Method Article

Regioselective O-Glycosylation of Nucleosides via the Temporary 2',3'-Diol Protection by a Boronic Ester for the Synthesis of Disaccharide Nucleosides

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DOI:

10.3791/57897

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July 26th, 2018

 ,  ,  , 
1Faculty of Pharmaceutical Sciences, Tokyo University of Science, 2Imaging Frontier Center, Tokyo University of Science

In This Article

Summary

Here, we present protocols for the synthesis of disaccharide nucleosides by the regioselective O-glycosylation of ribonucleosides via a temporary protection of their 2',3'-diol moieties utilizing a cyclic boronic ester. This method applies to several unprotected nucleosides such as adenosine, guanosine, cytidine, uridine, 5-methyluridine, and 5-fluorouridine to give corresponding disaccharide nucleosides.

Abstract

Disaccharide nucleosides, which consist of disaccharide and nucleobase moieties, have been known as a valuable group of natural products having multifarious bioactivities. Although chemical O-glycosylation is a commonly beneficial strategy to synthesize disaccharide nucleosides, the preparation of substrates such as glycosyl donors and acceptors requires tedious protecting group manipulations and a purification at each synthetic step. Meanwhile, several research groups have reported that boronic and borinic esters serve as a protecting or activating group of carbohydrate derivatives to achieve the regio- and/or stereoselective acylation, alkylation, silylation, and glycosylation. In this article, we demonstrate the procedure for the regioselective O-glycosylation of unprotected ribonucleosides utilizing boronic acid. The esterification of 2',3'-diol of ribonucleosides with boronic acid makes the temporary protection of diol, and, following O-glycosylation with a glycosyl donor in the presence of p-toluenesulfenyl chloride and silver triflate, permits the regioselective reaction of the 5'-hydroxyl group to afford the disaccharide nucleosides. This method could be applied to various nucleosides, such as guanosine, adenosine, cytidine, uridine, 5-metyluridine, and 5-fluorouridine. This article and the accompanying video represent useful (visual) information for the O-glycosylation of unprotected nucleosides and their analogs for the synthesis of not only disaccharide nucleosides, but also a variety of biologically relevant derivatives.

Introduction

Disaccharide nucleosides, which are conjugates of a nucleoside and a carbohydrate moiety linked via an O-glycosidic bond, constitute a valuable class of naturally-occurring carbohydrate derivatives1,2,3,4,5,6,7. For instance, they are incorporated in biological macromolecules such as tRNA (transfer ribonucleic acid) and poly(ADP-ribose) (ADP = adenosine diphosphate), as well as in some antibacterial agents and other biologically-active substances (e.g., adenophostins, amicetins, ezomycin)5,6,8,9,10,11,12,13,14,15,16,17,18,19. Hence, disaccharide nucleosides and their derivatives are expected to be lead compounds for drug discovery research. The methodologies for the synthesis of disaccharide nucleosides are classified into three categories; enzymatic O-glycosylation20,21, chemical N-glycosylation5,9,16,22,23,24, and chemical O-glycosylation7,9,14,16,18,19,24,25,26,27,28,29,30,31,32,33,34,35,36,37. In particular, chemical O-glycosylation would be an efficient method for the stereoselective synthesis and large-scale synthesis of disaccharide nucleosides. Previous research has shown that the O-glycosylation of 2'-deoxyribonucleoside 2 with the thioglycosyl donor 1, using the combination of p-toluenesulfenyl chloride and silver triflate, affords the desired disaccharide nucleoside 3 (Figure 1A; Ar = aryl and PG = protecting group)38.

Following these results, we decided to develop the O-glycosylation of ribonucleosides applying the p-toluenesulfenyl chloride/silver triflate promoter system. While several examples of the O-glycosylation of partially protected ribonucleosides have been demonstrated7,9,14,16,18,19,24,32,33,34,35,36,37, the use of unprotected or temporarily-protected ribonucleosides as a glycosyl acceptor for O-glycosylation has been negligibly reported. Therefore, the development of regioselective O-glycosylation of unprotected or temporarily-protected ribonucleosides would provide a more beneficial synthetic method without protecting group manipulations of ribonucleosides. In order to achieve the regioselective O-glycosylation of ribonucleosides, we focused on the boron compounds, because several examples of regio- and/or stereoselective acylation, alkylation, silylation, and glycosylation of carbohydrate derivatives assisted by boronic or borinic acid have been reported39,40,41,42,43,44,45,46,47,48,49,50. In this article, we demonstrate the procedure for the synthesis of disaccharide nucleosides utilizing regioselective O-glycosylation at the 5'-hydroxyl group of ribonucleosides via a boronic ester intermediate. In the strategy presented here, boronic ester intermediate 6 would be afforded by the esterification of the ribonucleoside 4 with the boronic acid 5, which allows the regioselective O-glycosylation at the 5'-hydroxyl group with thioglycosyl donor 7 to give the disaccharide nucleoside 8 (Figure 1B)51. We also studied the interaction of a ribonucleoside and boronic acid by nuclear magnetic resonance (NMR) spectroscopy, to observe the formation of a boronic ester. Esterification to make a boronic ester and a glycosylation reaction require anhydrous conditions to prevent the hydrolysis of the boronic ester and the glycosyl donor. In this article, we demonstrate the typical procedures to obtain the anhydrous conditions for successful glycosylation reactions for researchers and students not only in chemistry but also in other research fields.

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Protocol

NOTE: All experimental data [NMR, infrared spectroscopies (IR), mass spectroscopies (MS), optical rotations, and elemental analyses data] of the synthesized compounds were reported in a previous paper51.

1. Procedure for O-Glycosylation Reactions

  1. Synthesis of compound α/β-12 (Entry 12 in Table 1)
    NOTE: Entries 1 - 13 in Table 1 were carried out using a similar procedure.
    1. Temporary protection of 2',3'-diol of ribonucleoside40
      1. In a 10 mL pear-shaped flask (flask 1), dissolve mannosyl donor α-9 (28.4 mg, 0.0486 mmol)52, uridine 10 (7.9 mg, 0.0324 mmol) and 4-(trifluoromethyl)phenylboronic acid 11c (9.3 mg, 0.0490 mmol) in anhydrous pyridine (0.40 mL).
        NOTE: The use of a 10 mL pear-shaped flask is recommended because, in step 1.1.3.1, the reaction mixture will be transferred to flask 2 (a 10 mL two-neck round-bottom flask with a septum attached to it) containing molecular sieves powder.
      2. Co-evaporate the reaction mixture (obtained in step 1.1.1.1.) with anhydrous pyridine (0.40 mL, 3x) and anhydrous 1,4-dioxane (0.40 mL, 3x) at room temperature to ca. 40 °C to remove any water.
      3. Dissolve the residue (obtained in step 1.1.1.2.) in anhydrous 1,4-dioxane (0.32 mL) and stir the reaction mixture at its reflux temperature for 1 h to form a boronic ester (the temporary protection).
      4. Remove the solvent using a rotary evaporator followed by a vacuum pump.
    2. Activation of molecular sieves
      1. In a 10 mL two-neck round-bottom flask with a septum attached to it (flask 2), add 4 Å molecular sieves powder (64 mg).
        NOTE: Appropriate molecular sieves should be selected according to the solvent used for the glycosylation (3 Å for acetonitrile and 4 Å for 1,4-dioxane, dichloromethane, and propionitrile).
      2. Heat the molecular sieves in a microwave under atmospheric pressure and cool them under reduced pressure evacuated by a vacuum pump (3x), and then dry them with a heat gun under reduced pressure while replacing the air with argon gas several times.
    3. Glycosylation
      1. Dissolve the residue of step 1.1.1.4. in flask 1 in propionitrile (0.64 mL) or other solvents and transfer this solution to flask 2.
        NOTE: Acetonitrile, 1,4-dioxane, dichloromethane, and propionitrile were used for Entries 1 - 7 and 9, Entry 10, Entry 11, and Entries 8, 12, and 13, respectively.
      2. Stir the reaction mixture in flask 2 at room temperature for 0.5 h followed by cooling it to -40 °C.
        NOTE: The temperature was changed according to the solvent used for the glycosylation (-40 °C for dichloromethane and propionitrile, room temperature for 1,4-dioxane, and -20 °C for acetonitrile).
      3. Add silver triflate (49.9 mg, 0.194 mmol) and p-toluenesulfenyl chloride (12.8 µL, 0.0968 mmol) to the reaction mixture at the same temperature as used in step 1.1.3.2.
      4. Stir the reaction mixture at the same temperature for 1.5 h.
      5. Check the reaction by thin-layer chromatography (TLC) with hexane/ethyl acetate [3/1 (v/v)] to check the glycosyl donors [the retention factor (Rf) (donor α-9) = 0.63] and with chloroform/methanol [10/1 (v/v))] to check the glycosyl acceptors and products [Rf (acceptor 10) = 0.03, Rf (desired product) = 0.50].
      6. Quench the reaction mixture with saturated aqueous sodium bicarbonate (1.0 mL), dilute it with chloroform (2.0 mL), remove the insoluble materials with Celite, and carefully wash the Celite with chloroform (20 mL).
      7. Wash the filtrate (organic layer) with saturated aqueous sodium bicarbonate (20 mL, 3x) and brine (20 mL) using a 100 mL separatory funnel.
      8. Dry the resulting organic layer with sodium sulfate, filter the insoluble materials, and concentrate the filtrate using a rotary evaporator.
      9. Roughly purify the remaining residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 50/1 (v/v)] to afford crude 5'-O-(6"-O-acetyl-2",3",4"-tri-O-benzyl-α/β-ᴅ-mannopyranosyl)uridine containing a small quantity of byproducts (15.2 mg, colorless syrup).
    4. Acetylation
      1. In a 5 mL vial, dissolve the resulting crude compound prepared in step 1.1.3.9 in anhydrous pyridine (0.20 mL).
      2. Add N,N-dimethyl-4-aminopyridine (a catalytic amount) and acetic anhydride (20.4 µL, 0.0216 mmol: 10 equivalents based on the crude compound) to the solution at 0 °C.
      3. Stir the reaction mixture at the same temperature for 0.5 h followed by a warming to room temperature.
      4. After stirring overnight, check the reaction by TLC with chloroform/methanol [30/1 (v/v)] [Rf (α/β-12) = 0.45].
      5. Dilute the reaction mixture with chloroform (20 mL).
      6. Wash the organic layer with 1 M hydrochloric acid (20 mL, 3x), saturated aqueous sodium bicarbonate (20 mL, 3x), and brine (20 mL) using a 100 mL separatory funnel.
      7. Dry the resulting organic layer with sodium sulfate, filter the insoluble materials, and concentrate the filtrate using a rotary evaporator.
      8. Purify the remaining residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 90/1 (v/v)] to give α/β-12 (15.8 mg, 61%, α/β = 1.6/1, colorless amorphous solid).
  2. Synthesis of compounds β-22 to β-30 (Table 2) and β-33 (Table 3)
    NOTE: The synthesis of β-22 - β-30 and β-33 was carried out using a similar procedure.
    1. Synthesis of compound β-22 (Entry 1 in Table 2)
      1. Temporary protection of 2', 3'-diol of ribonucleoside
        1. In a 10 mL pear-shaped flask (flask 3), dissolve adenosine 13 (20.4 mg, 0.0763 mmol), galactosyl donor β-21 (80.4 mg, 0.114 mmol)53, and 4-(trifluoromethyl)phenylboronic acid 11c (21.7 mg, 0.114 mmol) in anhydrous pyridine (0.76 mL).
          NOTE: The use of a 10 mL pear-shaped flask is recommended because the reaction mixture will be transferred to flask 4 (a 10 mL two-neck round-bottom flask with a septum attached to it) containing molecular sieves powder in the step 1.2.1.3.1.
        2. Co-evaporate the reaction mixture (obtained in step 1.2.1.1.1.) with anhydrous pyridine (0.76 mL, 3x) and anhydrous 1,4-dioxane (0.76 mL, 3x) at room temperature to ca. 40 °C to remove any water.
        3. Dissolve the residue (obtained in step 1.2.1.1.2.) in anhydrous 1,4-dioxane (0.76 mL) and stir the reaction mixture at its reflux temperature for 1 h to form a boronic ester (a temporary protection).
        4. Remove the solvent using a rotary evaporator followed by a vacuum pump.
      2. Activation of molecular sieves
        1. In a 10 mL two-neck round-bottom flask with a septum attached to it (flask 4), add 4 Å molecular sieves powder (150 mg).
        2. Heat the molecular sieves in a microwave under atmospheric pressure and cool them under reduced pressure evacuated by a vacuum pump (3x), and then dry them with a heat gun under reduced pressure while replacing the air with argon gas several times.
      3. Glycosylation
        1. Dissolve the residue of step 1.2.1.1.4. in flask 3 in propionitrile (1.50 mL) and transfer this solution to flask 4.
        2. Stir the reaction mixture at room temperature for 0.5 h, followed by cooling it to -40 °C.
        3. Add silver triflate (117.6 mg, 0.458 mmol) and p-toluenesulfenyl chloride (30.3 µL, 0.229 mmol) to the reaction mixture at the same temperature as mentioned in step 1.2.1.3.2.
        4. Stir the reaction mixture, at the same temperature for 1.5 h.
        5. Check the reaction by TLC with hexane/ethyl acetate [2/1 (v/v)] to check the glycosyl donors [Rf (donor β-21) = 0.62] and with chloroform/methanol [10/1 (v/v)] to check the glycosyl acceptors and products [Rf (acceptor 13) = 0.05, Rf (desired product) = 0.30].
        6. Quench the reaction mixture with saturated aqueous sodium bicarbonate (2.0 mL), dilute it with chloroform (3.0 mL), remove the insoluble materials through Celite, and carefully wash the Celite with chloroform (30 mL).
        7. Wash the filtrate (organic layer) with saturated aqueous sodium bicarbonate (30 mL, 3x) and brine (30 mL) using a 100 mL separatory funnel.
        8. Dry the resulting organic layer with sodium sulfate, filter the insoluble materials, and concentrate the filtrate using a rotary evaporator.
        9. Purify the remaining residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 30/1 (v/v)] to afford β-22 (27.4 mg, 42%, colorless solid).
    2. Synthesis of compound β-23 (Entry 2 in Table 2)
      1. Conduct the reaction using 14 (28.4 mg, 0.0765 mmol)54, β-21 (80.5 mg, 0.115 mmol), 11c (21.8 mg, 0.115 mmol), p-toluenesulfenyl chloride (30.3 µL, 0.229 mmol), silver triflate (117.8 mg, 0.458 mmol), anhydrous 1,4-dioxane (0.76 mL), anhydrous propionitrile (1.50 mL), and 4 Å molecular sieves (150 mg). Purify the resulting residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 50/1 (v/v)] to give β-23 (21.9 mg, 30%, colorless solid). TLC: Rf (β-23) = 0.37 [chloroform/methanol = 10/1 (v/v)].
    3. Synthesis of compound β-24 (Entry 3 in Table 2)
      1. Conduct the reaction using 15 (21.6 mg, 0.0763 mmol), β-21 (80.5 mg, 0.115 mmol), 11c (21.8 mg, 0.115 mmol), p-toluenesulfenyl chloride (30.3 µL, 0.229 mmol), silver triflate (117.6 mg, 0.458 mmol), anhydrous 1,4-dioxane (0.76 mL), anhydrous propionitrile (1.50 mL), and 4 Å molecular sieves (150 mg). Purify the resulting residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 8/1 (v/v)] to give β-24 (8.1 mg, 12%, colorless solid). TLC: Rf (β-24) = 0.20 [chloroform/methanol = 10/1 (v/v)].
    4. Synthesis of compound β-25 (Entry 4 in Table 2)
      1. Conduct the reaction using 16 (27.0 mg, 0.0764 mmol)55, β-21 (80.5 mg, 0.115 mmol), 11c (21.8 mg, 0.115 mmol), p-toluenesulfenyl chloride (30.3 µL, 0.229 mmol), silver triflate (117.8 mg, 0.458 mmol), anhydrous 1,4-dioxane (0.76 mL), anhydrous propionitrile (1.50 mL), and 4 Å molecular sieves (150 mg). Purify the resulting residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 20/1 (v/v)] to give β-25 (31.4 mg, 44%, colorless solid). TLC: Rf (β-25) = 0.27 [chloroform/methanol = 10/1 (v/v)].
    5. Synthesis of compound β-26 (Entry 5 in Table 2)
      1. Conduct the reaction using 10 (18.6 mg, 0.0762 mmol), β-21 (80.4 mg, 0.114 mmol), 11c (21.7 mg, 0.114 mmol), p-toluenesulfenyl chloride (30.3 µL, 0.229 mmol), silver triflate (117.6 mg, 0.458 mmol), anhydrous 1,4-dioxane (0.76 mL), anhydrous propionitrile (1.50 mL), and 4 Å molecular sieves (150 mg). Purify the resulting residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 40/1 (v/v)] to give β-26 (26.1 mg, 42%, colorless solid). TLC: Rf (β-26) = 0.45 [chloroform/methanol = 10/1 (v/v)].
    6. Synthesis of compound β-27 (Entry 6 in Table 2)
      1. Conduct the reaction using 17 (19.7 mg, 0.0763 mmol), β-21 (80.5 mg, 0.115 mmol), 11c (21.8 mg, 0.115 mmol), p-toluenesulfenyl chloride (30.3 µL, 0.229 mmol), silver triflate (117.6 mg, 0.458 mmol), anhydrous 1,4-dioxane (0.76 mL), anhydrous propionitrile (1.50 mL), and 4 Å molecular sieves (150 mg). Purify the resulting residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 40/1 (v/v)] to give β-27 (33.8 mg, 53%, colorless solid). TLC: Rf (β-27) = 0.50 [chloroform/methanol = 10/1 (v/v)].
    7. Synthesis of compound β-28 (Entry 7 in Table 2)
      1. Conduct the reaction using 18 (20.0 mg, 0.0763 mmol), β-21 (80.4 mg, 0.114 mmol), 11c (21.7 mg, 0.114 mmol), p-toluenesulfenyl chloride (30.3 µL, 0.229 mmol), silver triflate (117.6 mg, 0.458 mmol), anhydrous 1,4-dioxane (0.76 mL), anhydrous propionitrile (1.50 mL), and 4 Å molecular sieves (150 mg). Purify the resulting residue by column chromatography [silica gel, chloroform then ethyl acetate/chloroform = 1/1 (v/v)] to give β-28 (38.8 mg, 61%, colorless solid). TLC: Rf (β-28) = 0.33 [chloroform/methanol = 10/1 (v/v)].
    8. Synthesis of compound β-29 (Entry 8 in Table 2)
      1. Conduct the reaction using 19 (18.5 mg, 0.0761 mmol), β-21 (80.4 mg, 0.114 mmol), 11c (21.7 mg, 0.114 mmol), p-toluenesulfenyl chloride (30.3 µL, 0.229 mmol), silver triflate (117.6 mg, 0.458 mmol), anhydrous 1,4-dioxane (0.76 mL), anhydrous propionitrile (1.50 mL), and 4 Å molecular sieves (150 mg). Purify the resulting residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 10/1 (v/v)] to give β-29 (34.1 mg, 55%, colorless solid). TLC: Rf (β-29) = 0.25 [chloroform/methanol = 10/1 (v/v)].
    9. Synthesis of compound β-30 (Entry 9 in Table 2)
      1. Conduct the reaction using 20 (26.6 mg, 0.0766 mmol)56, β-21 (80.6 mg, 0.115 mmol), 11c (21.8 mg, 0.115 mmol), p-toluenesulfenyl chloride (30.3 µL, 0.229 mmol), silver triflate (117.8 mg, 0.458 mmol), anhydrous 1,4-dioxane (0.76 mL), anhydrous propionitrile (1.50 mL), and 4 Å molecular sieves (150 mg). Purify the resulting residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 50/1 (v/v)] to give β-30 (28.0 mg, 40%, colorless solid). TLC: Rf (β-30) = 0.48 [chloroform/methanol = 10/1 (v/v)].
    10. Synthesis of compound β-33 (Entry 1 in Table 3)
      1. Conduct the reaction using 18 (20.0 mg, 0.0762 mmol), β-31 (80.4 mg, 0.114 mmol)57, 11c (21.7 mg, 0.114 mmol), p-toluenesulfenyl chloride (30.3 µL, 0.229 mmol), silver triflate (117.6 mg, 0.458 mmol), anhydrous 1,4-dioxane (0.76 mL), anhydrous propionitrile (1.50 mL), and 4 Å molecular sieves (150 mg). Purify the resulting residue by column chromatography [silica gel, chloroform/methanol = 1/0 - 30/1 (v/v)] to give β-33 (34.5 mg, 54%, colorless solid). TLC: Rf (β-33) = 0.33 [chloroform/methanol = 10/1 (v/v)].

2. Deprotection of β-28 (Figure 2)

  1. In a 5 mL vial, add β-28 (25.2 mg, 0.0300 mmol) and 10 M methylamine in methanol (2.0 mL)58.
  2. Stir the reaction mixture at 0 °C for 2 h followed by warming it to room temperature.
  3. After stirring the mixture for 13 h, check the reaction by TLC with chloroform/methanol [10/1 (v/v)] [Rf (β-35) = 0.20].
  4. Concentrate the reaction mixture using a rotary evaporator.
  5. Dissolve the resulting residue in water (15 mL) and wash the aqueous layer with dichloromethane (15 mL, 3x) using a 50 mL separatory funnel.
  6. Concentrate the aqueous layer using a rotary evaporator.
  7. Purify the remaining residue by preparative high-performance liquid chromatography (HPLC) [column: ODS (octadecylsilane) column (20Φ x 250 mm), eluent: water (contains 0.1% [v/v] trifluoroacetic acid), flow rate: 8.0 mL/min, detection: 266 nm, temperature: 25 °C, retention time: 20 min] to give β-35 (7.9 mg, 62%, colorless amorphous solid)59.

3. NMR Studies of Cyclic Boronic Ester (Figure 3 and 4)

  1. Preparation and measurement of 36
    1. In 10 mL pear-shaped flask, dissolve uridine 10 (34.3 mg, 0.140 mmol) and 4-(trifluoromethyl)phenylboronic acid 11c (40.0 mg, 0.211 mmol) in anhydrous pyridine (1.00 mL).
    2. Co-evaporate the reaction mixture with anhydrous pyridine (1.00 mL, 3x) and anhydrous 1,4-dioxane (1.00 mL, 3x) at room temperature to ca. 40 °C to remove any water.
    3. Dissolve the residue in anhydrous 1,4-dioxane (1.40 mL) and stir the reaction mixture at its reflux temperature for 1 h to form a boronic ester (a temporary protection).
    4. Dispense the reaction mixture (0.14 mL) to a 5 mL vial.
    5. Remove the solvent from the 5 mL vial using a rotary evaporator followed by a vacuum pump.
    6. Dissolve the resulting residue 36 in acetonitrile-d3 (0.64 mL).
    7. Measure 1H, 11B and 19F NMR spectroscopies using a quartz NMR tube at 25 °C.
  2. Preparation and measurement of 38
    1. Prepare the reaction mixture 38 from 11c (40.0 mg, 0.211 mmol) using the similar procedure as that of the step 3.1.

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Results

The results of the O-glycosylation of uridine 10 with thiomannoside α-9 are summarized in Table 160,61. In Entry 1, the O-glycosylation of 10 with α-9 in the absence of boronic acid derivatives resulted in the formation of a complicated mixture. In Entry 2, 10 and phenylboronic acid 11a...

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Discussion

The purpose of this manuscript is to show a convenient synthetic method to prepare disaccharide nucleosides using unprotected ribonucleosides without tedious protecting group manipulations. We report herein on the regioselective O-glycosylations of nucleosides via the temporary 2',3'-diol protection by a cyclic boronic ester (Figure 1B)51.

The preparation of the cyclic boronic ester intermediate is...

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Disclosures

The authors have nothing to disclose.

Acknowledgements

This research was financed by grants-in-aid from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (Nos. 15K00408, 24659011, 24640156, 245900425 and 22390005 for Shin Aoki), by a grant from the Tokyo Biochemical Research Foundation, Tokyo, Japan, and by the TUS (Tokyo University of Science) fund for strategic research areas. We would like to thank Noriko Sawabe (Faculty of Pharmaceutical Sciences, Tokyo University of Science) for the measurements of the NMR spectra, Fukiko Hasegawa (Faculty of Pharmaceutical Sciences, Tokyo University of Science) for the measurements of the mass spectra, and Tomoko Matsuo (Research Institute for Science and Technology, Tokyo University of Science) for the measurements of the elemental analyses.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Silver trifluoromethanesulfonateNacalai Tesque34945-61
Phenylboronic acid (contains varying amounts of anhydride)Tokyo Chemical IndustryB0857
p-Methoxyphenylboronic acidWako Pure Chemical Industries321-69201
4-(Trifluoromethyl)phenylboronic acid (contains varying amounts of anhydride)Tokyo Chemical IndustryT1788
2,4-Difluorophenylboronic acid (contains varying amounts of anhydride)Tokyo Chemical IndustryD3391
Cyclopentylboronic acid (contains varying amounts of Anhydride)Tokyo Chemical IndustryC2442
4-Nitrophenylboronic acid (contains varying amounts of anhydride)Tokyo Chemical IndustryN0812
4-Hexylphenylboronic acid (contains varying amounts of anhydride)Tokyo Chemical IndustryH1489
AdenosineMerck KGaA862.
GuanosineAcros Organics411130050
CytidineTokyo Chemical IndustryC0522
UridineTokyo Chemical IndustryU0020
5-FluorouridineTokyo Chemical IndustryF0636
5-MethyluridineSigmaM-9885
Methylamine (40% in Methanol, ca. 9.8mol/L)Tokyo Chemical IndustryM1016
N,N-dimethyl-4-aminopyridineWako Pure Chemical Industries044-19211
Acetic anhydrideNacalai Tesque00226-15
Pyridine, DehydratedWako Pure Chemical Industries161-18453
AcetonitrileKanto Chemical01031-96
1,4-DioxaneNacalai Tesque13622-73
DichloromethaneWako Pure Chemical Industries130-02457
PropionitrileWako Pure Chemical Industries164-04756
Molecular sieves 4A powderNacalai Tesque04168-65
Molecular sieves 3A powderNacalai Tesque04176-55
Celite 545RVSNacalai Tesque08034-85
Acetonitrile-D3 (D,99.8%)Cambridge Isotope LaboratoriesDLM-21-10
Trifluoroacetic acidNacalai Tesque34831-25
TLC Silica gel 60 F254Merck KGaA1.05715.0001
ChromatorexFuji Silysia ChemicalFL100D
Sodium hydrogen carbonateWako Pure Chemical Industries191-01305
Hydrochloric acidWako Pure Chemical Industries080-01061
Sodium sulfateNacalai Tesque31915-96
ChloroformKanto Chemical07278-81
Sodium chlorideWako Pure Chemical Industries194-01677
MethanolNacalai Tesque21914-74
JEOL Always 300JEOLMeasurement of NMR
Lamda 400JEOLMeasurement of NMR
PerkinElmer Spectrum 100 FT-IR SpectrometerPerkin ElmerMeasurement of IR
JEOL JMS-700JEOLMeasurement of MS
PerkinElmer CHN 2400 analyzerPerkin ElmerMeasurement of elemental analysis
JASCO P-1030 digital polarimeterJASCOMeasurement of optical rotation
JASCO PU-2089 Plus intelligent HPLC pumpJASCOFor HPLC
Jasco UV-2075 Plus Intelligent UV/Vis DetectorJASCOFor HPLC
Rheodyne Model 7125 InjectorSigma-Aldrich58826For HPLC
Chromatopac C-R8AShimadzuFor HPLC
Senshu Pak Pegasil ODSSenshu ScientificFor HPLC
p-Toluenesulfenyl chloridePrepared  Ref. 38
Phenyl 6-O-acetyl-2,3,4-tri-O-benzyl-1-thio-a-D-mannopyranoside (a-9)Prepared  Ref. 52
4-Metylphenyl 2,3,4,6-tetra-O-benzoyl-1-thio-b-D-galactopyranoside (b-21)Prepared  Ref. 53
4-Metylphenyl 2,3,4,6-tetra-O-benzoyl-1-thio-b-D-glucopyranoside (b-31)Prepared  Ref. 57
4-Metylphenyl 2,3,4,6-tetra-O-benzoyl-1-thio-a-D-Mannopyranoside (a-32)Prepared  Ref. 67
6-N-Benzoyladenosine (14)Prepared  Ref. 54
2-N-Isobutyrylguanosine (16)Prepared  Ref. 55
4-N-Benzoylcytidine (20)Prepared  Ref. 56

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Boronic Ester ProtectionRibonucleoside GlycosylationSilver Triflate CatalysisPara Toluenesulfonyl ChlorideMolecular Sieves DryingSilica Gel ChromatographyNMR AnalysisAnhydrous Conditions

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