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Molecules 2014, 19(2), 2602-2611; doi:10.3390/molecules19022602

Article
Glycosylation of a Newly Functionalized Orthoester Derivative
Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama 223-8522, Japan
*
Author to whom correspondence should be addressed.
Received: 26 January 2014; in revised form: 20 February 2014 / Accepted: 21 February 2014 / Published: 24 February 2014

Abstract

:
Tandem glycosylation of the 6-O-Fmoc-substituted benzyl orthoester derivative 2a was carried out in moderate yields by electrogenerated acid (EGA). The Fmoc group was effectively removed under mild basic conditions, and the product was submitted to the subsequent glycosylation.
Keywords:
glycosylation; orthoester; electrogenerated acid (EGA); Fmoc group; selective deprotection

1. Introduction

With the discovery of the numerous biologically important roles of sugar chains, a number of glycosylation protocols have been reported [1,2]. Generally, the glycosylation process consists of efficient generation of an oxocarbenium ion species as a glycosyl donor and modulation of its selective nucleophilic attack on aglycons. Enhancement of β-selectivity is supported by the participation of acyl-protected hydroxyl groups. In particular, construction of a cyclic carboxonium ion between the C1 and C2 positions is an effective method to obtain the corresponding β-glycosidic linkage of D-sugars, although orthoesters are produced as side products when sterically hindered functional groups are used in glycosyl donors and acceptors [3,4,5,6]. Orthoesters were known to be converted to the corresponding glycosides under acidic conditions, and relatively stable derivatives were employed as glycoside precursors by Kochetkov [7,8,9,10,11] and Kunz [12,13]. We reported the synthesis and reaction of the orthoester 1 (Figure 1), which could be purified by silica gel column chromatography and kept for one month at room temperature [14]. Upon glycosylation with appropriate alcohols in the presence of the electrogenerated acid (EGA) [15,16,17,18,19,20], which is considered to be anhydrous HClO4 produced by anodic oxidation of cyclohexanol and Bu4NClO4, the corresponding glycosides were produced in high (primary OH) to moderate (secondary OH) yields, even on tertiary OH groups [14]. Upon comparison with Lewis acids and Brønsted acids, EGA provided better results in glycosylation reactions. To synthesize sugar chains by repeated glycosylation, devices providing effective activation of the anomeric center and the subsequent selective deprotection of an appropriate hydroxyl group in sugar units are required. The orthoester 1 was modified to examine its applicability to practical tandem glycosylation. Here, we describe a synthesis of the orthoester derivative 2a carrying an Fmoc group at the C-6 position, and its glycosylation reaction.
Figure 1. The benzyl orthoesters 14.
Figure 1. The benzyl orthoesters 14.
Molecules 19 02602 g001

2. Results and Discussion

To examine the chemical properties of the orthoester 1 for the modulation of protecting groups, the acyl protecting groups were removed under basic conditions to give an unstable triol. Although removal of the TBS group in the benzylic region provided the stable phenol derivative 4 [14], the triol was labile under standard work-up conditions, and only detected by ESI mass spectrum. These observations prompted us to synthesize the orthoester derivative 2a, in which a protecting group at the C-6 position can be selectively removed. After assessment of various protecting groups, we adopted the Fmoc group, which is removed under mild basic conditions.

2.1. Synthesis of the Orthoester 2a

Synthesis of the C-6 modified orthoester 2a commenced with conversion of 6-O-trityl-1,2,3,4-tetra-O-benzoyl-β-d-glucopyranose, which was synthesized by the standard procedure from D-glucose, into 6-O-Fmoc-2,3,4-tri-O-benzoyl-α-d-glucopyranosyl bromide (5) using a two-step manipulation, followed by the anomeric bromination (Scheme 1). Glycosylation of 5 with the benzyl alcohol 6 under Königs–Knorr reaction conditions provided the expected orthoester 2a (50%) [21], along with the corresponding benzyl glycoside 2b (30%). Stability of 2a under standard work-up and chromatographic conditions was similar to the previously reported derivative 1. The Fmoc group in 2a was selectively removed under basic conditions to give 6-OH orthoester 3, which will be used as a glycosyl acceptor under neutral glycosylation conditions.
Scheme 1. Synthesis of the orthoester derivative 2a.
Scheme 1. Synthesis of the orthoester derivative 2a.
Molecules 19 02602 g002
Reagents and yields: (a) TrCl, Et3N, pyr.; BzCl. (b) H2Pd/C. (c) FmocCl, pyr. (d) HBr-AcOH, Ac2O, AcOH (43% in four steps. (e) 6, Hg(CN)2, 4Å MS (2a: 50%, 2b: 30%). (f) pyr. (81%).

2.2. Glycosylation of the Orthoester 2a

When the orthoester 2a was submitted to reaction with cyclohexanol in the presence of EGA [14] and 4Å MS in 1,2-DCM at 40 °C, the expected β-glycoside 7 was obtained in 62% yield (Scheme 2). Next selective removal of the Fmoc group in 7 under basic conditions was carried out to give 8 in 95% yield, without any acyl migration from C-4 to C-6 position or removal of the acyl group. Repeated glycosylation of 8 with 2a under the same conditions as mentioned above, gave the disaccharide 9 in 38% yield. Alternatively, glycosylation of 10 with 2a provided the disaccharide 11 in 53% yield. After removal of the Fmoc group, further glycosylation with 2a provided the corresponding trisaccharide 12 [22].
Scheme 2. Glycosylation reactions of the orthoester derivative 2a.
Scheme 2. Glycosylation reactions of the orthoester derivative 2a.
Molecules 19 02602 g003
Reagents and yields: (a) cyclohexanol, EGA, 4Å MS (62%). (b) Et3N, pyr. (95%). (c) 2a, EGA, 4Å MS (38%). (d) 10, EGA, 4Å MS (63%). (e) Et3N, pyr. (82%). (f) 2a, EGA, 4Å MS (12%).

3. Experimental

General Information

All reactions were carried out under an argon atmosphere unless otherwise noted. When necessary, solvents were dried prior to use. Dry THF, dry Et2O and dry CH2Cl2 were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). Optical rotations were measured on a JASCO P-2200 digital polarimeter with a sodium (D line) lamp. IR spectra were recorded on a JASCO Model A-202 spectrophotometer. 1H-NMR spectra and 13C-NMR spectra were obtained on JEOL JNM-GX400, JNM-α400, JNM-AL400 or JNM-ECX400 spectrometers in deuterated solvent using tetramethylsilane as an internal standard. Deuteriochloroform was used as a solvent, unless otherwise stated. High-resolution mass spectra were obtained on a Waters LCT Premier XE (ESI) or JEOL JMS-700 (FAB). Preparative and analytical TLC were carried out on silica gel plate (Kieselgel 60 F254, E. Merck AG., Darmstadt, Germany) using UV light, 1 M aq. sulfuric acid, and/or 5% molybdophosphoric acid in ethanol for detection. Kanto silica 60N (spherical, neutral, 105–210 μm) was used for column chromatography. All anodic oxidation was conducted using HA-151A (Hokuto Denko, Tokyo, Japan) as a potentiostat and galvanostat, using glassy carbon plate as anodes, and platinum plate or wire as cathode.
2,3,4-Tri-O-benzoyl-6-O-9-fluorenylmethyloxycarboxyl-D-glucopyranosyl bromide (5). To a suspension of d-glucose (4.55 g, 0.025 mol) in pyridine (50 mL) were added TrCl (10.6 g, 0.038 mol) and Et3N (18 mL, 0.13 mol), and the mixture was stirred at room temperature overnight. After the addition of BzCl (23 mL, 0.20 mol) at the same temperature, stirring was continued overnight. The reaction was quenched with sat. aq. NaHCO3, and the mixture was extracted with CHCl3. The organic layer was dried (Na2SO4), and concentrated in vacuo. After removal of the high polarity byproducts by silica gel short column (hexane/EtOAc = 5:1), a crude was used in the next step without further purification. The crude product was solved in MeOH/EtOAc (1:1, 100 mL). After the addition of 5% Pd-C (cat.), the mixture was stirred overnight under H2 atmosphere. The mixture was filtered and the filtrate was concentrated in vacuo. After silica gel short column chromatography (hexane/EtOAc = 3:1), a crude was used in the next step without further purification.
A mixture of the crude and FmocCl (9.80 g, 0.038 mol) in pyridine (100 mL) was stirred overnight. The reaction was quenched by sat. aq. NaHCO3, and the mixture was extracted with CHCl3. The organic layer was dried (Na2SO4), and concentrated in vacuo. After purification by silica gel short column chromatography (hexane/EtOAc = 10:1), a crude was used in the next step without further purification. A mixture of the crude, HBr-AcOH (250 mL), AcOH (60 mL), and Ac2O (60 mL) was stirred overnight. The mixture was extracted with CHCl3. The organic layer was dried (Na2SO4), and concentrated in vacuo. The residue was purified by silica gel column chromatography (hexane/EtOAc = 5:1) to give 5 as a colorless oil (8.43 g, 43% in 4 steps): [α]23D +76.4 (c 1.00, CHCl3); IR (film) 1732, 1601, 1261 cm−1; 1H-NMR (400 MHz) δ 4.40 (5H, m, H-6a,6b, Fmoc), 4.67 (1H, m, H-5), 5.36 (1H, dd, J = 9.8, 4.0 Hz, H-2), 5.78 (1H, t, J = 9.8 Hz, H-4), 6.28 (1H, t, J = 9.8 Hz, H-3), 6.89 (1H, d, J = 4.0 Hz, H-4), 7.40 (11H, m, Ar), 7.54 (2H, m, Ar), 7.64 (2H, m, Ar), 7.78 (2H, m, Ar), 7.89 (2H, m, Ar), 8.00 (4H, m, Ar); 13C-NMR (100 MHz) δ 46.5 (Fmoc), 64.9 (C-6), 67.7 (C-4), 70.4 (C-3), 70.5 (C-2), 71.3 (C-5), 72.5 (FmocCH2), 86.7 (C-1), 120.0 (Ar), 125.2 (Ar), 125.3 (Ar), 127.2 (Ar), 127.9 (Ar), 128.3 (Ar), 128.4 (Ar), 128.5 (Ar), 128.7 (Ar), 129.7 (Ar), 129.9 (Ar), 130.0 (Ar), 133.3 (Ar), 133.7 (Ar), 133.85 (Ar), 141.2 (Ar), 141.3 (Ar), 143.2 (Ar), 143.4 (Ar), 154.7 (C=O), 165.1 (C=O), 165.2 (C=O), 165.5 (C=O). ESI-MS: calcd for C42H33O10NaBr 799.1155 (M+Na)+, found m/z 799.1151.
(2R,5R,6R,7S)-5-(((((9H-Fluoren-9-yl)methoxy)carbonyl)oxy)methyl)-2-((2-((tert-butyldimethylsilyl)oxy)-4-methoxybenzyl)oxy)-2-phenyltetrahydro-3aH-[1,3]dioxolo[4,5-b]pyran-6,7-diyl dibenzoate (2a) and 2-tert-Buthyldimethylsyloxy-4-methoxyphenylmethyl2,3,4-tetra-O-benzoyl-6-O-9-fluorenylmethyloxycarboxyl-β-d-glucopyranoside (2b). To a solution of 5 (71 mg, 0.092 mmol) in anhydrous toluene (0.9 mL) were added 6 (74 mg, 0.27 mmol), Hg(CN)2 (46 mg, 0.18 mmol), and MS 4A at 100 °C, and the mixture was stirred overnight. The mixture was filtered through a Celite pad, and the filtrate was washed with sat. aq. NaHCO3. The organic extracts were dried (Na2SO4), and concentrated in vacuo. The residue was purified by silica gel column chromatography (hexane/acetone = 7:1) to give 2a (45 mg, 50%) and 2b (26 mg, 30%) as oil: 2a: [α]23D +1.2 (c 1.00, CHCl3); IR (film) 1728, 1613, 1257 cm−1; 1H-NMR (400 MHz) δ 0.15 (6H, s, SiCH3), 0.93 (9H, s, tBu), 3.74 (3H, s, OCH3), 4.16 (2H, m, Fmoc), 4.29 (6H, m, H-5,6a,6b, ArCH2, Fmoc), 4.78 (1H, m, H-2), 5.37 (1H, bd, H-4), 5.76 (1H, br, H-3), 6.04 (1H, bd, H-1), 6.31 (1H, d, J = 2.2 Hz, Ar), 6.48 (1H, dd, J = 8.3, 2.2 Hz, Ar), 7.18 (1H, d, J = 8.3 Hz, Ar), 7.41 (11H, m, Ar), 7.57 (4H, m, Ar), 7.75 (2H, m, Ar), 7.82 (2H, m, Ar), 7.97 (2H, m, Ar), 8.11 (2H, m, Ar); 13C-NMR (100 MHz) δ −4.3 (SiCH3), 18.1 (tBu), 25.6 (tBu), 46.6 (Fmoc), 55.2 (OCH3), 61.3 (ArCH2), 67.3 (C-6), 68.3 (C-2), 69.2 (C-3), 70.1 (C-4), 72.3 (FmocCH2), 77.2 (C-5), 97.4 (C-1), 105.3 (Ar), 105.9 (Ar), 119.9 (Ar), 120.5 (Ar), 121.4 (Ar), 125.3 (orthoester-C), 126.5 (Ar), 127.1 (Ar), 127.2 (Ar), 127.8 (Ar), 128.3 (Ar), 1283.4 (Ar), 128.5 (Ar), 129.1 (Ar), 129.6 (Ar), 129.9 (Ar), 130.0 (Ar), 133.5 (Ar), 133.6 (Ar), 135.3 (Ar), 141.2 (Ar), 143.2 (Ar), 154.0 (C=O), 154.9 (C=O), 159.9 (C=O), 165.2 (C=O). ESI-MS: calcd for C56H56O13SiNa 987.3388 (M+Na)+, found m/z 987.3373. 2b: [α]23D −14.9 (c 1.00, CHCl3); IR (film) 1735, 1610, 1262 cm−1; 1H-NMR (400 MHz) δ 0.10 (3H, s, SiCH3), 0.16 (3H, s, SiCH3), 0.94 (9H, s, tBu), 3.72 (3H, s, OCH3), 4.06 (1H, ddd, J = 9.8, 5.3, 3.2 Hz, H-5), 4.23 (1H, t, J = 7.6 Hz, Fmoc), 4.40 (4H, m, H-6a,6b, Fmoc), 4.76 (1H, d, J = 12.4 Hz, ArCH2), 4.83 (1H, d, J = 12.4 Hz, ArCH2), 4.86 (1H, d, J = 8.0 Hz, H-1), 5.58 (1H, dd, J = 9.8, 8.0 Hz, H-2), 5.60 (1H, t, J = 9.8 Hz,H-4), 5.83 (1H, t, J = 9.8 Hz, H-3), 6.27 (1H, dd, J = 8.7, 2.5 Hz, Ar), 6.30 (1H, d, J = 2.5 Hz, Ar), 7.12 (1H, d, J = 8.7 Hz, Ar), 7.35 (14H, m, Ar), 7.50 (2H, m, Ar), 7.62 (2H, m, Ar), 7.76 (2H, m, Ar), 7.83 (4H, m, Ar), 7.92 (1H, m, Ar); 13C-NMR (100 MHz) δ −4.4 (SiCH3), −4.3 (SiCH3), 18.1 (tBu), 25.6 (tBu), 29.7 (tBu), 46.6 (Fmoc), 55.2 OCH3), 65.6 (ArCH2), 66.3 (C-6), 69.6 (C-4), 70.2 (C-3), 71.6 (C-2), 72.0 (C-5), 72.9 (FmocCH2), 99.1 (C-1), 105.4 (Ar), 105.9 (Ar), 119.6 (Ar), 119.9 (Ar), 125.2 (Ar), 125.3 (Ar), 127.2 (Ar), 127.8 (Ar), 128.1 (Ar), 128.2 (Ar), 128.4 (Ar), 128.8 (Ar), 129.3 (Ar), 129.7 (Ar), 129.8 (Ar), 129.9 (Ar), 131.2 (Ar), 133.0 (Ar), 133.2 (Ar), 133.4 (Ar), 141.2 (Ar), 143.2 (Ar), 143.4 (Ar), 154.7 (C=O), 154.9 (C=O), 160.2 (C=O), 164.9 (C=O), 165.2 (C=O), 165.8 (C=O). ESI-MS: calcd for C56H56O13SiNa 987.3388 (M+Na)+, found m/z 987.3381.
(2R,5R,6R,7S)-2-((2-((tert-Butyldimethylsilyl)oxy)-4-methoxybenzyl)oxy)-5-(hydroxymethyl)-2-phenyl-tetrahydro-3aH-[1,3]dioxolo[4,5-b]pyran-6,7-diyl dibenzoate (3). To a solution of 2a (30 mg, 0.032 mmol) in pyridine (0.5 mL) was added Et3N (13 µL, 0.095 mmol) at room temperature. After being stirred overnight, the reaction mixture was concentrated in vacuo. The residue was purified by silica gel column chromatography (hexane/acetone = 2:1) to give 3 as a clear oil (19 mg, 81%): [α]23D +9.5 (c 1.00, CHCl3); IR (film) 3527, 1725, 1613, 1261 cm−1; 1H-NMR (400 MHz) δ 0.15 (6H, s, SiCH3), 0.90 (9H, s, tBu), 3.65 (1H, m, H-6), 3.75 (3H, s, OCH3), 3.78 (1H, m, H-6'), 3.89 (1H, m, H-5),4.29 (1H, d, J = 11.2 Hz, ArCH2), 4.36 (1H, d, J = 11.2 Hz, ArCH2), 4.75 (1H, ddd, J = 5.2, 2.9, 0.8 Hz, H-2), 5.37 (1H, m, H-4), 5.74 (1H, m, H-3), 6.03 (1H, bd, H-1), 6.31 (1H, d, J = 2.5 Hz, Ar), 6.48 (1H, dd, J = 8.5, 2.5 Hz, Ar), 7.18 (1H, d, J = 8.5 Hz, Ar), 7.44 (7H, m, Ar), 7.60 (2H, m, Ar), 7.81 (2H, m, Ar), 7.98 (2H, m, Ar), 8.05 (2H, m, Ar); 13C-NMR (100 MHz) δ −4.3 (SiCH3), 18.1 (tBu), 25.6 (tBu), 55.2 (OCH3), 61.2 (ArCH2), 62.4 (C-6), 68.3 (C-2), 69.4 (C-3), 69.8 (C-4), 72.6 (C-5), 97.5 (C-1), 105.3 (Ar), 105.9 (Ar), 120.5 (Ar), 121.1 (Ar), 126.4 (orthoester-C), 128.3 (Ar), 128.4 (Ar), 128.5 (Ar), 129.0 (Ar), 129.1 (Ar), 129.6 (Ar), 129.9 (Ar), 130.0 (Ar), 133.5 (Ar), 133.6 (Ar), 135.3 (Ar), 154.0 (C=O), 159.9 (C=O), 164.6 (C=O), 165.6 (C=O). ESI-MS: calcd for C41H46O11NaSi 765.2707 (M+Na)+, found m/z 765.2710.
Cyclohexyl 2,3,4-tri-O-benzoyl-6-O-9-fluorenylmethyloxycarboxyl-d-glucopyranoside (7). A 0.1 M solution of cyclohexanol in 1,2-DCE (10 mL) containing 4Å MS and Bu4NClO4 (3.42 g, 1 M) as a supporting salt, was electrolyzed by constant current electrolysis (C.C.E., 6 mA/cm2) at 40 °C, using a glassy carbon plate (1.5 cm × 1.5 cm) as an anode and a Pt plate (1.8 cm × 1.8 cm) as a cathode. After the electrolysis, the reaction mixture (3.4 mL, 0.1 M EGA solution, 3 equiv.) was added to a solution of 2a (0.11 g, 0.11 mmol), cyclohexanol (0.4 mL, 0.34 mmol), and 4Å MS in 1,2-DCE (1.1 mL). After being stirred 20 min, the mixture was filtered through a Celite pad and the filtrate was washed with sat. aq. NaHCO3. The organic extracts were dried (Na2SO4), and concentrated in vacuo. The residue was purified by preparative TLC (hexane/acetone = 5:1) to give 7 as an oil (0.056 g, 62%): [α]23D −4.10 (c 1.00, CHCl3); IR (film) 1732, 1601, 1262 cm−1; 1H-NMR (400 MHz) δ 1.22 (5H, m, cyclohexyl), 1.46 (2H, m, cyclohexyl), 1.70 (2H, m, cyclohexyl), 1.88 (1H, m, cyclohexyl), 3.72 (1H, m, cyclohexyl), 4.09 (1H, m, Fmoc), 4.22 (1H, m, Fmoc), 4.36 (3H, m, H-5,6a, Fmoc), 4.46 (1H, d, J = 11.4, 6.5 Hz, H-6b), 4.93 (1H, d, J = 8.0 Hz, H-1), 5.51 (1H, dd, J = 9.6, 8.0 Hz, H-2), 5.55 (1H, t, J = 9.6 Hz, H-4), 5.90 (1H, t, J = 9.6 Hz, H-3), 7.30 (11H, m, Ar), 7.51 (2H, m, Ar), 7.60 (2H, m, Ar), 7.77 (2H, m, Ar), 7.84 (2H, m, Ar), 7.95 (4H, m, Ar); 13C-NMR (100 MHz) δ 23.4 (cyclohexyl), 23.6 (cyclohexyl), 25.3 (cyclohexyl), 31.5 (cyclohexyl), 33.1 (cyclohexyl), 46.6 (Fmoc), 66.5 (C-6), 69.8 (C-4), 70.2 (C-2,3), 72.0 (FmocCH2), 72.9 (C-5), 78.2 (cyclohexyl), 99.7 (C-1), 120.0 (Ar), 125.2 (Ar), 127.2 (Ar), 127.9 (Ar), 128.3 (Ar), 128.4 (Ar), 128.7 (Ar), 128.8 (Ar), 129.5 (Ar), 129.7 (Ar), 129.8 (Ar), 133.1 (Ar), 133.2 (Ar), 133.5 (Ar), 141.2 (Ar), 143.2 (Ar), 123.3 (Ar), 154.8 (C=O), 164.9 (C=O), 165.3 (C=O), 165.8 (C=O). ESI-MS: calcd for C48H45O117997.2962 (M+H)+, found m/z 797.2936.
Cyclohexyl 2,3,4-tri-O-benzoyl-β-d-glucopyranoside (8). To a solution of 7 (54 mg, 0.068 mmol) in pyridine (1 mL) was added Et3N (19 µL, 0.14 mmol) at room temperature, and the mixture was stirred overnight. The mixture was concentrated in vacuo. The residue was purified by silica gel column chromatography (hexane/acetone = 2:1) to give 8 as an oil (37 mg, 95%): [α]23D −11.5 (c 1.00, CHCl3); IR (film) 3523, 1731, 1601, 1263 cm−1; 1H-NMR (400 MHz) δ 1.05 (3H, m, cyclohexyl), 1.48 (6H, m, cyclohexyl), 1.77 (1H, m, cyclohexyl), 2.41 (1H, bs, OH), 3.68 (4H, m, cyclohexyl, H-5,6a,6b), 4.38 (1H, d, J = 8.0 Hz, H-1), 5.37 (2H, bt, H-2,4), 5.82 (1H, t, J = 9.6 Hz, H-3), 7.18 (3H, m, Ar), 7.31 (4H, m, Ar), 7.41 (2H, m, Ar), 7.74 (2H, m, Ar), 7.85 (4H, m, Ar); 13C-NMR (100 MHz) δ 23.4 (cyclohexyl), 23.6 (cyclohexyl), 25.4 (cyclohexyl), 31.5 (cyclohexyl), 33.2 (cyclohexyl), 61.5 (C-6), 69.7 (C-4,4',6'), 72.1 (C-2,3,3'), 72.9 (C-2',5'), 74.5 (C-5), 78.1 (cyclohexyl), 99.6 (C-1,1'), 128.2 (Ar), 128.3 (Ar), 128.5 (Ar), 128.6 (Ar), 128.9 (Ar), 129.5 (Ar), 129.6 (Ar), 129.7 (Ar), 129.9 (Ar), 133.1 (Ar), 133.2 (Ar), 133.6 (Ar), 164.9 (C=O), 165.8 (C=O), 165.9 (C=O). ESI-MS: calcd for C33H34O9Na 597.2101 (M+Na)+, found m/z 597.2112.
Cyclohexyl 2,3,4-tri-O-benzoyl-β-d-glucopyranosyl-(1→6)-2,3,4-tri-O-benzoyl-6-O-9-fluorenyl-methyloxycarboxyl-β-d-glucopyranoside (9). A solution of cyclohexanol in 1,2-DCE (10 mL, 0.1 M) containing 4Å MS and Bu4NClO4 (3.42 g, 1 M) as a supporting salt, was electrolyzed by constant current electrolysis (C.C.E., 6 mA/cm2) at 40 °C, using a glassy carbon plate (1.5 cm × 1.5 cm) as an anode and a Pt plate (1.8 cm × 1.8 cm) as a cathode. The reaction mixture (0.6 mL, 0.1 M EGA solution, 3 equiv.) was added to a solution of 2a (31 mg, 0.032 mmol) and 8 (37 mg, 0.064 mmol), and MS 4A in 1,2-DCE (0.5 mL). After being stirred 20 min, the mixture was filtered through a Celite pad, and the filtrate was washed with sat. aq. NaHCO3. The organic extract was dried (Na2SO4), and concentrated in vacuo. The residue was purified by preparative TLC (hexane/acetone = 2:1) to give 9 as an oil (16 mg, 38%): [α]23D −9.8 (c 1.00, CHCl3); IR (film) 1734, 1601, 1261 cm−1; 1H-NMR (400 MHz) δ 1.41 (10H, m, cyclohexyl), 3.62 (1H, m, cyclohexyl), 4.00 (4H, m, Fmoc, H-5,5'), 4.29 (5H, m, H-6a,6b,6a',6b'), 4.78 (1H, d, J = 7.8 Hz, H-1'), 5.04 (1H, d, J = 7.8 Hz, H-1), 5.34 (1H, t, J = 9.8 Hz, H-4'), 5.41 (1H, dd, J = 9.8, 7.8 Hz, H-2'), 5.48 (1H, dd, J = 9.8, 7.8 Hz, H-2), 5.52 (1H, t, J = 9.8 Hz, H-4), 5.80 (1H, t, J = 9.8 Hz, H-3'), 5.83 (1H, t, J = 9.8 Hz, H-3), 7.41 (22H, m, Ar), 7.62 (2H, m, Ar), 7.78 (6H, m, Ar), 7.92 (8H, m, Ar); 13C-NMR (100 MHz) δ 22.9 (cyclohexyl), 23.2 (cyclohexyl), 25.4 (cyclohexyl), 31.2 (cyclohexyl), 32.9 (cyclohexyl), 46.6 (Fmoc), 66.0 (C-6), 68.3 (C-6'), 69.3(C-4), 69.9 (C-4'), 70.3 (C-3), 71.7 (C-2,3'), 71.9 (C-2'), 72.1 C-5'), 72.8 (C-5), 72.9 (cyclohexyl), 74.1 (FmocCH2), 99.4 (C-1'), 100.8 (C-1), 119.9 (Ar), 125.3 (Ar), 125.4 (Ar), 127.2 (Ar), 127.8 (Ar), 128.2 (Ar), 128.3 (Ar), 128.4 (Ar), 128.7 (Ar), 128.9 (Ar), 129.3 (Ar), 129.5 (Ar), 129.6 (Ar), 129.7 (Ar), 129.8 (Ar), 133.0 (Ar), 133.1 (Ar), 133.2 (Ar), 133.4 (Ar), 133.5 (Ar), 141.2 (Ar), 143.3 (Ar), 143.4 (Ar), 154.8 (C=O), 164.9 (C=O), 165.1 (C=O), 165.2 (C=O), 165.4 (C=O), 165.6 (C=O), 165.7 (C=O). ESI-MS: calcd for C75H66O19Na 1293.4096 (M+Na)+, found m/z 1293.4078.
Phenyl 2,3,4-tri-O-benzoyl-6-O-9-fluorenylmethyloxycarboxyl-β-d-glucopyranosyl-(1→6)-2,3,4-tri-O-benzoyl-1-thio-β-d-glucopyranoside (11). A solution of cyclohexanol in 1,2-DCE (10 mL, 0.1 M) containing 4Å MS and Bu4NClO4 (3.42 g, 1 M) as a supporting salt was electrolyzed by constant current electrolysis (C.C.E., 6 mA/cm2) at 40 °C, using a glassy carbon plate (1.5 cm × 1.5 cm) as an anode and a Pt plate (1.8 cm × 1.8 cm) as a cathode. After the electrolysis, the reaction mixture (1.3 mL, 0.1 M EGA solution, 3 equiv.) was added to a solution of 2a (34 mg, 0.036 mmol), 10 (42 mg, 0.072 mmol), m and 4Å MS in 1,2-DCE (0.5 mL). After being stirred for 20 min, the reaction mixture was filtered through a Celite pad, and the filtrate was washed with sat. aq. NaHCO3. The organic extracts were dried (Na2SO4), and concentrated in vacuo. The residue was purified by preparative TLC (hexane/acetone = 2:1) to give 11 as an oil (29 mg, 63%): [α]23D +8.9 (c 1.00, CHCl3); IR (film) 1733, 1601, 1261 cm−1; 1H-NMR (400 MHz) δ 3.99 (4H, m, Fmoc, H-5,5'), 4.24 (1H, m, Fmoc), 4.38 (4H, m, H-6a,6b,6a',6b'), 4.92 (1H, d, J = 9.8 Hz, H-1'), 4.97 (1H, d, J = 8.0 Hz, H-1), 5.31 (1H, t, J = 9.8 Hz, H-4'), 5.34 (1H, t, J = 9.8 Hz, H-3'), 5.51 (2H, m, H-2,4), 5.81 (1H, t, J = 9.8 Hz, H-3'), 5.85 (1H, t, J = 9.8 Hz, H-3), 7.23 (2H, m, Ar), 7.39 (25H, m, Ar), 7.61 (2H, m, Ar), 7.74 (4H, m, Ar), 7.85 (4H, m, Ar), 7.93 (6H, m, Ar); 13C-NMR (100 MHz) δ 46.6 (Fmoc), 65.9 (C-6), 68.4 (C-6'), 69.4 (C-4'), 69.5 (C-4'), 70.2 (C-3'), 70.4 (C-3), 71.6 (C-2), 72.0 (C-2'), 72.8 (C-5'),74.0 (FmocCH2), 78.3 (C-5), 85.9 (C-1'), 101.0 (C-1), 119.9 (Ar), 125.3 (Ar), 125.4 (Ar), 127.2 (Ar), 128.2 (Ar), 128.3 (Ar), 128.4 (Ar), 128.5 (Ar), 128.6 (Ar), 128.7 (Ar), 128.8 (Ar), 129.1 (Ar), 129.2 (Ar), 129.7 (Ar), 129.8 (Ar), 131.7 (Ar), 133.2 (Ar), 133.4 (Ar), 133.5 (Ar), 141.2 (Ar), 141.3 (Ar), 143.3 (Ar), 143.4 (Ar), 154.8 (C=O), 164.9 (C=O), 165.1 (C=O), 165.2 (C=O), 165.3 (C=O), 165.6 (C=O), 165.7 (C=O). ESI-MS: calcd for C75H60O18NaS 1303.3398 (M+Na)+, found m/z 1303.3385.
Phenyl 2,3,4-tri-O-benzoyl-6-O-9-fluorenylmethyloxycarboxyl-β-d-glucopyranosyl-(1→6)-2,3,4-tri-O-benzoyl-β-d-glucopyranosyl-(1→6)-2,3,4-tri-O-benzoyl-1-thio-β-d-glucopyranoside (12). To a solution of 11 (29 mg, 0.022 mmol) in pyridine (0.5 mL) was added Et3N (6.3 µL, 0.045 mmol) at room temperature. After being stirred overnight, the mixture was concentrated in vacuo. The residue was purified by silica gel column chromatography (hexane/acetone = 2:1) to give an alcohol as an oil (19 mg, 82%): [α]23D −2.1 (c 1.00, CHCl3); IR (film) 3062, 1731, 1601, 1262 cm−1; 1H-NMR (400 MHz) δ 2.81 (1H, dd, J = 8.0, 5.6 Hz, OH), 3.60 (1H, m, H-5), 3.74 (2H, m, H-5',6a'), 3.95 (1H, dd, J = 10.5, 5.6 Hz, H-6b'), 4.05 (2H, m, H-6a,6b), 4.92 (2H, bt, H-1,1'), 5.23 (1H, t, J = 9.6 Hz, H-4'), 5.40 (3H, m, H-2,2'), 5.81 (1H, t, J = 9.6 Hz, H-3'), 5.85 (1H, t, J = 9.6 Hz, H-3), 7.38 (19H, m, Ar), 7.53 (4H, m, Ar), 7.76 (2H, m, Ar), 7.81 (2H, m, Ar), 7.93 (8H, m, Ar); 13C-NMR (100 MHz) δ 61.2 (C-6), 67.9 (C-6'), 69.4 (C-4'), 69.9 (C-4), 70.4 (C-3'), 71.5 (C-3), 72.8 (C-2), 73.9 (C-2'), 74.6 (C-5'), 77.8 (C-5), 86.0 (C-1'), 100.5 (C-1), 128.2 (Ar), 128.3 (Ar), 128.4 (Ar), 128.5 (Ar), 128.7 (Ar), 128.8 (Ar), 129.1 (Ar), 129.2 (Ar), 129.7 (Ar), 129.8 (Ar), 129.9 (Ar), 131.5 (Ar), 133.2 (Ar), 133.3 (Ar), 133.5 (Ar), 133.6 (Ar), 164.9 (C=O), 165.1 (C=O), 165.6 (C=O), 165.7 (C=O), 165.9 (C=O). ESI-MS: calcd for C60H50O16NaS 1081.2717 (M+Na)+, found m/z 1081.2732.
A solution of cyclohexanol in 1,2-DCE (10 mL, 0.1 M) containing 4Å MS and Bu4NClO4 (3.42 g, 1 M) as a supporting salt, was electrolyzed by constant current electrolysis (C.C.E., 6 mA/cm2) at 40 °C, using a glassy carbon plate (1.5 cm × 1.5 cm) as an anode and a Pt plate (1.8 cm × 1.8 cm) as a cathode. The reaction mixture (0.6 mL, 0.1 M EGA solution, 3 equiv.) was added to a solution of 2a (37 mg, 0.038 mmol), the alcohol (20 mg, 0.019 mmol), and 4Å MS in 1,2-DCE (0.5 mL). After being stirred at 40 °C for 20 min, the reaction mixture was filtered through a Celite pad, and the filtrate was washed with saturated aqueous NaHCO3. The organic extracts were dried (Na2SO4), and concentrated in vacuo. The residue was purified by preparative TLC (Et2O/hexane = 2:1) to give 12 as a clear oil (4.1 mg, 12%): [α]23D −5.4 (c 1.00, CHCl3); IR (film) 1733, 1601, 1261 cm−1; 1H-NMR (400 MHz) δ 3.62 (1H, dd, J = 11.2, 5.0 Hz, H-6a), 3.84 (3H, m, Fmoc, H-5',6), 4.01 (2H, m, H-5,5''), 4.31 (6H, m, H-6a,6b,6a',6b'), 4.61 (1H, d, J = 8.0 Hz, H-1'), 4.99 (1H, d, J = 10.0 Hz, H-1''), 5.08 (1H, d, J = 8.0 Hz, H-1), 5.10 (1H, t, J = 10.0 Hz, H-4), 5.21 (1H, dd, J = 9.8, 8.0 Hz, H-2''), 5.51 (4H, m, H-2,2',4',4''), 5.68 (1H, t, J = 9.8 Hz, H-3'), 5.84 (1H, t, J = 9.8 Hz, H-3''), 6.06 (1H, t, J = 9.8 Hz, H-3), 7.18 (2H, m, Ar), 7.36 (34H, m, Ar), 7.58 (2H, m, Ar), 7.76 (2H, m, Ar), 7.92 (14H, m, Ar); 13C-NMR (100 MHz) δ 46.6 (Fmoc), 66.2 (C-6), 68.3 (C-6''), 69.4 (C-6'), 69.7 (C-4''), 70.1 (C-4), 70.2 (C-4'), 70.5 (C-3''), 71.7 (C-3), 71.9 (C-2,2',3'), 72.0 (C-2''), 72.6 (C-5'), 72.7 (C-5''), 73.9 (C-5), 74.1 ((FmocCH2), 86.3 (C-1''), 100.5 (C-1), 101.2 (C-1'), 119.9 (Ar), 120.0 (Ar), 125.3 (Ar), 125.4 (Ar), 127.2 (Ar), 127.8 (Ar), 128.1 (Ar), 128.2 (Ar), 128.3 (Ar), 128.4 (Ar), 128.5 (Ar), 128.6 (Ar), 128.7 (Ar), 128.8 (Ar), 129.0 (Ar), 129.1 (Ar), 129.3 (Ar), 129.4 (Ar), 129.7 (Ar), 129.8 (Ar), 130.0 (Ar), 132.1 (Ar), 132.8 (Ar), 133.0 (Ar), 133.2 (Ar), 133.4 (Ar), 141.2 (Ar), 143.3 (Ar), 143.4 (Ar), 154.8 (C=O), 164.9 (C=O), 165.2 (C=O), 165.3 (C=O), 165.7 (C=O). ESI-MS: calcd for C102H82O26NaS 1777.4713 (M+Na)+, found m/z 1777.4736.

4. Conclusions

The orthoester 2a bearing the selectively removable Fmoc group at the C-6 position was synthesized, and its utility as a glycosyl donor and acceptor after deprotection of the Fmoc group was demonstrated.

Acknowledgments

We thanked for MEXT-Supported Programs for the Strategic Research Foundation at Private Universities. 2009-2013, 2012-2016; Scientific Research (C) from MEXT; Sasakawa Research Grant for Financial Support (to KK).

Author Contributions

The listed authors contributed to this work as described in the following. Kohei Kawa synthesized the sugar derivatives. Tsuyoshi Saitoh made technical supports of the chemical and electrochemical procedures. Eisuke Kaji contributed with discussions of carbohydrate chemistry and the research direction. Shigeru Nishiyama conducted the research progress. All authors helped preparing manuscript and approved the final version.

Conflicts of Interest

The authors declare no conflict of interest.

References and Notes

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  22. The reaction condition has not yet optimized.
  • Sample Availability: Samples are available from the authors.
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