A Facile α-Glucuronidation Using Methyl 1,2,3,4-Tetra-O-acetyl-D-glucuronate as Glycosyl Donor

Abstract

Some terpenyl 2,3,4-tri-O-acetyl-α-D-glucuronide methyl esters were facilely synthesized from commercially available methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate and terpenoid alcohols in the presence of bis(trifluoromethanesulfonyl)imide (Tf₂NH) in dichloromethane (DCM) in good yields. The predominant α-selectivity at the anomer position is caused via transition state in which the neighboring group participation of the methoxycarbonyl group at C-6 stabilizes the oxonium intermediate by forming ¹C₄ conformation. The intermediate accelerates the glucuronidation reaction despite the use of the acetyl group, which is not a good activating group in general glycosylation reactions, as the activating group.

Introduction

Glucuronide bonds, particularly β-glucuronides, are abundantly found in biologically active complex oligosaccharides in nature. Glycosaminoglycans (GAGs), such as hyaluronic acid, chondroitin, and heparin present in the extracellular matrix in animal tissue, are examples of glucuronide-containing complex carbohydrates.^1,2) GAGs display an array of physical properties and biological functions by forming repeating disaccharide units that are composed of a pair of glucuronic acid and a basic monosaccharide. A search of the plant kingdom has revealed a vast number of glucuronate-containing saponins particularly in Family Leguminosae.³⁾ For instance, glycyrrhetin, which was isolated from the roots of Glycyrrhiza glabra var. grandiflorum as a potent anti-inflammatory active component, is a triterpenoid glycoside in which the glycosyl part is composed of β(1→2)-linked two molecules of glucuronic acid.⁴⁾ Moreover, it is well known that many drugs undergo glucuronidation in liver to facilitate excretion into urine during drug metabolism. In this regard, extensive efforts have been made to design reactions for forming glucuronide bonds.

As strategies for β-glucuronidation, the preparation of 2-O-acetyl-6,3-lactone of D-glucuronic acid (1)⁵⁾ (Strategy A, Chart 1) and the method of pre-glucosylation followed by oxidation^6–10) (Strategy B, Chart 1) have been employed to provide the desired β-glucuronides (Chart 1).

Chart 1. Strategies for β-Glucuronidation

On the other hand, the synthesis of α-glucuronides remains now a difficult task because the 2-O-acetyl-6,3-lactone method (Strategy A, Chart 1) cannot form the α-bond owing to the participation of the neighboring acetoxy group at C-2⁵⁾ and the pre-glucosidation-oxidation method (Strategy B, Chart 1) is not so reliable due to the lack of a predominant α-glucosylation protocol that does not produce the β-isomer.

As regards Strategy A, van der Marel and colleagues prepared 2-O-benzyl-6,3-lactone derivative (2) with the expectation that the anomeric effect will afford α-glucuronide.¹¹⁾ In spite of the fact that lactone (2) behaved as a good glycosyl donor in the reaction with diphenyl sulfoxide and trifluoro-methanesulfonic anhydride (Ph₂SO, Tf₂O), 2 must be carefully prepared from 2,4-O-protected-1-thio-D-glucoside (3) via 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) oxidation in order to suppress the formation of sulfoxide or sulfone by-products. In actual experiments, the oxidation-lactonization of thio-glucoside (3) to generate 2 proceeded in moderate yield (Chart 2).

Chart 2. Strategies for α-Glucuronidation

With regard to Strategy B, the synthesis of α-glucuronide-containing pentasaccharide, which is a component of the repeating units of biofilms produced by Klebsiella pneumoniae, was accomplished by using the “pre-glucosidation-TEMPO oxidation” method by Mirsa and colleagues.¹²⁾ The reaction proceeded in satisfactory yield; however, it required the tedious task of selective protection and deprotection of the primary alcohol at C-6 position in the D-glucose moiety and successive oxidation with TEMPO. To present another example, Nishimura and colleagues reported that glycosylation using methyl ester of 1-phenylthio-2,3,4-tri-O-benzyl-β-D-glucuronide (4) as the donor substrate and activation with NIS/TfOH in dichloromethane (DCM) gave a mixture of α- and β-isomers (7 : 3).⁵⁾ To overcome these drawbacks, a direct α-glucuronidation must be developed for the synthesis of diverse carbohydrates including α-glucuronide. We report herein, a facile method that affords α-glucuronides without producing β-glucuronides, in which commercially available methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate is used as the glycosyl donor.

Results and Discussion

We have made a serendipitous discovery that commercially available methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate (5) can be used as the donor substrate for glucuronidation. In the beginning, (−)-menthol (6a) was treated with 5 in the presence of BF₃·Et₂O in DCM at room temperature to afford (−)-menthyl 1,2,3,4-tetra-O-acetyl-α-D-glucuronide methyl ester (6b) in 40% yield. Several attempts to replace the Lewis acid from BF₃·Et₂O with other acids, such as TfOH, trimethylsilyl trifluoromethanesulfonate (TMSOTf), AlCl₃, and SnCl₄ were performed; however, the reaction provided 6b in lower yields, i.e., 8, 12, 17, and 33%, respectively. Finally, the reaction with bis(trifluoromethanesulfonyl)imide (Tf₂NH) was attained and gave the best result (50%). Moreover, the reaction with BF₃·Et₂O at 0 °C did not proceed at all and that with Tf₂NH at 0 °C did not finish in 4 h, while these reactions at room temperature (r.t.) completed within 2 h.

Based on the result, the reactions using other terpenoid alcohols, including cholestanol (7a), 1-adamantanol (8a), 2-adamantanol (9a), nor-ent-kauranol (10a),¹³⁾ and (+)-borneol (11a), as the acceptor substrates were carried out in the presence of Tf₂NH at r.t. (Table 1). In general, the yields of α-glucuronides (6b-9b, 11b) were higher than 50% and the best yield was 70% (Table 1, entry 2). The stereochemistry of the anomer carbon in 6b-11b and 10c was determined based on the study using ¹H- and ¹³C-NMR, HMQC, and ¹H–¹H-correlation spectroscopy (COSY) spectra; i.e., the coupling constant of H-1′ around 3.6–3.8 Hz meant that the proton presented at equatorial orientation and the glycosyl bond formed at α-configuration. Moreover, it is worth noting that β-glucuronides were not produced except in the reaction with 11a.

Table 1. α-Glucuronidation of Terpenoid Alcohols (6a–11a) with Tf₂NH in DCM

The reason why almost no β-glucuronides are obtained in the glycosylation reaction is speculated by considering the reaction mechanism. First, a proton of Tf₂NH accelerates the elimination of the acetyl group at the anomer position to generate oxonium intermediate (A). Next, the half-chair form of intermediate (A) is converted into ¹C₄ form (B), which is stabilized by the participation of the neighboring group attributed to the methoxycarbonyl group. Finally, S_N 2 substitution by the attack of the hydroxy group of alcohols takes place to afford α-glucuronides (Chart 3). In general, the acetyl group at the anomer position is not a good activating group in the glycosylation reaction; however, the formation of intermediate (B) will reduce the energy level of the transition state and accelerate the reaction in the present case.

Chart 3. Plausible Mechanism of the α-Glucuronidation in Bis(trifluoro-methanesulfonyl)imide System

Conclusion

We have established an excellent method for synthesizing α-glucuronides without the production of β-isomers by using methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate (5) as the glycosyl donor in the presence of Tf₂NH as the activator. Both glycosyl donor (5) and the activator are commercially available and the reaction can be performed at room temperature. This facile procedure is expected to make the synthesis of biologically active α-glucuronic acid containing oligosaccharides much easier than before.

Experimental

All ¹H- and ¹³C-NMR spectra of the products were measured in CDCl₃ by spectrometers operating at 400 MHz (100 MHz for ¹³C-NMR) at 25 °C. The ¹H-NMR chemical shifts were recorded in parts per million (ppm, δ) relative to tetramethylsilane (δ = 0.00 ppm) as the internal standard. The ¹³C-NMR chemical shifts were reported in ppm with the solvent as the reference peak (CDCl₃: δ = 77.0 ppm). Mass spectra were recorded on an electrospray ionization-time of flight (ESI-TOF) mass spectrometer (Micromass LCT). Column chromatography was performed with Merck Silica Gel 60 (60–230 mesh) and n-hexane and ethyl acetate as eluents for the isolation of the products. Analytical TLC was carried out by using Merck Silica Gel F254 plates (0.25 mm). Spots were detected by UV irradiation (254 nm) and/or by staining with 5% phosphomolybdic acid followed by heating. Melting points were measured with Yanaco Micro Melting Point Apparatus without any corrections.

Materials: Methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate (5), and Tf₂NH are commercially available and used as received. All other starting materials are purchased and used without further purification.

(−)-Menthyl 2,3,4-Tri-O-acetyl-α-D-glucuronide Methyl Ester (6b)

To a solution of methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate (5) (91.5 mg, 0.243 mmol) and (−)-menthol (2a) (58.6 mg, 0.376 mmol) in DCM (5.0 mL), Tf₂NH (105.1 mg, 0.375 mmol) was added at room temperature. After stirring the mixture for 2 h at room temperature, the mixture was poured into ice water and extracted with ethyl acetate. The organic layer was treated as usual. The residue was purified by silica gel column chromatography (n-hexane : ethyl acetate = 4 : 1) to afford the desired α-glucuronide (6b) (57.5 mg, 50%) as a colorless oil. [α]²⁶_D=+68.6 (c 1.38, CHCl₃). ¹H-NMR (400 MHz, CDCl₃) δ: 5.55 (1H, dd, J = 9.5, 10.4 Hz, H-3′), 5.24 (1H, d, J = 3.8 Hz, H-1′), 5.18 (1H, dd, J = 9.5, 10.4 Hz, H-4′), 4.85 (1H, dd, J = 3.8, 10.4 Hz, H-2′), 4.46 (1H, d, J = 10.4 Hz, H-5′), 3.77 (3H, s, CO₂Me), 3.31 (1H, td, J = 10.8, 4.4 Hz), 2.04 (3H, s, OAc), 2.03 (6H, s, 2 x OAc), 1.07 (1H, q, J = 11.7 Hz), 0.91 (3H, d, J = 7.2 Hz), 0.89 (3H, d, J = 6.4 Hz), 0.69 (3H, d, J = 7.2 Hz); ¹³C-NMR (100 MHz, CDCl₃) δ: 170.11, 170.03, 169.59, 168.28, 97.67 (C-1′), 83.02, 70.86 (C-2′), 69.81 (C-4′), 69.34 (C-3′), 68.23 (C-5′), 52.85, 48.28, 42.64, 33.99, 31.52, 24.83, 22.68, 22.13, 20.95, 20.73, 20.55, 20.47, 15.70; high resolution (HR)MS (ESI): Calcd for C₂₃H₃₆O₁₀Na [M + Na]⁺ 495.2206, Found 495.2205.

Cholestan-3β-yl 2,3,4-Tri-O-acetyl-α-D-glucuronide Methyl Ester (7b)

To a solution of methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate (5) (82.6 mg, 0.220 mmol) and β-cholestanol (7a) (126.0 mg, 0.325 mmol) in DCM (5.0 mL), Tf₂NH (128.2 mg, 0.456 mmol) was added at room temperature. After stirring the mixture for 3 h at room temperature, the mixture was poured into ice water and extracted with ethyl acetate. The organic layer was treated as usual. The residue was purified by silica gel column chromatography (n-hexane : ethyl acetate = 4 : 1) to afford the desired α-glucuronide (7b) (108.8 mg, 70%) as crystal needles. mp 178.8–179.0 °C. [α]²⁷_D=+94.2 (c 1.30, CHCl₃). ¹H-NMR (400 MHz, CDCl₃) δ: 5.53 (1H, t, J = 10.2 Hz, H-3′), 5.31 (1H, d, J = 3.7 Hz, H-1′), 5.14 (1H, t, J = 10.2 Hz, H-4′), 4.80 (1H, dd, J = 3.7, 10.2 Hz, H-2′), 4.42 (1H, d, J = 10.2 Hz, H-5′), 3.75 (3H, s, CO₂Me), 3.53 (1H, m), 2.06, 2.04, 2.03 (each 3H, s, OAc), 0.89, 0.86, 0.84 (each 3H, d, J = 6.7 Hz); ¹³C-NMR (100 MHz, CDCl₃) δ: 170.12, 169.96, 169.65, 168.39, 94.18 (C-1′), 78.31, 70.70 (C-2′), 69.76 (C-4′), 69.25 (C-3′), 68.13 (C-5′), 56.35, 56.18, 54.15, 52.86, 44.82, 42.52, 39.90, 39.45, 36.59, 36.10, 35.75, 35.67, 35.47, 35.40, 31.95, 28.60, 28.21, 27.97, 27.43, 24.16, 23.77, 22.80, 22.53, 21.15, 20.72, 20.67, 20.54, 18.61, 12.27, 12.01; HRMS (ESI) C₄₀H₆₄O₁₀Na [M + Na]⁺ 727.4397, Found 727.4400.

1-Adamantyl 2,3,4-Tri-O-acetyl-α-D-glucuronide Methyl Ester (8b)

To a solution of methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate (5) (51.0 mg, 0.136 mmol) and 1-adamantanol (8a) (31.5 mg, 0.207 mmol) in DCM (2.5 mL), Tf₂NH (77.2 mg, 0.274 mmol) was added at room temperature. After stirring the mixture for 1 h at room temperature, the mixture was poured into ice water and extracted with ethyl acetate. The organic layer was treated as usual. The residue was purified by silica gel column chromatography (n-hexane : ethyl acetate = 4 : 1) to afford the desired α-glucuronide (8b) (35.8 mg, 56%) as a colorless oil. [α]²⁶_D=+94.6 (c 2.04, CHCl₃). ¹H-NMR (400 MHz, CDCl₃): δ; 5.57 (1H, d, J = 3.8 Hz, H-1′), 5.55 (1H, t, J = 10.2 Hz, H-3′), 5.14 (1H, t, J = 10.2 Hz, H-4′), 4.78 (1H, dd, J = 3.8, 10.2 Hz, H-2′), 4.52 (1H, d, J = 10.2 Hz, H-5′), 3.75 (3H, s, CO₂Me), 2.15 (3H, br s), 2.05, 2.04, 2.03 (each 3H, s, OAc), 1.77 (3H, br d, A type of AB, J = 11.8 Hz), 1.71 (3H, br d, B type of AB, J = 11.8 Hz), 1.63 (3H, br d, A′ type of A′B′, J = 12.4 Hz), 1.58 (3H, br d, B′ type of A′B′, J = 12.4 Hz),: ¹³C-NMR (100 MHz, CDCl₃) δ: 170.19, 170.06, 169.67, 168.55, 88.78 (C-1′), 75.69, 70.79 (C-2′), 69.95 (C-4′), 69.37 (C-3′), 67.95 (C-5′), 52.81, 42.02 (3C), 35.95 (3C), 30.44 (3C), 20.74 (2C), 20.56. HRMS (ESI) Calcd for C₂₃H₃₀O₁₀Na [M + Na]⁺ 491.1893, Found 491.1895.

2-Adamantyl 2,3,4-Tri-O-acetyl-α-D-glucuronide Methyl Ester (9b)

To a solution of methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate (5) (50.9 mg, 0.135 mmol) and 2-adamantanol (9a) (33.5 mg, 0.220 mmol) in DCM (2.5 mL), Tf₂NH (75.7 mg, 0.269 mmol) was added at room temperature. After stirring the mixture for 1.5 h at room temperature, the mixture was poured into ice water and extracted with ethyl acetate. The organic layer was treated as usual. The residue was purified by silica gel column chromatography (n-hexane : ethyl acetate = 3 : 1) to afford the desired α-glucuronide (9b) (34.6 mg, 55%) as crystal needles. mp 113.6–113.8 °C. [α]²⁶_D=+134.9 (c 0.66, CHCl₃). ¹H-NMR (400 MHz, CDCl₃) δ: 5.60 (1H, t, J = 10.0 Hz, H-3′), 5.33 (1H, d, J = 3.8 Hz, H-1′), 5.16 (1H, t, J = 10.0 Hz, H-4′), 4.86 (1H, dd, J = 3.8, 10.0 Hz, H-2′), 4.43 (1H, d, J = 10.0 Hz, H-5′), 3.78 (1H, br s), 3.75 (3H, s, CO₂Me), 2.14 (1H, br d, J = 12.4 Hz), 2.10–1.95 (1H, m), 1.90–1.78 (5H, m), 1.75–1.52 (6H, m), 1.46 (1H, br d, J = 12.4 Hz), 2.050, 2.047, 2.045 (each 3H, s, OAc); ¹³C-NMR (100 MHz, CDCl₃) δ: 170.82, 170.70, 170.35, 169.08, 94.29 (C-1′), 81.02, 71.46 (C-2′), 70.52 (C-4′), 69.99 (C-3′), 68.91 (C-5′), 53.56, 37.89, 37.15, 36.64, 34.11, 32.31, 31.66, 31.48, 27.85, 27.66, 21.44, 21.29, 21.23; HRMS (ESI) Calcd for C₂₃H₃₀O₁₀Na [M + Na]⁺ 491.1893, Found 491.1895.

16-Nor-ent-kauryl 2,3,4-Tri-O-acetyl-α-D-glucuronide Methyl Ester (10b)

To a solution of methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate (5) (44.5 mg, 0.117 mmol) and 16-nor-ent-kauranol (10a)¹³⁾ (48.8 mg, 0.177 mmol) in DCM (2.5 mL), Tf₂NH (64.8 mg, 0.231 mmol) was added at room temperature. After stirring the mixture for 2 h at room temperature, the mixture was poured into ice water and extracted with ethyl acetate. The organic layer was treated as usual. The residue was purified by silica gel column chromatography (n-hexane : ethyl acetate = 3 : 1–2 : 1) to afford the desired α-glucuronide (10b) (27.0 mg, 39%) and its 2-deacetylated derivative (10c) (12.2 mg, 18%) as crystal needles. mp 126.5–127.6 °C. [α]²⁵_D=+69.8 (c 0.90, CHCl₃). ¹H-NMR (400 MHz, CDCl₃) δ: 5.51 (1H, t, J = 10.0 Hz, H-3′), 5.24 (1H, d, J = 3.6 Hz, H-1′), 5.15 (t, J = 10.0 Hz, H-4′), 4.84 (1H, dd, J = 3.6, 10.0 Hz, H-2′), 4.35 (1H, d, J = 10.0 Hz, H-5′), 4.22 (1H, m), 3.75 (3H, s, CO₂Me), 2.27 (1H, br s), 2.06, 2.04, 2.03 (each 3H, s, OAc), 1.99 (1H, dd, J = 2.4, 11.9 Hz), 1.02, 0.84, 0.80 (each 3H, s, CH₃); ¹³C-NMR (100 MHz, CDCl₃) δ: 170.05, 169.99, 169.64, 168.32, 94.32 (C-1′), 76.82, 70.76 (C-2′), 69.73 (C-4′), 69.34 (C-3′), 68.21 (C-5′), 57.43, 56.18, 52.86, 46.77, 43.66, 41.97, 41.63, 40.39, 39.28 (2C), 37.51, 33.57, 33.20, 25.88, 21.53, 20.75, 20.63, 20.55, 19.95, 18.57, 18.40, 17.62. HRMS (ESI) Calcd for C₃₂H₄₈O₁₀Na [M + Na]⁺ 615.3145, Found 615.3148.

16-Nor-ent-kauryl 3,4-di-O-acetyl-α-D-glucuronide Methyl Ester (10c)

Crystal needles. mp 133.0–133.8 °C. [α]²⁷_D=+60.6 (c 0.24, CHCl₃). ¹H-NMR (400 MHz, CDCl₃) δ: 5.26 (1H, t, J = 9.8 Hz, H-3′), 5.10 (1H, t, J = 10.0 Hz, H-4′), 5.04 (1H, d, J = 3.8 Hz, H-1′), 4.27 (1H, d, J = 10.0 Hz, H-5′), 4.28 (1H, m), 3.75 (3H, s, CO₂Me), 3.71 (1H, ddd, J = 3.8, 10.0, 12.0 Hz, H-2′), 2.30 (1H, br s), 2.10, 2.04 (each 3H, s, OAc), 2.01 (1H, m), 1.93 (1H, d, J = 11.8 Hz), 1.02, 0.84, 0.80 (each 3H, s, CH₃); ¹³C-NMR (100 MHz, CDCl₃) δ: 171.11, 169.68, 168.38, 97.11 (C-1′), 76.82, 72.58 (C-3′), 70.54 (C-2′), 69.15 (C-4′), 68.72 (C-5′), 57.0, 56.15, 52.91, 46.52, 43.66, 41.95, 41.66, 40.28, 39.35, 39.27, 37.51, 33.57, 33.21, 25.86, 21.55, 20.92, 20.57, 19.93, 19.13, 18.56, 17.71. HRMS (ESI) Calcd for C₃₀H₄₆O₁₀Na [M + Na]⁺ 573.3040, Found 573.3044.

(+)-Bornyl 2,3,4-Tri-O-acetyl-α-D-glucuronide Methyl Ester (11b)

To a solution of methyl 1,2,3,4-tetra-O-acetyl-β-D-glucuronate (5) (50.6 mg, 0.133 mmol) and (+)-borneol (11a) (31.0 mg, 0.201 mmol) in DCM (2.5 mL), Tf₂NH (72.5 mg, 0.258 mmol) was added at room temperature. After stirring the mixture for 2 h at room temperature, the mixture was poured into ice water and extracted with ethyl acetate. The organic layer was treated as usual. The residue was purified by silica gel column chromatography (n-hexane : ethyl acetate = 3 : 1) to afford the desired α-glucuronide (11b) and its β-anomer (11 : 1) (32.3 mg, 51%) as a colorless oil. [α]²⁵_D=+124.4 (c 1.48, CHCl₃). ¹H-NMR (400 MHz, CDCl₃) δ: 5.53 (1H, t, J = 10.0 Hz, H-3′), 5.22 (1H, d J = 3.8 Hz, H-1′), 5.14 (1H, t, J = 10.0 Hz, H-4′), 4.84 (1H, dd, J = 3.8, 10.0 Hz, H-2′), 4.35 (1H, d, J = 10.0 Hz, H-5′), 3.97 (1H, dt, J = 2.0, 8.4 Hz), 3.75 (3H, s, CO₂Me), 2.12–2.02 (2H, m), 2.07 (3H, s, OAc), 2.04 (6H, s, 2 x OAc), 1.80–1.70 (1H, m), 1.67 (1H, t, J = 4.6 Hz), 1.33–1.24 (1H, m), 1.17–1.09 (1H, m), 0.91, 0.87, 0.82 (each 3H, s, 3 x CH₃), 0.80 (1H, dd, J = 3.2, 13.2 Hz); ¹³C-NMR (100 MHz, CDCl₃) δ: 170.06, 169.98, 169.65, 168.25, 93.95 (C-1′), 82.21, 70.81 (C-5′), 69.71 (C-3′), 69.37 (C-4′), 68.12 (C-5′), 52.89, 49.03, 47.99, 44.76, 35.58, 28.34, 26.44, 20.76, 20.62, 20.54, 19.61, 18.83, 13.88. HRMS (ESI) Calcd for C₂₃H₃₄O₁₀Na [M + Na]⁺ 493.2050, Found 493.2050.

Conflict of Interest

The authors declare no conflicts of interest.

Supplementary Materials

This article contains supplementary materials.

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