Chemistry
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Regioselective O-Glycosylation of Nucleosides via the Temporary 2',3'-Diol Protection by a Boronic Ester for the Synthesis of Disaccharide Nucleosides
Chapters
Summary July 26th, 2018
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.
Transcript
This method can be expected to help drug discovery research because of short process needed for the preparation of biologically-active disaccharide nucleosides as compared with traditional existing methods. The main advantage of this technique is that unprotected nucleosides can be applied for glycosylation without a tedious protecting group manipulation to synthesize disaccharide nucleosides. Visual demonstration of this method will present useful information for successful glycosylation of unprotected nucleosides for researchers and students, not only in chemistry but also in other research fields.
To begin, optimize the reaction conditions for the synthesis of compound Alpha-and Beta-12 as described in the text protocol. Next, apply the optimized reaction conditions to the synthesis of Beta-22 to Beta-30, as well as for Beta-33, using a similar procedure. In this video, the synthesis of compound Beta-22 is demonstrated.
Begin with the temporary protection of two prime, three prime-Diol of ribonucleoside by dissolving 20.4 milligrams of Adenosine-13 in 0.76 milliliters of anhydrous pyridine in a 10 milliliter pear-shaped flask. Then, add 80.4 milligrams of galactosyl donor Beta-21. Now add 21.7 milligrams of four-Trifluoromethyl phenylboronic acid 11C.
This boronic acid alternative is used because it gave Alpha-and Beta-12 in the highest chemical yield during optimization. Evaporate the mixture, dissolved in 0.76 milliliters of anhydrous pyridine, using the rotary evaporator. Then, co-evaporate the resulting reaction mixture with 0.76 milliliters of anhydrous pyridine twice, followed by 0.76 milliliters of anhydrous one four-dioxane three times at room temperature to approximately 40 degrees Celsius to remove any water.
Dissolve the residue in 0.76 milliliters of anhydrous one four-dioxane and stir the reaction mixture at its reflux temperature for one hour to form a boronic ester as a temporary protection. Remove the solvent using a rotary evaporator, followed by a vacuum pump. Meanwhile, in a 10 milliliter two-neck round-bottom flask with a septum attached to it, add 150 milligrams of four-angstrom molecular sieves powder.
Heat the molecular sieves in a microwave under atmospheric pressure. Then, cool them under reduced pressure, evacuated by a vacuum pump three times. After cooling, dry the sieves with a heat gun under reduced pressure, while replacing the air with argon gas several times.
To perform the glycosylation, dissolve the residue resulting from vacuum decompression in 1.50 milliliters of anhydrous propionitrile and transfer this solution to a flask containing the molecular sieves. Stir the reaction mixture at room temperature for 0.5 hours, followed by cooling it to minus 40 degrees Celsius. Now, add 117.6 milligrams of silver triflate followed by 30.3 microliters of para toluenesulfonyl chloride to the reaction mixture at the same temperature.
Stir the reaction mixture at minus 40 degrees Celsius for 1.5 hours. Next, check the reaction by thin-layer chromatography with two to one hexane diethyl acetate to check the glycosyl donors and with 10 to one chloroform to methanol to check the glycosyl acceptors and products. Quench the reaction mixture with 2.0 milliliters of saturated aqueous sodium bicarbonate.
Then, dilute the reaction with 3.0 milliliters of chloroform. Now, remove the insoluble materials through celite and carefully wash the celite with 30 milliliters chloroform. Using a 100 milliliter separatory funnel, wash the filtrate with 30 milliliters of saturated aqueous sodium bicarbonate three times.
Then, wash the filtrate with 30 milliliters of brine. Dry the resulting organic layer with sodium sulfate. After filtering the insoluble materials, concentrate the filtrate using a rotary evaporator.
Purify the remaining residue by silica gel column chromatography with a chloroform methanol gradient ranging from a one to zero ratio to a 30 to one ratio to afford the Beta-22. Continue synthesis of other compounds as detailed in the text protocol. In a 10 milliliter pear-shaped flask, dissolve 34.3 milligram of Uridine 10 in one milliliter of anhydrous pyridine.
Add 40.0 milligrams of four trifluoromethyl phenylboronic acid 11C. Co-evaporate the reaction mixture with one milliliter of anhydrous pyridine three times and then one milliliter of anhydrous one four dioxane three times at room temperature to approximately 40 degrees Celsius to remove any water. Dissolve the residue in anhydrous 1.40 milliliters of one four dioxane.
Then, stir the reaction mixture at its reflux temperature for one hour to form a boronic ester as a temporary protection. Now, dispense the 0.14 milliliters of the reaction mixture to a five milliliter vial. Remove the solvent from the five milliliter vial using a rotary evaporator, followed by a vacuum pump.
Dissolve the resulting residue in 0.64 milliliters of tridutero acetonitrile. Finally, measure proton, boron 11 and fluorine 19 NMR spectrum using a quartz NMR tube at 25 degrees Celsius. Reaction conditions for the glycosylation of Uridine 10 with glycosyl donor Alpha-nine were optimized.
The condition, as shown in entry 12, gave the disaccharide nucleoside Alpha-and Beta-12 in highest chemical yield. Shown here are the results of the glycosylations of various nucleosides 10 and 13 through 20 with glycosyl donor Beta-21, that when used with optimized reaction conditions, afford the corresponding disaccharide nucleosides Beta-22 through Beta-30 in the highest yields. While attempting this procedure, it's very important to obtain the anhydrous conditions using sufficient redried flask, molecular sieves and solvents to avoid the hydrolysis of boronic ester on the glycosyl donor.
The application of the temporary protection of hydroxyl groups by boronic ester will be useful for the synthesis of a variety of natural and artificial compounds, as well as disaccharide nucleosides.
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