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Abstract

The first synthesis of a natural triterpenoid saponin bearing N-acetylglucosamine, albidoside A, which is isolated from the roots of Acacia albida, is concisely achieved in a convergent strategy. Preliminary pharmacological research shows anticancer activity against HL-60, MCF-7, MDA-MB-231, Hep-2, and Hela cell lines. In particular, it exhibited good selectivity, which is five times more cytotoxic toward Hep-2 cells (IC₅₀ = 8.91 μM) than the Hela cell line.

Keywords

albidoside A oleanolic acid bidesmoside saponin glycosylation antitumor activity

Introduction

Triterpene saponins, which can be found as glycosylated secondary metabolites in natural medicinal plants and some lower marine species, have tremendous structural diversity and impart a multitude of biological and physiological activities, such as antiviral, anti-HIV, antitumor, and anti-inflammatory.^1–6 Triterpene saponins containing an N-acetylglucosamine moiety are rare, but these naturally occurring saponins have been drawing significant attention due to their remarkable cytotoxicity and antiproliferative effects. In 2001, Yu and coworkers reported the synthesis of an N-acetylglucosamine-containing plant saponin, oleanolic acid 3-yl α-L-arabinopyranosyl-(1→2)-α-L-arabinopyranosyl-(1→6)-2-acetamido-2-deoxy-β-D-glucopyranoside, isolated from Acacia tenuifolia and Albizia subdimidiata, which exhibited significant activity against the A2780 and M109 lung cancer cell lines with IC₅₀ values of 0.8 and 1.0 μg mL⁻¹, respectively.^7–12 In 2006, Cheng and coworkers completed the synthesis of two N-acetylglucosamine-bearing saponins, lotoidosides D and E, from the roots of Glinus lotoides, showing prominent antiproliferative activity against a murine fibrosarcoma cell lines WEHI-164 with IC₅₀ values of 0.36 and 0.03 μM, respectively.¹³,¹⁴ In 2010, Li and coworkers designed and synthesized 14 ursolic acid and oleanolic acid saponins containing the N-acetyl-β-D-glucosamine unit, and (1→4)-linked and (1→6)-linked N-acetyl-β-D-glucosamine oligosaccharide residues, evaluating their cytotoxicity against HL-60, BGC-823, and Hep-2.¹⁵ In addition, a glycosyl library of N-acetylglucosamine-bearing oleanolic acids and a series of N-acyl, N-alkoxycarbonyl, and N-alkylcarbamoyl derivatives of 20-deoxy-glucosyl-bearing oleanolic saponins were designed and synthesized by Liang and coworkers, which were evaluated against HL-60, PC-3, and HT29 tumor cancer cells.¹⁶,¹⁷

Recently, Ngadjui and coworkers reported seven novel bidesmosidic triterpenoid saponins, albidosides A–G, isolated from the roots of Acacia albida with cytotoxicity against Hela and HL-60 cells.¹⁸ However, the formidable task of isolating these compounds turned out to be a great obstacle for further pharmacological research, so it is valuable to complete the synthesis of these N-acetylglucosamine-bearing bidesmosidic triterpenoid saponins for further structure–activity relationships (SAR) investigations. In this paper, we report the first concise synthesis and the in vitro antitumor activity of the bidesmosidic triterpenoid saponin, albidoside A (1) (Figure 1).

Figure 1.

Structure of the natural triterpenoid saponin, albidoside A (1).

Results and discussion

The target molecule 1 could be retrosynthetically disconnected into an oleanolic acid saponin acceptor 2,¹⁴ a monosaccharide donor 3,¹⁹ and a trisaccharide donor 4. The trisaccharide moiety 4 could be synthesized from three readily accessible monosaccharide building blocks, 6, 7, and 8, through standard glycosylation reactions (Scheme 1).

Scheme 1.

Retrosynthesis of the target saponin 1.

As shown in Scheme 2, trisaccharide donor 4 was first prepared through a sequential glycosylation strategy. Thus, 2,3,4-tri-O-benzoyl-α-D-xylopyranosyl trichloroacetimidate (7)²⁰ was coupled with 2-methyl-5-tert-butylphenyl 2,3-O-isopropylidene-1-thio-α-L-rhamnopyranoside (8)²¹ in dry CH₂Cl₂ in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) at −30 °C to provide stereoselectively 2,3,4-tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-2,3-O-isopropylidene-1-(2-methyl-5-tert-butylphenyl)thio-α-L-rhamnopyranoside (9), which was confirmed by 1H nuclear magnetic resonance (NMR) data (H-1′: 5.32, d, J = 8.0 Hz). Compound 9 was followed by deprotection of the 2,3-isopropylidene acetal with p-toluenesulfonic acid in CH₂Cl₂-CH₃OH to afford compound 10 with 87% yield over the two steps. Treatment of compound 10 with triethylorthoacetate and p-toluenesulfonic acid followed by addition of aqueous acetic acid (aq. AcOH) afforded the C-2 monoacetylated compound 5 as a glycosyl acceptor with a free hydroxyl group at the C-3 position. Glycosylation of 5 with 6 in dry CH₂Cl₂ at −30 °C using TMSOTf as the promoter successfully gave stereoselectively trisaccharide 11 with a yield of 85%, which was confirmed by 1H NMR data (H-1′′: 5.33, d, J = 7.9 Hz). Treatment of 11 with N-bromosuccinimide (NBS) in acetone-H₂O (v/v, 4:1), followed by trichloroacetimidation [Cl3CCN, DBU(1,8-Diazabicyclo[5.4.0]undec-7-ene), CH2Cl2], afforded the trisaccharide building block 4 with 89% yield over two steps.

Scheme 2.

Reagents and conditions. (a) 7, TMSOTf, CH₂Cl₂, -30 °C. (b) p-TsOH, CH₂Cl₂-CH₃OH. (c) Triethyl orthoacetate, p-TsOH; AcOH-H₂O (v/v, 4:1). (d) 6, TMSOTf, CH₂Cl₂, −30 °C. (e) NBS, acetone-H₂O (v/v, 4:1), 0 °C. (f) Cl₃CCN, DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene), CH₂Cl₂.

With an effective synthetic access to the trisaccharide donor 4, we then set about formally assembling the target saponin 1. According to Cheng’s strategy,¹⁴ the key intermediate 13 was readily obtained from oleanolic acid benzyl ester and a 2-phthalimido-2-deoxyglucose donor; subsequent deprotection of the benzyl group through catalytic hydrogenolysis provided compound 2 with 90% yield. Coupling of saponin acceptor 2 with 3,4-di-O-benzoyl-2-O-levulinoyl-β-D-xylopyranosyl trichloroacetimidate donor (3) was carried out in dry CH₂Cl₂ in the presence of a catalytic amount of TMSOTf at 0 °C, affording the stereoselectively saponin derivative 14 with 83% yield, which was confirmed by 1H NMR data (H-1′′: 5.75, d, J = 8.3 Hz). Selective removal of the Lev protecting group by treatment with NH₂NH₂·HOAc in CH₂Cl₂-CH₃OH gave saponin acceptor 15 with 86% yield. Glycosylation of 15 and 4 afforded stereoselectively saponin 16 with a yield of 82%, which was confirmed by 1H NMR data (H-1′′′: 5.43, d, J = 1.5 Hz). Finally, removal of the acetyl and benzoyl groups with NaOMe in MeOH-CH₂Cl₂ provided the target saponin 1 with 90% yield (Scheme 3). All the analytical data for the synthesized triterpene saponin 1 were identical in all respects to those reported in the literature (Table 1).¹⁸

Table 1.

1H and 13C NMR of glycoside moieties of natural product albidoside A (1) compared with synthesized compound 1.

Position	1 (ref: literature 18)			1
Position	1H (ppm)	J (Hz)	13C (ppm)	1H (ppm)	J (Hz)	13C (ppm)
1′	4.40 d	8.3	103.5	4.41 d	8.3	103.5
2′	3.63		56.2	3.63 dd	8.3, 9.6	56.3
3′	3.41		74.5	3.40 t	9.6	74.5
4′	3.32		70.5	3.33 t	9.6	70.6
5′	3.29		76.5	3.29 m		76.5
6′-1	3.83		61.4	3.82 dd	3.3, 12.3	61.4
6′-2	3.67			3.68 dd	5.8, 12.3
NHCOCH₃	1.92 s		21.8	1.93 s		21.9
NHCOCH₃			172.1			172.0
1′′	5.45 d	6.6	94.2	5.45 d	6.6	94.1
2′′	3.47		75.4	3.48 dd	6.6, 9.3	75.3
3′′	3.51		74.4	3.50 t	9.3	74.4
4′′	3.30		69.9	3.30 m		69.9
5′′-1	3.29		65.0	3.28 m		65.1
5′′-2	3.89 dd	4.3, 11.5		3.89 dd	4.3, 11.5
1′′′	5.10 d	1.6	100.0	5.10 d	1.6	100.0
2′′′	4.24		69.7	4.25 dd	1.6, 3.7	69.7
3′′′	3.98		81.3	3.98 dd	3.7, 9.1	81.4
4′′′	3.63		77.6	3.64 t	9.1	77.6
5′′′	3.80		67.8	3.80 m		67.7
6′′′	1.26 d	6.3	17.2	1.25 d	6.5	17.3
1′′′′	4.65 d	7.6	103.8	4.65 d	7.6	103.9
2′′′′	3.05		74.3	3.06 dd	7.6, 9.3	74.3
3′′′′	3.21		76.5	3.22 t	9.3	76.5
4′′′′	3.22		70.4	3.23 m		70.5
5′′′′-1	3.13 t	11.1	65.6	3.12 dd	3.3, 11.1	65.7
5′′′′-2	3.79			3.79 dd	5.1, 11.1
1′′′′′	4.55 d	7.6	103.8	4.55 d	7.6	103.8
2′′′′′	3.26		74.0	3.26 dd	7.6, 9.5	74.1
3′′′′′	3.29		76.6	3.30 t	9.5	76.6
4′′′′′	3.45		70.0	3.46 t	9.5	70.1
5′′′′′	3.21		76.5	3.21 m		76.4
6′′′′′-1	3.83		61.2	3.84 dd	3.6, 12.0	61.3
6′′′′′-2	3.67			3.67 dd	5.1, 12.0

To examine the biological activities of the natural oleanolic acid bidesmoside, albidoside A (1), its in vitro cytotoxic activity against human promyelocytic leukemia cancer (HL-60), non-small-cell lung cancer (A549), human melanoma cancer (A375), human breast cancer (MCF-7), human breast cancer (MDA-MB-231), human laryngeal epithelioid cancer (Hep-2), human gastric adenocarcinoma cancer (BGC-823), and human epidermal cancer (Hela) was assessed using the methylthiazol tetrazolium (MTT) reduction test and the sulforhodamine B (SRB) staining methods for protein. The cytotoxicity results presented in Table 2 show the IC₅₀ values. The results clearly indicate that the anticancer activity of albidoside A (1) is against the HL-60, MCF-7, MDA-MB-231, Hep-2, and Hela cell lines, with IC₅₀ values of less than 50 μM. However, albidoside A (1) displayed no anticancer activity toward the A-549, A-375, and BGC-823 cancer cell lines, which suggests that albidoside A (1) demonstrates tumor specificity. In particular, it exhibited good selectivity because it was five times more cytotoxic toward Hep-2 cells (IC₅₀ = 8.91 μM) than the Hela cell line (Table 2).

Table 2.

In vitro cytotoxicity of the natural oleanolic acid bidesmoside, albidoside A (1), against eight human cancer cell lines.

Compound	IC₅₀ (μmol L⁻¹)^a
Compound	HL-60	A-549	A-375	MCF-7	MDA-MB-231	Hep-2	BGC-823	Hela
1	16.5	>50	>50	39.1	26.7	8.91	>50	43.1

Data obtained from mean values ± standard deviation for three independent experiments made in triplicate.

Scheme 3.

Reagents and conditions: (a) H₂, 10% Pd-C, EtOAc. (b) TMSOTf, CH₂Cl₂, 0 °C. (c) NH₂NH₂·HOAc, CH₂Cl₂-CH₃OH (v/v, 1:1). (d) TMSOTf, CH₂Cl₂, −30 °C. (e) NaOMe, CH₂Cl₂-CH₃OH (v/v, 1:2).

Conclusion

In conclusion, we have succeeded in the first total synthesis of the natural oleanolic acid bidesmoside, albidoside A (1), from commercially available oleanolic acid in a convergent strategy. Preliminary pharmacological research showed good anticancer activity against HL-60, MCF-7, MDA-MB-231, Hep-2, and Hela cell lines. In particular, it exhibited dramatic selectivity, which was five times more cytotoxic toward Hep-2 cells (IC₅₀ = 8.91 μM) than the Hela cell line. Further studies on the preparation and bioactivity evaluation of analogs and derivatives of this natural product are in progress, and the results will be reported in due course.

Experimental

Chemistry

Commercial reagents were used without further purification unless specified. Solvents were dried and redistilled prior to use. Thin-layer chromatography (TLC) was performed on precoated E. Merck Silica Gel 60 F254 plates. Flash column chromatography was performed on Merck silica gel (200–300 mesh). Optical rotations were determined with a Perkin–Elmer Model 241 MC polarimeter. 1H NMR and 13C NMR spectra were recorded on a JEOL JNM-ECP 600 spectrometer with tetramethylsilane as the internal standard, and chemical shifts are recorded in δ values. Mass spectra were recorded on a Q-TOF Global mass spectrometer.

2,3,4-Tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-2,3-O-isopropylidene-1-(2-methyl-5-tert-butylphenyl)thio-α-L-rhamnopyranoside (9)

A solution of 7 (991 mg, 1.64 mmol), 8 (500 mg, 1.36 mmol), and pre-activated 4 Å MS in dry CH₂Cl₂ (50 mL) was stirred for 30 min at room temperature, and then cooled to −30 °C. TMSOTf (45 μL, 0.27 mmol) was slowly added at −30 °C. The resulting mixture was stirred for 1.5 h, and then brought to room temperature. The reaction was quenched by addition of Et₃N (1 mL) and then filtered. The filtrate was concentrated and purified by silica gel column chromatography (petroleum ether–EtOAc, 4:1) to give 9 (1.00 g, 91%) as a white solid. R_f = 0.38 (petroleum ether–EtOAc, 2:1); [α]_D²⁵ −70.1 (c 0.15, CH₂Cl₂). 1H NMR (600 MHz, CDCl₃): δ 7.22-8.05 (m, 18 H, Ph-H), 5.95 (t, J = 9.6 Hz, 1 H, H-3′), 5.68 (m, 1 H, H-4′), 5.67 (d, J = 0.9 Hz, 1 H, H-1), 5.51 (dd, J = 9.6, 8.0 Hz, 1 H, H-2′), 5.32 (d, J = 8.0 Hz, 1 H, H-1′), 4.63 (dd, J = 3.2, 12.1 Hz, 1 H, H-5′-1), 4.46 (dd, J = 5.5, 12.1 Hz, 1 H, H-5′-2), 4.21 (dd, J = 0.9, 5.6 Hz, 1 H, H-2), 4.03 (m, 1 H, H-5), 4.01 (dd, J = 5.6, 9.1 Hz, 1 H, H-3), 3.67 (t, J = 9.1 Hz, 1 H, H-4), 2.41 (s, 3 H, CH₃ thio), 1.48, 1.26 (each s, each 3 H, O₂C(CH₃)₂), 1.29 (s, 9 H, t-Bu), 1.23 (d, J = 6.0 Hz, 3 H, H-6); 13C NMR (150 MHz, CDCl₃): δ 166.2, 165.8, 165.3, 133.5, 133.2, 129.9, 129.3, 128.1, 127.7, 109.6 (O₂C(CH₃)₂), 100.4 (C-1′), 83.5 (C-1), 81.0, 77.8, 76.6, 73.1, 72.1, 71.9, 69.8, 65.4, 34.6, 27.9, 26.3, 21.3, 17.3; HRESIMS: m/z calcd for [M + Na]⁺: C₄₆H₅₀NaO₁₁S: 833.2966; found: 833.2989.

2,3,4-Tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-1-(2-methyl-5-tert-butylphenyl)thio-α-L-rhamnopyranoside (10)

A solution of 9 (800 mg, 0.99 mmol) in 80% AcOH (15 mL) was stirred for 5 h at 80 °C, brought to room temperature and concentrated. The residue was purified by a silica gel column chromatography (petroleum ether–EtOAc, 1:1) to give 10 (739 mg, 96%) as a colorless oil. R_f = 0.31 (petroleum ether–EtOAc, 1:1); [α]_D²⁵ −100.3 (c 0.15, CH₂Cl₂). 1H NMR (600 MHz, CDCl₃): δ 7.23-8.04 (m, 18 H, Ph-H), 5.93 (t, J = 9.6 Hz, 1 H, H-3′), 5.69 (m, 1 H, H-4′), 5.53 (dd, J = 9.6, 7.8 Hz, 1 H, H-2′), 5.43 (d, J = 1.3 Hz, 1 H, H-1), 5.30 (d, J = 7.8 Hz, 1 H, H-1′), 4.65 (dd, J = 3.0, 12.1 Hz, 1 H, H-5′-1), 4.49 (dd, J = 5.2, 12.1 Hz, 1 H, H-5′-2), 4.16 (m, 1 H, H-5), 4.07 (dd, J = 1.3, 3.7 Hz, 1 H, H-2), 4.01 (dd, J = 3.7, 9.3 Hz, 1 H, H-3), 3.69 (t, J = 9.3 Hz, 1 H, H-4), 2.40 (s, 3 H, CH₃ thio), 1.29 (s, 9 H, t-Bu), 1.25 (d, J = 6.0 Hz, 3 H, H-6); 13C NMR (150 MHz, CDCl₃): δ 166.2, 165.8, 165.3, 133.5, 133.2, 129.9, 129.3, 128.1, 127.7, 100.2 (C-1′), 87.2 (C-1), 81.3, 73.1, 72.5, 72.4, 71.5, 69.8, 67.4, 26.3, 21.3, 17.3; HRESIMS: m/z calcd for [M + Na]⁺: C₄₃H₄₆NaO₁₁S: 793.2653; found: 793.2674.

2,3,4-Tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-2-O-acetyl-1-(2-methyl-5-tert-butylphenyl)thio-α-L-rhamnopyranoside (5)

A solution of 10 (700 mg, 0.91 mmol), triethyl orthoacetate (0.33 mL, 1.82 mmol) and p-TsOH (77 mg, 0.45 mmol) in dimethylformamide (DMF) (15 mL) was stirred for 5 h and concentrated. The residue was treated with 80% AcOH for 1 h, and then concentrated. The residue was purified by silica gel column chromatography (petroleum ether–EtOAc, 2:1) to give 5 (665 mg, 90%) as a white solid. R_f = 0.43 (petroleum ether–EtOAc, 1:1); [α]_D²⁵ −125.1 (c 0.10, CH₂Cl₂). 1H NMR (600 MHz, CDCl₃): δ 7.22-8.01 (m, 18 H, Ph-H), 5.95 (t, J = 9.6 Hz, 1 H, H-3′), 5.63 (m, 1 H, H-4′), 5.55 (dd, J = 9.6, 7.9 Hz, 1 H, H-2′), 5.41 (d, J = 1.1 Hz, 1 H, H-1), 5.33 (d, J = 7.9 Hz, 1 H, H-1′), 4.67 (dd, J = 3.0, 12.0 Hz, 1 H, H-5′-1), 4.47 (dd, J = 5.5, 12.0 Hz, 1 H, H-5′-2), 4.17 (m, 1 H, H-5), 4.05 (dd, J = 1.1, 3.7 Hz, 1 H, H-2), 4.03 (dd, J = 3.7, 9.1 Hz, 1 H, H-3), 3.63 (t, J = 9.1 Hz, 1 H, H-4), 2.43 (s, 3 H, CH₃ thio), 2.21 (s, 3 H, CH₃CO), 1.27 (s, 9 H, t-Bu), 1.23 (d, J = 6.0 Hz, 3 H, H-6); 13C NMR (150 MHz, CDCl₃): δ 170.1, 166.3, 165.9, 165.4, 133.6, 133.2, 129.8, 129.3, 128.3, 127.7, 100.1 (C-1′), 87.9 (C-1), 81.3, 73.3, 72.5, 72.3, 71.5, 69.7, 67.4, 26.5, 22.0, 21.3, 17.3; HRESIMS: m/z calcd for [M + Na]⁺: C₄₅H₄₈NaO₁₂S: 835.2759; found: 835.2781.

2,3,4-Tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-2-O-acetyl-1-(2-methyl-5-tert-butylphenyl)thio-α-L-rhamnopyranoside (11)

A solution of 5 (500 mg, 0.62 mmol), 6 (546 mg, 0.74 mmol), and pre-activated 4 Å MS in dry CH₂Cl₂ (30 mL) was stirred for 30 min at room temperature, and then cooled to −30 °C. TMSOTf (15 μL, 0.09 mmol) was slowly added at −30 °C. The resulting mixture was stirred for 1.5 h, and then brought to room temperature. The reaction was quenched by addition of Et₃N (0.3 mL) and then filtered. The filtrate was concentrated and purified by silica gel column chromatography (petroleum ether–EtOAc, 3:1) to give 11 (733 mg, 85%) as a white solid. R_f = 0.39 (petroleum ether–EtOAc, 2:1); [α]_D²⁵ −98.5 (c 0.10, CH₂Cl₂). 1H NMR (600 MHz, CDCl₃): δ 7.18-8.11 (m, 38 H, Ph-H), 6.03 (t, J = 9.7 Hz, 1 H, H-3′), 5.95 (t, J = 9.6 Hz, 1 H, H-3′′), 5.77 (t, J = 9.7 Hz, 1 H, H-4′), 5.63 (m, 1 H, H-4′′), 5.59 (dd, J = 9.7, 7.8 Hz, 1 H, H-2′), 5.55 (dd, J = 9.6, 7.9 Hz, 1 H, H-2′′), 5.48 (d, J = 7.8 Hz, 1 H, H-1′), 5.41 (d, J = 1.1 Hz, 1 H, H-1), 5.33 (d, J = 7.9 Hz, 1 H, H-1′′), 4.97 (dd, J = 3.3, 12.3 Hz, 1 H, H-6′-1), 4.73 (m, 1 H, H-5′), 4.67 (dd, J = 3.0, 12.0 Hz, 1 H, H-5′′-1), 4.58 (dd, J = 5.6, 12.3 Hz, 1 H, H-5′′-2), 4.47 (dd, J = 5.5, 12.0 Hz, 1 H, H-5′′-2), 4.19 (m, 1 H, H-5), 4.09 (dd, J = 1.1, 3.7 Hz, 1 H, H-2), 4.07 (dd, J = 3.7, 9.1 Hz, 1 H, H-3), 3.69 (t, J = 9.1 Hz, 1 H, H-4), 2.44 (s, 3 H, CH₃ thio), 2.20 (s, 3 H, CH₃CO), 1.23 (s, 9 H, t-Bu), 1.21 (d, J = 6.1 Hz, 3 H, H-6); 13C NMR (150 MHz, CDCl₃): δ 170.1, 166.5, 166.4, 166.3, 165.9, 165.8, 165.7, 165.4, 133.9, 133.7, 133.6, 133.2, 133.0, 129.8, 129.7, 129.5, 129.3, 129.0, 128.7, 128.4, 128.3, 128.1, 127.7, 127.3, 105.3 (C-1′′), 103.5 (C-1′), 89.9 (C-1), 82.5, 81.3, 75.3, 73.3, 72.9, 72.5, 72.3, 71.5, 69.9, 69.7, 67.4, 66.5, 35.1, 26.5, 21.3, 17.3; HRMALDIMS: m/z calcd for [M + Na]⁺: C₇₉H₇₄NaO₂₁S: 1413.4336; found: 1413.4311.

2,3,4-Tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-2-O-acetyl-α-L-rhamnopyranosyl trichloroacetimidate (4)

To a solution of compound 11 (500 mg, 0.36 mmol) in 10 mL of 9:1 acetone-H₂O, was added NBS (167 mg, 0.94 mmol) at −20 °C. The mixture was stirred for 5 min and quenched with sat aq NaHCO₃. The reaction mixture was concentrated, and the residue was diluted with EtOAc, washed with sat aq NaHCO₃ and brine, dried over Na₂SO₄, and concentrated. A solution of the above-obtained product, CNCCl₃ (0.17 mL, 1.77 mmol) and DBU (0.04 mL, 0.11 mmol) in dry CH₂Cl₂ (5 mL) was stirred for 3 h at room temperature, and then the solvent was evaporated in vacuo to give a residue, which was purified by silica gel flash column chromatography (petroleum ether–EtOAc, 4:1) to give 4 (440 mg, 89%) as a white solid. R_f = 0.42 (petroleum ether–EtOAc, 2:1); [α]_D²⁵ −76.3 (c 0.13, CH₂Cl₂). 1H NMR (600 MHz, CDCl₃): δ 8.74 (s, 1 H, NH), 7.18-8.05 (m, 35 H, Ph-H), 6.00 (t, J = 9.6 Hz, 1 H, H-3′), 5.96 (t, J = 9.6 Hz, 1 H, H-3′′), 5.78 (t, J = 9.6 Hz, 1 H, H-4′), 5.65 (m, 1 H, H-4′′), 5.58 (dd, J = 9.6, 7.9 Hz, 1 H, H-2′), 5.53 (dd, J = 9.6, 7.9 Hz, 1 H, H-2′′), 5.46 (d, J = 7.9 Hz, 1 H, H-1′), 5.43 (d, J = 1.3 Hz, 1 H, H-1), 5.29 (d, J = 7.9 Hz, 1 H, H-1′′), 4.99 (dd, J = 3.4, 12.1 Hz, 1 H, H-6′-1), 4.76 (m, 1 H, H-5′), 4.63 (dd, J = 3.3, 12.0 Hz, 1 H, H-5′′-1), 4.56 (dd, J = 5.8, 12.1 Hz, 1 H, H-6′-2), 4.45 (dd, J = 5.5, 12.0 Hz, 1 H, H-5′′-2), 4.18 (m, 1 H, H-5), 4.07 (dd, J = 1.3, 3.6 Hz, 1 H, H-2), 4.04 (dd, J = 3.6, 9.0 Hz, 1 H, H-3), 3.67 (t, J = 9.0 Hz, 1 H, H-4), 2.19 (s, 3 H, CH₃CO), 1.20 (d, J = 6.3 Hz, 3 H, H-6); 13C NMR (150 MHz, CDCl₃): δ 170.3, 166.6, 166.3, 165.9, 165.8, 165.7, 165.4, 133.9, 133.6, 133.2, 133.1, 129.8, 129.6, 129.5, 129.3, 129.1, 128.5, 128.4, 128.1, 127.9, 127.3, 105.5 (C-1′′), 103.3 (C-1′), 94.1 (C-1), 82.7, 81.6, 75.4, 73.8, 72.8, 72.5, 72.1, 71.5, 69.9, 69.6, 67.5, 66.5, 35.3, 26.7, 21.5, 17.1; HRMALDIMS: m/z calcd for [M + H]⁺: C₇₀H₆₁Cl₃NO₂₂: 1372.2745; found: 1372.2766.

2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-D-glucopyranoside oleanolic acid (2)

A mixture of 13 (400 mg, 0.46 mmol) and 10% Pd-C (300 mg) in CH₂Cl₂-MeOH (v/v, 1/1, 100 mL) in the presence of AcOH (1 mL) was stirred under 1 atm of H₂ for 4 h. The reaction mixture was then filtered, and the filtrate was concentrated to dryness to give a white solid, which was purified by silica gel flash column chromatography (petroleum ether–EtOAc, 3:1) to give 2 (340 mg, 90%) as a white solid. R_f = 0.32 (petroleum ether–EtOAc, 3:1); [α]_D²⁵ +25.9 (c 0.41, acetone). 1H NMR (600 MHz, CDCl₃): δ 5.56 (d, J = 8.3 Hz, 1 H, NH), 5.32 (t, J = 3.6 Hz, 1 H, H-12), 5.25 (t, J = 9.6 Hz, 1 H, H-3′), 5.01 (t, J = 9.6 Hz, 1 H, H-4′), 4.67 (d, J = 8.0 Hz, 1 H, H-1′), 4.25 (m, 1 H, H-5), 4.10 (dd, J = 2.7, 12.0 Hz, 1 H, H-6′-1), 3.88 (dd, J = 8.0, 9.6 Hz, 1 H, H-3′), 3.70 (dd, J = 5.5, 12.0 Hz, 1 H, H-6′-2), 3.12 (dd, J = 4.1, 11.3 Hz, 1 H, H-3), 2.84 (dd, J = 3.7, 13.7 Hz, 1 H, H-18), 2.06, 2.03, 2.01, 1.91 (s each, 3 H each, CH₃CO × 4), 2.02 (m, 1 H, H-16-1), 1.87-1.90 (m, 2 H, H-2-1, H-11-1), 1.68-1.71 (m, 2 H, H-19-1, H-22-1), 1.60-1.64 (m, 4 H, H-1-1, H-2-2, H-15-1, H-16-2), 1.47-1.52 (m, 4 H, H-6-1, H-7-1, H-9, H-22-2), 1.35-1.37 (m, 3 H, H-6-2, H-7-2, H-21-1), 1.10-1.18 (m, 3 H, H-15-2, H-19-2, H-21-2), 0.90-0.92 (m, 2 H, H-1-2, H-11-2), 0.74 (m, 1H, H-5), 1.10, 0.93, 0.90, 0.88, 0.87, 0.76, 0.61 (s each, 3 H each, CH₃ × 7); HRESIMS: m/z calcd for [M + H]⁺: C₄₄H₆₈NO₁₁: 786.4787; found: 786.4753.

3-O-[2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-D-glucopyranosyl]oleanolic acid 28-O-[2-O-levulinoyl-3,4-di-O-benzoyl-β- D-xylopyranosyl] ester (14)

A solution of 2 (300 mg, 0.38 mmol), 3 (275 mg, 0.46 mmol), and pre-activated 4 Å MS in dry CH₂Cl₂ (20 mL) was stirred for 30 min at room temperature, and then cooled to −30 °C. TMSOTf (10 μL, 0.06 mmol) was slowly added at −30 °C. The resulting mixture was stirred for 1.5 h, and then brought to room temperature. The reaction was quenched by addition of Et₃N (0.2 mL), and then filtered. The filtrate was concentrated and purified by silica gel column chromatography (petroleum ether–EtOAc, 4:1) to give 14 (386 mg, 83%) as a white solid. R_f = 0.37 (petroleum ether–EtOAc, 3:1); [α]_D²⁵ +45.1 (c 0.33, CHCl₃). 1H NMR (600 MHz, CDCl₃): δ 7.21-8.03 (m, 10 H, Ph-H), 5.85 (t, J = 9.6 Hz, 1 H, H-3′′), 5.75 (d, J = 8.3 Hz, 1 H, H-1′′), 5.53 (d, J = 8.1 Hz, 1 H, NH), 5.50 (t, J = 8.3, 9.6 Hz, 1 H, H-2′′), 5.46 (m, 1 H, H-4′′), 5.39 (m, 1 H, H-5′′-1), 5.32 (t, J = 3.6 Hz, 1 H, H-12), 5.28 (t, J = 5.3, 12.0 Hz, 1 H, H-5′′-2), 5.25 (t, J = 9.6 Hz, 1 H, H-3′), 5.01 (t, J = 9.6 Hz, 1 H, H-4′), 4.67 (d, J = 8.0 Hz, 1 H, H-1′), 4.25 (m, 1 H, H-5), 4.10 (dd, J = 2.7, 12.0 Hz, 1 H, H-6′-1), 3.88 (dd, J = 8.0, 9.6 Hz, 1 H, H-3′), 3.70 (dd, J = 5.5, 12.0 Hz, 1 H, H-6′-2), 3.12 (dd, J = 4.1, 11.3 Hz, 1 H, H-3), 2.84 (dd, J = 3.7, 13.7 Hz, 1 H, H-18), 2.61-2.65 (m, 2 H, Lev-CH₂), 2.48-2.53 (m, 2 H, Lev-CH₂), 2.04, 2.03, 2.00, 1.93 (s each, 3 H each, CH₃CO × 4), 2.01 (m, 1 H, H-16-1), 1.86-1.90 (m, 2 H, H-2-1, H-11-1), 1.69-1.72 (m, 2 H, H-19-1, H-22-1), 1.62-1.66 (m, 4 H, H-1-1, H-2-2, H-15-1, H-16-2), 1.61 (s, 3 H, Lev-CH₃), 1.50-1.52 (m, 4 H, H-6-1, H-7-1, H-9, H-22-2), 1.34-1.37 (m, 3 H, H-6-2, H-7-2, H-21-1), 1.10-1.18 (m, 3 H, H-15-2, H-19-2, H-21-2), 0.90-0.94 (m, 2 H, H-1-2, H-11-2), 0.79 (m, 1H, H-5), 1.11, 0.93, 0.91, 0.88, 0.87, 0.73, 0.60 (s each, 3 H each, CH₃ × 7); 13C NMR (150 MHz, CDCl₃): δ 207.3, 176.5, 172.1, 170.3, 170.1, 170.0, 169.9, 165.9, 165.8, 165.5, 144.3, 133.9, 133.6, 133.2, 129.8, 129.6, 129.3, 129.1, 128.4, 128.1, 127.9, 127.3, 123.0, 107.5 (C-1′), 95.7 (C-1′′), 93.2 (C-3), 82.7, 81.6, 75.4, 73.8, 72.8, 72.5, 72.1, 71.5, 69.9, 69.6, 67.5, 66.5, 55.7, 47.7, 47.3, 46.4, 42.1, 39.3, 39.1, 38.8, 37.9, 37.6, 34.2, 33.1, 35.3, 29.5, 29.0, 26.9, 26.7, 23.8, 23.7, 23.6, 21.5, 18.6, 17.1, 16.2; HRMALDIMS: m/z calcd for [M + Na]⁺: C₆₈H₈₉NO₁₉Na: 1246.5921; found: 1246.5953.

3-O-[2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-D- glucopyranosyl] oleanolic acid 28-O-[3,4-di-O-benzoyl-β-D-xylopyranosyl] ester (15)

To a stirred solution of 14 (300 mg, 0.25 mmol) in CH₂Cl₂ (50 mL) and CH₃OH (50 mL) was added NH₂NH₂·HOAc (80 mg, 0.81 mmol). After 2 h, the solution was concentrated, and the residue was purified by silica gel column chromatography (petroleum ether–EtOAc, 3:1) to give 15 (237 mg, 86%) as a white solid. R_f = 0.33 (petroleum ether–EtOAc, 3:1); [α]_D²⁵ +56.1 (c 0.21, CHCl₃). 1H NMR (600 MHz, CDCl₃): δ 7.23-8.01 (m, 10 H, Ph-H), 5.83 (t, J = 9.7 Hz, 1 H, H-3′′), 5.76 (d, J = 8.0 Hz, 1 H, H-1′′), 5.55 (d, J = 8.0 Hz, 1 H, NH), 5.49 (t, J = 8.0, 9.7 Hz, 1 H, H-2′′), 5.45 (m, 1 H, H-4′′), 5.42 (m, 1 H, H-5′′-1), 5.36 (t, J = 3.2 Hz, 1 H, H-12), 5.31 (t, J = 5.5, 12.3 Hz, 1 H, H-5′′-2), 5.21 (t, J = 9.7 Hz, 1 H, H-3′), 5.06 (t, J = 9.7 Hz, 1 H, H-4′), 4.63 (d, J = 7.8 Hz, 1 H, H-1′), 4.29 (m, 1 H, H-5), 4.08 (dd, J = 2.5, 12.3 Hz, 1 H, H-6′-1), 3.96 (dd, J = 7.8, 9.7 Hz, 1 H, H-3′), 3.70 (dd, J = 5.5, 12.3 Hz, 1 H, H-6′-2), 3.11 (dd, J = 3.7, 13.7 Hz, 1 H, H-3), 2.86 (dd, J = 4.3, 13.7 Hz, 1 H, H-18), 2.05 (m, 1 H, H-16-1), 2.04, 2.01, 2.00, 1.95 (s each, 3 H each, CH₃CO × 4), 1.89-1.93 (m, 3 H, H-2-1, H-11-1, H-19-1), 1.73 (m, 1 H, H-22-1), 1.63-1.68 (m, 4 H, H-1-1, H-2-2, H-15-1, H-16-2), 1.45-1.51 (m, 4 H, H-6-1, H-7-1, H-9, H-22-2), 1.35-1.39 (m, 3 H, H-6-2, H-7-2, H-21-1), 1.13-1.20 (m, 3 H, H-15-2, H-19-2, H-21-2), 0.91-0.97 (m, 2 H, H-1-2, H-11-2), 0.78 (m, 1H, H-5), 1.13, 0.95, 0.91, 0.89, 0.87, 0.73, 0.61 (s each, 3 H each, CH₃ × 7); 13C NMR (150 MHz, CDCl₃): δ 176.5, 170.3, 170.1, 170.0, 169.9, 165.9, 165.8, 165.5, 144.3, 133.9, 133.6, 133.2, 129.8, 129.6, 129.5, 129.1, 128.4, 128.1, 127.8, 127.2, 123.0, 106.9 (C-1′), 95.9 (C-1′′), 93.0 (C-3), 82.1, 80.3, 75.4, 73.8, 72.8, 72.5, 72.1, 71.5, 69.9, 69.6, 67.5, 66.5, 55.7, 47.7, 47.3, 46.4, 42.3, 39.3, 39.0, 38.8, 37.7, 34.3, 33.1, 35.3, 29.1, 26.9, 26.5, 23.9, 23.7, 23.5, 21.3, 18.6, 17.0, 16.3; HRMALDIMS: m/z calcd for [M + Na]⁺: C₆₃H₈₃NO₁₇Na: 1148.5553; found: 1148.5571.

3-O-(2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-D-glucopyranosyl) oleanolic acid 28-O-{2,3,4,6-tetra-O- benzoyl-β-D-glucopyranosyl-(1→3)-(2,3,4-tri-O-benzoyl-β-D-xylopyranosyl-(1→4))-2-O-acetyl-α-L-rhamnopyranosyl-(1→2)-3,4-di-O-benzoyl-β- D-xylopyranosyl} ester (16)

A solution of 15 (200 mg, 0.18 mmol), 4 (365 mg, 0.27 mmol) and pre-activated 4 Å MS in dry CH₂Cl₂ (20 mL) was stirred for 30 min at room temperature, and then cooled to −30 °C. TMSOTf (10 μL, 0.06 mmol) was slowly added at −30 °C. The resulting mixture was stirred for 1.5 h, and then brought to room temperature. The reaction was quenched by addition of Et₃N (0.2 mL), and then filtered. The filtrate was concentrated and purified by silica gel column chromatography (petroleum ether–EtOAc, 3:1) to give 16 (345 mg, 82%) as a white solid. R_f = 0.31 (petroleum ether–EtOAc, 2:1); [α]_D²⁵ +67.3 (c 0.21, CH₂Cl₂). 1H NMR (600 MHz, CDCl₃): δ 7.11-8.17 (m, 45 H, Ph-H), 6.03 (t, J = 9.6 Hz, 1 H, H-3′′′′), 5.93 (t, J = 9.6 Hz, 1 H, H-3′′′′′), 5.85 (t, J = 9.6 Hz, 1 H, H-3′′), 5.79 (t, J = 9.6 Hz, 1 H, H-4′′′′), 5.77 (d, J = 8.0 Hz, 1 H, H-1′′), 5.69 (m, 1 H, H-4′′′′′), 5.61 (dd, J = 9.6, 7.9 Hz, 1 H, H-2′′′′), 5.56 (d, J = 8.0 Hz, 1 H, NH), 5.54 (dd, J = 9.6, 7.9 Hz, 1 H, H-2′′′′′), 5.47 (t, J = 8.0, 9.6 Hz, 1 H, H-2′′), 5.45 (d, J = 7.9 Hz, 1 H, H-1′′′′), 5.43 (d, J = 1.5 Hz, 1 H, H-1′′′), 5.41 (m, 1 H, H-4′′), 5.40 (m, 1 H, H-5′′-1), 5.32 (t, J = 3.7 Hz, 1 H, H-12), 5.29 (d, J = 7.9 Hz, 1 H, H-1′′′′′), 5.26 (t, J = 5.6, 12.0 Hz, 1 H, H-5′′-2), 5.23 (t, J = 9.6 Hz, 1 H, H-3′), 5.07 (t, J = 9.6 Hz, 1 H, H-4′), 5.01 (dd, J = 3.4, 12.1 Hz, 1 H, H-6′′′′-1), 4.79 (m, 1 H, H-5′′′′), 4.69 (dd, J = 3.3, 12.0 Hz, 1 H, H-5′′′′′-1), 4.61 (d, J = 7.8 Hz, 1 H, H-1′), 4.58 (dd, J = 5.8, 12.1 Hz, 1 H, H-6′′′′-2), 4.55 (dd, J = 5.5, 12.0 Hz, 1 H, H-5′′′′′-2), 4.27 (m, 1 H, H-5), 4.21 (m, 1 H, H-5′′′), 4.11 (dd, J = 1.3, 3.6 Hz, 1 H, H-2′′′), 4.07 (dd, J = 2.9, 12.0 Hz, 1 H, H-6′-1), 4.02 (dd, J = 3.6, 9.0 Hz, 1 H, H-3′′′), 3.95 (dd, J = 7.8, 9.6 Hz, 1 H, H-3′), 3.77 (dd, J = 5.7, 12.0 Hz, 1 H, H-6′-2), 3.69 (t, J = 9.0 Hz, 1 H, H-4′′′), 3.12 (dd, J = 3.7, 13.7 Hz, 1 H, H-3), 2.87 (dd, J = 4.1, 13.7 Hz, 1 H, H-18), 2.09 (m, 1 H, H-16-1), 2.07, 2.04, 2.03, 2.01, 1.99 (s each, 3 H each, CH₃CO × 5), 1.23 (d, J = 6.5 Hz, 3 H, H-6′′′), 1.93 (m, 1 H, H-2-1), 1.65-1.73 (m, 4 H, H-2-2, H-11-1, H-19-1, H-22-1), 1.59-1.65 (m, 3 H, H-1-1, H-15-1, H-16-2), 1.48-1.54 (m, 4 H, H-6-1, H-7-1, H-9, H-22-2), 1.33-1.38 (m, 3 H, H-6-2, H-7-2, H-21-1), 1.13-1.19 (m, 3 H, H-15-2, H-19-2, H-21-2), 0.91-0.94 (m, 2 H, H-1-2, H-11-2), 0.72 (m, 1H, H-5), 1.16, 0.99, 0.96, 0.89, 0.87, 0.77, 0.66 (s each, 3 H each, CH₃ × 7); 13C NMR (150 MHz, CDCl₃): δ 176.5, 170.3, 170.1, 170.0, 169.9, 166.6, 166.3, 165.9, 165.8, 165.7, 165.5, 165.4, 144.3, 133.9, 133.6, 133.2, 133.1, 129.8, 129.6, 129.5, 129.3, 129.1, 128.5, 128.4, 128.1, 127.9, 127.8, 127.3, 127.2, 123.0, 106.6 (C-1′), 105.5 (C-1′′′′′), 103.3 (C-1′′′′), 96.1 (C-1′′), 94.9 (C-1′′′), 93.0 (C-3), 82.7, 82.1, 81.6, 80.3, 76.3, 75.5, 74.4, 73.7, 72.9, 72.6, 72.3, 71.6, 70.9, 69.9, 69.5, 68.9, 67.5, 66.5, 55.7, 47.7, 47.3, 46.4, 42.3, 39.3, 39.0, 38.8, 37.7, 34.3, 33.1, 35.3, 29.3, 26.9, 26.7, 23.9, 23.7, 23.5, 21.3, 18.6, 17.1, 16.5; HRMALDIMS: m/z calcd for [M + Na]⁺: C₁₃₁H₁₄₁NO₃₈Na: 2358.9024; found: 2358.9065.

3-O-2-Acetamido-2-deoxy-β-D-glucopyranosyl oleanolic acid 28-O-β-D-glucopyranosyl-(1→3)-(β-D-xylopyranosyl-(1→4))-α-L-rhamnopyranosyl-(1→2)-β-D-xylopyranosyl ester (Albidoside A) (1)

To a solution of 16 (100 mg, 0.04 mmol) in dry 1:2 CH₂Cl₂-MeOH (20 mL) was added a freshly prepared solution of NaOMe in MeOH (1.0 mol L⁻¹, 0.20 mL). The mixture was stirred at room temperature for 5 h and neutralized with Dowex H⁺ resin to pH 7 and then filtered. The filtrate was concentrated, and the resulting residue was purified by silica gel column chromatography (MeOH-CH₂Cl₂-H₂O, 1:25:0.1) to give 1 (47 mg, 90%) as a white solid. R_f = 0.33 (MeOH-CH₂Cl₂-H₂O, 1:25:0.1); [α]_D²⁵ +197 (c 0.03, MeOH). 1H NMR (600 MHz, CD₃OD): δ 5.45 (d, J = 6.6 Hz, 1 H, H-1′′), 5.10 (d, J = 1.6 Hz, 1 H, H-1′′′), 4.65 (d, J = 7.6 Hz, 1 H, H-1′′′′), 4.55 (d, J = 7.6 Hz, 1 H, H-1′′′′′), 4.41 (d, J = 8.3 Hz, 1 H, H-1′), 4.25 (dd, J = 3.7, 1.6 Hz, 1 H, H-2′′′), 3.98 (dd, J = 3.7, 9.1 Hz, 1 H, H-3′′′), 3.89 (dd, J = 4.3, 11.5 Hz, 1 H, H-5′′-1), 3.84 (dd, J = 3.6, 12.0 Hz, 1 H, H-6′′′′′-1), 3.82 (dd, J = 3.3, 12.3 Hz, 1 H, H-6′-1), 3.80 (m, 1 H, H-5′′′), 3.79 (dd, J = 5.1, 11.1 Hz, 1 H, H-5′′′′-1), 3.68 (dd, J = 5.8, 12.3 Hz, 1 H, H-6′-1), 3.67 (dd, J = 5.1, 12.0 Hz, 1 H, H-6′′′′′-2), 3.64 (t, J = 9.1 Hz, 1 H, H-4′′′), 3.63 (dd, J = 9.6, 8.3 Hz, 1 H, H-2′), 3.50 (t, J = 9.3 Hz, 1 H, H-3′′), 3.48 (dd, J = 6.6, 9.3 Hz, 1 H, H-2′′), 3.46 (t, J = 9.5 Hz, 1 H, H-4′′′′′), 3.40 (t, J = 9.6 Hz, 1 H, H-3′), 3.33 (t, J = 9.6 Hz, 1 H, H-4′), 3.30 (m, 1 H, H-4′′), 3.30 (t, J = 9.5 Hz, 1 H, H-3′′′′′), 3.29 (m, 1 H, H-5′), 3.28 (m, 1 H, H-5′′-1), 3.26 (dd, J = 9.5, 7.6 Hz, 1 H, H-2′′′′′), 3.23 (m, 1 H, H-4′′′′), 3.22 (t, J = 9.3 Hz, 1 H, H-3′′′′), 3.21 (m, 1 H, H-5′′′′′), 3.12 (dd, J = 3.3, 11.1 Hz, 1 H, H-5′′′′-1), 3.06 (dd, J = 3.7, 11.6 Hz, 1 H, H-3), 3.06 (dd, J = 9.3, 7.6 Hz, 1 H, H-2′′′′), 2.82 (dd, J = 4.3, 13.7 Hz, 1 H, H-18), 1.93 (s, 3 H, CH₃CO), 2.01 (m, 1 H, H-16-1), 1.86-1.90 (m, 2 H, H-2-1, H-11-1), 1.67-1.70 (m, 2 H, H-19-1, H-22-1), 1.60-1.64 (m, 4 H, H-1-1, H-2-2, H-15-1, H-16-2), 1.47-1.51 (m, 4 H, H-6-1, H-7-1, H-9, H-22-2), 1.35-1.37 (m, 3 H, H-6-2, H-7-2, H-21-1), 1.25 (d, J = 6.5 Hz, 3 H, H-6′′′), 1.11-1.17 (m, 3 H, H-15-2, H-19-2, H-21-2), 0.91-0.923 (m, 2 H, H-1-2, H-11-2), 0.75 (m, 1H, H-5),1.13, 0.94, 0.92, 0.90, 0.88, 0.76, 0.75 (s each, 3 H each, CH₃ × 7); 13C NMR (150 MHz, CD₃OD): δ 176.4 (C-28), 172.0 (CH₃CO), 143.5 (C-13), 122.4 (C-12), 103.9 (C-1′′′′), 103.8 (C-1′′′′′), 103.5 (C-1′), 100.0 (C-1′′′), 94.1 (C-1′′), 89.8 (C-3), 81.4, 77.6, 76.6, 76.5, 76.4, 75.3, 74.5, 74.4, 74.3, 74.1, 70.6, 70.5, 70.1, 69.9, 69.7, 67.7, 65.7, 65.1, 61.4, 61.3, 56.3, 55.6, 47.6, 46.8, 45.9, 41.7, 41.4, 39.4, 38.6, 38.4, 36.5, 33.5, 32.7, 32.2, 31.9, 30.2, 27.6, 27.3, 25.5, 24.8, 23.2, 22.8, 22.7, 21.9, 18.1, 17.3, 16.4, 15.7, 14.7; HRESIMS: m/z calcd for [M + Na]⁺: C₆₀H₉₇NO₂₅Na: 1254.6242; found: 1254.6253.

Cytotoxic assay

The cytotoxicity of albidoside A (1) was examined using a panel of human tumor cell lines, including one human promyelocytic leukemia cell line (HL-60), one human non-small-cell lung cancer cell line (A549), and one human melanoma cancer cell line (A375) and human breast cancer (MCF-7), human breast cancer (MDA-MB-231), human laryngeal epithelioid cancer (Hep-2), human gastric adenocarcinoma cancer (BGC-823), and human epidermal cancer (Hela). Cells were seeded into 96-well plates and treated in triplicate with gradient concentrations of tested compound at 37 °C for 72 h. Cytotoxicity to HL-60, MCF-7, and MDA-MB-231 cells was assessed by the MTT assay, and cytotoxicity to A549, A375, Hep-2, and BGC-823 was assessed by the SRB assay as previously described.²²,²³ The cytotoxicity of the tested compound was expressed as an IC₅₀ value, determined by the logit method from at least three independent experiments.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This project was financially supported by the Foundation of Natural Science Basic Research and Social Development Plan of Shaanxi Province of China (2018JM2060 and 2016SF-240), Shaanxi Youth Science and Technology Star Project (2015KJXX-28), and the Foundation of Shaanxi Educational Committee (15JK1717 and 16JK1768).

ORCID iD

Qingchao Liu

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Total synthesis and anticancer activity of the natural oleanolic acid bidesmoside saponin,albidoside A,isolated from the roots of Acacia albida

Abstract

Keywords

Introduction

Results and discussion

Conclusion

Experimental

Chemistry

2,3,4-Tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-2,3-O-isopropylidene-1-(2-methyl-5-tert-butylphenyl)thio-α-L-rhamnopyranoside (9)

2,3,4-Tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-1-(2-methyl-5-tert-butylphenyl)thio-α-L-rhamnopyranoside (10)

2,3,4-Tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-2-O-acetyl-1-(2-methyl-5-tert-butylphenyl)thio-α-L-rhamnopyranoside (5)

2,3,4-Tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-2-O-acetyl-1-(2-methyl-5-tert-butylphenyl)thio-α-L-rhamnopyranoside (11)

2,3,4-Tri-O-benzoyl-α-D-xylopyranosyl-(1→4)-2-O-acetyl-α-L-rhamnopyranosyl trichloroacetimidate (4)

2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-D-glucopyranoside oleanolic acid (2)

3-O-[2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-D-glucopyranosyl]oleanolic acid 28-O-[2-O-levulinoyl-3,4-di-O-benzoyl-β- D-xylopyranosyl] ester (14)

3-O-[2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-D- glucopyranosyl] oleanolic acid 28-O-[3,4-di-O-benzoyl-β-D-xylopyranosyl] ester (15)

3-O-(2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-D-glucopyranosyl) oleanolic acid 28-O-{2,3,4,6-tetra-O- benzoyl-β-D-glucopyranosyl-(1→3)-(2,3,4-tri-O-benzoyl-β-D-xylopyranosyl-(1→4))-2-O-acetyl-α-L-rhamnopyranosyl-(1→2)-3,4-di-O-benzoyl-β- D-xylopyranosyl} ester (16)

3-O-2-Acetamido-2-deoxy-β-D-glucopyranosyl oleanolic acid 28-O-β-D-glucopyranosyl-(1→3)-(β-D-xylopyranosyl-(1→4))-α-L-rhamnopyranosyl-(1→2)-β-D-xylopyranosyl ester (Albidoside A) (1)

Cytotoxic assay

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References