C-35 (Fig. 4). Therefore, the absolute configuration of 1 was determined as 1S,5R,7S,30S,31S,35S.

Fig. 3 Key NOESY correlations of 1 and 3.

Fig. 4 Δδ values (in ppm) = δ_S − δ_R values (in ppm) obtained for (S)- and (R)-MTPA esters (1a and 1b).

Hypersampsone T (2), colorless oil, had a molecular formula C₃₃H₄₂O₄ evidenced by HR-ESI MS (m/z 503.3158 [M + H]⁺, calcd 503.3161), indicating 13 degrees of unsaturation. Extensive analysis of the 1D and 2D NMR spectra revealed that 2 was closely related to the hypersampsone H (10),¹⁸ except for the loss of a prenyl group at C-33. It was further confirmed by ¹H–¹H COSY and HMBC correlations (Fig. 2). The chemical shifts of H₂-6 (Δδ_H 0.06), C-7 (δ_C 48.9), C-17 (δ_C 23.6) and C-18 (δ_C 27.4) suggested that H-7 was situated at the equatorial position.^25,26 Based on the known relative configuration, a pair of enantiomers (1R,5S,7S)-2a and (1S,5R,7R)-2b were calculated for their ECD spectra. As a result, the overall pattern of calculated ECD spectra of 2a were well matched the experimental data of 2 (Fig. 5). Therefore, the absolute configurations of 2 were established as 1R,5S,7S.

Fig. 5 Calculated and experimental ECD spectra of compound 2 and 3.

Hypersampsone U (3), colorless oil, gave the molecular formula C₃₃H₄₂O₅ with 13 degrees of unsaturation, as revealed by its HR-ESI MS at m/z 519.3115 [M + H]⁺ (calcd 519.3110). The NMR data of 3 were similar to those of sampsonione O (7). The ¹H–¹H COSY and HMBC correlations suggested that compound 3 had the same planar structure with 7 (Fig. 2). According to the chemical shifts of H₂-6 (Δδ_H 0.03), C-7 (δ_C 49.0), C-17 (δ_C 22.8) and C-18 (δ_C 26.9), the isopentenyl group at C-7 was also determined to be situated at the axial orientation. In the NOESY spectrum of 3, the correlations of H-25/H-6eq and H-29/H₃-28 suggested the β-orientation of H-25 (Fig. 3). The quantum chemical ECD calculation method was also used to determine the absolute configuration of 3. Calculated ECD curves of a pair of enantiomers 3a-(1R,5S,7S,25S) and 3b-(1S,5R,7R,25R), together with an experimental ECD curve are shown in Fig. 5. The overall pattern of the theoretically calculated ECD data of 3a were in agreement with the experimental ECD data, which indicated the assignment of the absolute configuration of 3 as 1R,5S,7S,25S.

Hypersampsone V (4) and W (5) were both obtained as colorless oil with the same molecular formula assigned as C₃₃H₄₄O₇ by HR-ESI MS (m/z 553.3157 [M + H]⁺ and 553.3171 [M + H]⁺, calcd 553.3165). The NMR data of 4 and 5 were identical to those of sampsonione P (6),¹⁷ except that the double bond Δ^21,22 was replaced by two hydroxy groups situated at C-20 and C-21 in 4 and 5. It was further confirmed by the correlations in the ¹H–¹H COSY and HMBC spectra. In the NOESY spectra of 4 and 5, the correlations between H-30 and H-6eq suggested the β-orientation of H-30 in both compounds. In addition, the small coupling constant of J_H-6ax,H-7 = 6.8 Hz in 4 and J_H-6ax,H-7 = 6.4 Hz in 5 indicated that H-7 was situated at the equatorial position in both compounds.²⁵ The absolute configurations of the 1,2-diol moiety in 4 and 5 were determined by in situ dimolybdenum CD method.^28,29 On the basis of the empirical rule proposed by Snatzke, the negative Cotton effect in 4 and the positive Cotton effect in 5 at around 310 nm (Fig. 6), observed in the induced CD spectrum, permitted the assignment of 20R and 20S absolute configurations for 4 and 5, respectively.

Fig. 6 The CD spectra of the Rh complex of 4 and 5 with the inherent CD spectrum subtracted.

Compounds 1–5 with bicyclo[3.3.1]nonane skeleton enriched the structural diversity of PPAPs, which also shed new light on the synthesis of PPAPs. The biogenetic pathway for 1–10 were proposed to be biosynthesized from the common precursor 2,4,6-trihydroxybenzophenone (i).¹⁶ It was further reacted with dimethylallyl diphosphate (DMAPP) followed by different ways of cyclization to yield the intermediate ii or iii. Compounds 1–10 were presumably biosynthesized from ii or iii through a series of reactions, such as epoxidation, intramolecular cyclization, dehydration, reduction, and so on (Fig. 7).^16,20,21

Fig. 7 Plausible biogenetic pathway for of 1–10.

Retinoid X receptor-α (RXRα), a unique intracellular target for pharmacologic interventions, regulates diverse biologic processes. The ligands for RXRα show promise as therapeutic agents for many diseases, such as cancer.³⁰ All the isolated compounds were evaluated for RXRα transcriptional-inhibitory activities and cytotoxicity against HeLa cells. As a result, compounds 6–8 (5–20 μM) showed potent RXRα transcriptional-inhibitory activities in a dose dependent manner. In addition, compounds 6–9 (5–20 μM) showed a dose-dependent cytotoxicity against HeLa cells, while compounds 1, 3 and 10 inhibited cell proliferation in HeLa cells at a concentration of 20 μM (Fig. 8).

Fig. 8 Effects of compounds 1–5 (5, 10, and 20 μM) on the cytotoxic effects.

Conclusions

In summary, ten PPAPs were isolated from H. sampsonii, including five new ones. Their structures were elucidated by spectroscopic analysis, modified Mosher's method, ECD calculation and in situ dimolybdenum CD method. Among them, compound 1 presents the first example of PPAPs using modified Mosher's method to solve its absolute configuration. Besides, all the compounds were evaluated for RXRα transcriptional-inhibitory activities and cytotoxicity against HeLa cells. As a consequence, compounds 6–8 showed dose dependent effects in both two bioassays. Preliminary structure–activity relationship (SAR) analysis have suggested that PPAPs with a double bond at C-20 and C-21 exhibited more potent effects than the ones replaced by two hydroxy groups, which was confirmed by the activity results that compound 6 showed more potent effects than its corresponding hydroxy group substituted ones 4 and 5. Besides, compound 10 showed more potent cytotoxic effects than 2 at a concentration of 20 μM, which indicated that the additional prenyl group at C-33 increased the cytotoxicity. Thus, the unsaturated hydrophobic prenyl group at C-7 may be therefore essential for potent cytotoxicity against HeLa cells. We also found that the number of unsaturated prenyl substitutes can affect the cytotoxic activity of PPAPs with bicyclo[3.3.1]nonane skeleton: the more prenyl groups, the more potent effect. Previous study also suggested that increasing the polarity of the prenyl group may actually reduce the cytotoxic activity of the prenylated xanthones,^31,32 which further confirmed our prediction. Furthermore, the biogenetic pathway for 1–10 were proposed, which enriched the awareness of biosynthesis of PPAPs with bicyclo[3.3.1]nonane skeleton in H. sampsonii.

Experimental

General experimental information

Optical rotations were measured on a Jasco P-1020 polarimeter with a 1 cm cell at room temperature. UV spectra were recorded on a JASCO V-550 UV/Vis spectrometer. IR spectra were obtained using a JASCO FT/IR-480 plus spectrometer. CD spectra were obtained on a Jasco J-810 spectropolarimeter at room temperature. HR-ESI-MS spectra were acquired using a Waters Synapt G2 mass spectrometer. The NMR spectra were measured with a Bruker AV-300/400/600 spectrometer at room temperature. Silica gel (200–300 mesh, Qingdao Marine Chemical Ltd., China), octadecylsilanized (ODS) silica gel (YMC Ltd., Japan), HP-20 (Mitsubishi Plastics, Inc. Japan), HW-40 (Tosoh, Japan) and Sephadex LH-20 (Amersham Pharmacia Biotech, Sweden) were used for open column chromatography (CC). TLC was performed on precoated silica gel plates (SGF254, 0.2 mm, Yantai Chemical Industry Research Institute, China). The semi-preparative HPLC was carried out on a Shimadzu LC-6AD Liquid Chromatography system with a SPD-20A Detector (Shimadzu, Japan) using a reversed-phase C18 column (5 μm, 10 × 250 mm; YMC, Japan).

Plant material

The aerial parts of Hypericum sampsonii were collected from Wuming, Guangxi Province, China, in June, 2012, and authenticated by Professor Songji Wei (Guangxi University of Traditional Chinese Medicine, Guangxi, China).

Extraction and isolation

The air-dried aerial parts of H. sampsonii (15 kg) were refluxed twice with 60% EtOH for 2 hours each time. The crude extract (1.4 kg) was column chromatographed over a macroporous resin HP-20 eluted with EtOH–H₂O in gradient. The 90% EtOH–H₂O eluent (198.2 g) was fractionated by silica gel column chromatography eluted with CHCl₃–CH₃OH (100 : 0 → 0 : 100) to afford twelve fractions (Fr. 1–12). Fr. 1 (4.7 g, CHCl₃/CH₃OH 100 : 0) was chromatographed on a silica gel column using cyclohexane–EtOAc gradient elution to yield 12 subfractions (Fr. 1.1–1.12). Fr. 1.4 (336.1 mg, C/E 95 : 5) was further purified by preparative HPLC on ODS column with 85% ACN–H₂O to yield compound 10 (5.7 mg). Fr. 1.5 (578 mg, C/E 9 : 1) was purified by preparative HPLC on ODS column with 90% MeOH–H₂O to afford compound 2 (5.8 mg). Fr. 5 (43.1 g, CHCl₃/CH₃OH 98 : 2) was chromatographed on a silica gel column using cyclohexane–EtOAc gradient elution to yield 11 subfractions (Fr. 5.1–5.11). Fr. 5.4 (7.1 g, C/E 9 : 1) was subjected to ODS column chromatography to afford 8 finefractions (Fr. 5.4.1–5.4.8). Fr. 5.4.5 (1.2 g, MeOH/H₂O 90 : 10) was further subjected to a silica gel column using cyclohexane–EtOAc–acetone gradient elution to yield 9 finefractions (Fr. 5.4.5.1–5.4.5.9). Fr. 5.4.5.2 was purified by preparative HPLC on ODS column with 85% MeOH–H₂O to afford 8 (11.7 mg). The subfraction Fr. 5.5 (1.1 g, C/E 85 : 15) was further subjected to ODS column chromatography eluted with MeOH–H₂O (70 : 30 → 100 : 0), and then purified by preparative HPLC on ODS column with 75% MeOH–H₂O to afford compound 6 (19.3 mg) and 9 (7.3 mg). Furthermore, the subfraction Fr. 5.6 (4.8 g, C/E 80 : 20) was subjected to ODS column chromatography eluted with MeOH–H₂O (70 : 30 → 100 : 0) to yield 9 finefractions (Fr. 5.6.1–5.6.9). The subfraction Fr. 5.6.6 (1.3 g, MeOH/H₂O 80 : 20) was further subjected to Sephadex LH-20 column chromatography eluted with MeOH and purified by preparative HPLC on ODS column with 80% MeOH–H₂O to yield compound 3 (6.0 mg). Fr. 5.6.7 was purified by preparative HPLC on ODS column with 75% ACN–H₂O to yield compound 7 (36.2 mg). Moreover, Fr. 5.6.8 was purified by preparative HPLC on ODS column with 80% MeOH–H₂O to yield compound 1 (24.3 mg). Fr. 7 (9.2 g, CHCl₃/CH₃OH 95 : 5) was subjected to ODS column chromatography eluted with MeOH–H₂O (40 : 60 → 100 : 0) to yield 13 subfractions. The subfraction Fr. 5.7.11 was further subjected to HW-40 column chromatography eluted with MeOH–H₂O (60 : 40 → 100 : 0), and then purified by preparative HPLC on ODS column with 40% ACN–H₂O to afford compound 4 (26.7 mg) and 5 (16.5 mg).

Hypersampsone R (1). Colorless oil; [α]²³_D +62.0 (c 0.50, CHCl₃); UV (CH₃OH) λ_max (log ε) 208 (4.11), 247 (3.95), 286 (3.92) nm; IR (KBr) ν_max 3435, 2924, 1722, 1697, 1630, 1592, 1448, 1222, 689 cm⁻¹; HR-ESI-MS m/z 587.3734 [M + H]⁺ (calcd for C₃₈H₅₁O₅, 587.3736). ¹H (300 MHz, CDCl₃) and ¹³C NMR (75 MHz, CDCl₃) data, see Table 1.

Table 1 ¹H (400 Hz) NMR and ¹³C NMR (100 Hz) data for 1–3 (CDCl₃)

No.	1^a		2^b		3^c
	δC	δH (J in Hz)	δC	δH (J in Hz)	δC	δH (J in Hz)
a Recorded at 75 M for ^13C and 300 M for ^1H.b Recorded at 75 M for ^13C and 600 M for ^1H.c Recorded at 100 M for ^13C and 300 M for ^1H.
1	77.0		71.4		78.1
2	192.9		167.9		188.7
3	123.5		113.8		118.4
4	167.9		195.4		177.0
5	52.6		64.0		54.1
6ax	40.0	2.10, m	41.1	2.15, dd (13.8, 7.8)	40.1	2.24, m
6eq		2.38, br.d (14.4)		2.05, br.d (13.8)		2.21, m
7	47.9	1.43, m	48.9	1.40, m	49.0	1.50, m
8	48.4		49.3		49.7
9	206.4		208.7		207.7
10	194.0		194.1		193.6
11	137.2		137.2		137.1
12, 16	128.4	7.45, dd (7.2, 1.2)	128.6	7.53, br.d (7.8)	128.5	7.56, dd (7.2, 1.5)
13, 15	128.0	7.20, br.t (7.2)	127.9	7.22, br.t (7.8)	128.1	7.26, br.t (7.2)
14	132.0	7.34, br.t (7.2)	132.1	7.38, br.t (7.8)	132.1	7.40, br.t (7.2)
17	22.4	1.41, s	23.6	1.46, s	22.8	1.47, s
18	27.0	1.32, s	27.4	1.38, s	26.9	1.32, s
19	29.2	2.09, m 1.22, m	29.8	2.22, br.d (16.2) 1.92, m	30.6	1.78, m 2.28, m
20	125.0	4.84, br.t (7.2)	125.1	4.84, br.t (7.2)	125.0	5.03, br.t (6.9)
21	132.7		132.9		132.6
22	18.2	1.51, s	18.1	1.51, s	18.2	1.55, s
23	26.0	1.65, s	26.3	1.65, s	26.0	1.66, s
24	22.6	2.92, dd (13.5, 7.2) 3.04, dd (13.5, 7.2)	16.5	2.35, m	28.2	2.86, m 2.82, m
25	120.6	4.96, br.t (7.2)	31.4	1.52, m 1.36, m	93.5	4.70, dd (11.1, 9.6)
26	132.3		80.5		71.0
27	18.1	1.56, s	25.3	0.49, s	25.3	1.25, s
28	26.1	1.58, s	27.8	1.22, s	27.1	1.42, s
29	23.1	3.12, br.d (14.4) 1.13, m	30.3	2.53, dd (14.4, 6.6) 2.48, dd (14.4, 7.2)	29.8	2.52, m
30	48.9	1.12, m	119.8	5.05, br.t (7.2)	118.4	5.11, br.t (7.5)
31	83.0		134.6		135.5
32	22.0	1.47, s	18.4	1.68, s	18.4	1.68, s
33	39.1	2.08, m 1.61, m	26.0	1.64, s	26.3	1.71, s
34	28.7	1.86, m 1.52, m
35	77.6	3.34, dd (11.1, 4.5)
36	39.0
37	14.5	0.80, s
38	27.5	1.04, s

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Hypersampsone T (2). Colorless oil; [α]²³_D +62.8 (c 0.50, CHCl₃); UV (CH₃OH) λ_max (log ε) 203 (4.32), 247 (4.07), 286 (3.89) nm; IR (KBr) ν_max 3406, 2969, 2924, 1699, 1615, 1446, 1386, 1219, 688 cm⁻¹; CD (CH₃OH) λ_max (Δε) 208 (0.81), 216 (1.09), 247 (−1.48), 266 (−0.19), 277 (−0.38), 306 (0.44) nm; HR-ESI-MS m/z 503.3158 [M + H]⁺ (calcd for C₃₃H₄₃O₄, 503.3161). ¹H (600 MHz, CDCl₃) and ¹³C NMR (75 MHz, CDCl₃) data, see Table 1.

Hypersampsone U (3). Colorless oil; [α]²³_D +28.2 (c 0.50, CHCl₃); UV (CH₃OH) λ_max (log ε) 208 (4.37), 248 (4.29), 277 (4.17) nm; IR (KBr) λ_max 3434, 2977, 2933, 1725, 1698, 1616, 1447, 1385, 1224, 688 cm⁻¹; CD (CH₃OH) λ_max (Δε) 224 (+0.33), 248 (−0.17), 270 (+0.62), 286 (+0.43), 296 (+0.52), 316 (−0.21), 343 (+0.36) nm; HR-ESI-MS m/z 519.3115 [M + H]⁺ (calcd for C₃₃H₄₃O₅, 519.3110). ¹H (300 MHz, CDCl₃) and ¹³C NMR (100 MHz, CDCl₃) data, see Table 1.

Hypersampsone V (4). Colorless oil; [α]²³_D +13.0 (c 0.50, CHCl₃); UV (CH₃OH) λ_max (log ε) 204 (4.45), 253 (4.31) nm; IR (KBr) λ_max 3438, 2978, 2931, 1732, 1673, 1627, 1449, 1372, 1212, 954 cm⁻¹; HR-ESI-MS m/z 553.3157 [M + H]⁺ (calcd for C₃₃H₄₅O₇, 553.3165); ¹H (400 MHz, CDCl₃) and ¹³C NMR (100 MHz, CDCl₃) data, see Table 2.

Table 2 ¹H NMR and ¹³C NMR data for 4–5 (CDCl₃)

No.	4^a		5^a
	δC	δH (J in Hz)	δC	δH (J in Hz)
a Recorded at 100 M for ^13C and 400 M for ^1H.
1	68.8		68.9
2	194.7		194.8
3	116.9		116.8
4	176.6		177.9
5	59.1		59.5
6ax	36.5	2.21, dd (14.4, 6.8)	38.2	2.25, dd (14.0, 6.4)
6eq		2.31, br.d (14.4)		2.83, br.d (14.0)
7	41.7	1.87, m	44.2	1.76, m
8	47.7		48.2
9	205.2		205.9
10	193.0		194.4
11	135.4		136.6
12, 16	129.2	7.64, br.d (7.6)	129.7	7.69, br.d (7.6)
13, 15	128.7	7.35, br.t (7.6)	128.8	7.37, br.t (7.6)
14	133.8	7.50, br.t (7.6)	134.1	7.52, br.t (7.6)
17	22.4	1.16, s	22.2	1.12, s
18	26.6	1.05, s	26.6	1.05, s
19	30.8	1.98, br.t (12.4) 1.44, br.t (12.4)	33.6	1.79, m 1.52, dd (10.4, 6.4)
20	77.0	3.21, br.d (10.0)	80.5	3.13, br.d (10.0)
21	73.5		73.4
22	24.3	1.17, s	23.8	1.18, s
23	25.7	1.23, s	26.0	1.15, s
24	25.5	2.63, m 2.42, dd (13.6, 4.8)	25.2	2.65, dd (8.8, 4.4) 2.40, dd (13.6, 4.4)
25	119.5	4.90, br.t (6.0)	119.6	4.96, br.t (6.4)
26	135.4		135.4
27	18.2	1.54, s	18.3	1.56, s
28	26.4	1.62, s	26.4	1.68, s
29	31.7	2.68, m 1.78, dd (13.2, 5.6)	30.6	2.68, m 1.75, m
30	92.4	4.61, dd (10.8, 5.6)	92.1	4.71, dd (10.8, 5.2)
31	70.8		70.7
32	24.0	1.05, s	24.3	1.10, s
33	26.8	1.08, s	27.0	1.18, s

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Hypersampsone W (5). Colorless oil; [α]²³_D +0.60 (c 0.50, CHCl₃); UV (CH₃OH) λ_max (log ε) 204 (4.44), 253 (4.29) nm; IR (KBr) λ_max 3436, 2973, 2939, 1732, 1670, 1627, 1451, 1369, 1213, 949 cm⁻¹; HR-ESI-MS m/z 553.3157 [M + H]⁺ (calcd for C₃₃H₄₅O₇, 553.3165). ¹H (400 MHz, CDCl₃) and ¹³C NMR (100 MHz, CDCl₃) data, see Table 2.

Preparation of (S)-MTPA (1a) and (R)-MTPA (1b) esters

A solution of 1 (0.5 mg) in pyridine-d₅ (0.5 mL) was treated with (S)-MTPA chloride (15 μL) under an atmosphere of nitrogen in an NMR tube. The mixture was stirred at room temperature for 4 h to obtain the (R)-MTPA ester (1b). The same procedure was used to prepare the (S)-MTPA ester (1a) with (R)-MTPA chloride.

Quantum chemical ECD calculation method

In theoretical calculations, the geometry of the molecules was optimized with Gaussian 09 package³³ at B3LYP/6-31G(d) computational level. The minimum nature of the structure was confirmed by frequency calculations at the same computational level. Then ECD calculations were carried out in the methanol solvent medium using time-dependent density functional theory (TDDFT) with B3LYP functional and DGDZVP basis set.

Absolute configuration of the 1,2-diol moiety in 4 and 5

HPLC grade DMSO was dried with 4 Å molecular sieves. According to the published procedure,^28,29 a mixture of 1 : 1.2 diol/Mo₂(OAc)₄ for 4 and 5 was subjected to CD measurement at a concentration of 1 mg mL⁻¹. The first CD spectrum was recorded immediately after mixing, and its time evolution is controlled with a rate of about one spectrum every 10 minutes, until a stationary ICD is reached (usually, 30–40 min after the mixing). The inherent CD was subtracted. The observed signs of the diagnostic bands at around 310 nm in the induced CD spectrum were correlated to the absolute configuration of the 1,2-diol moiety.

Acknowledgements

This project was financially supported by the Programme of Introducing Talents of Discipline to Universities (B13038), and grants from State Key Laboratory of Drug Research (SIMM1203KF-09).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR spectra of compounds 1–5, computational details of 2–3 and biological assays details. See DOI: 10.1039/c5ra26332h

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