Name: one-pot microwave-assisted conversion of anomeric nitrate-esters to trichloroacetimidates
Uploaded: 2018-01-15T13:00:00
Description: Watch this Scientific Journal Video about One-pot Microwave-assisted Conversion of Anomeric Nitrate-esters to Trichloroacetimidates at JoVE.com

The authors would like to acknowledge Vanderbilt University and the Institute of Chemical Biology for financial support. Mr. Berkley Ellis and Prof. John McLean are acknowledged for High-Resolution Mass Spectral Analysis. 
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1. Representative Microwave-Assisted Denitration
<ol>
	<li>Place the azido nitrate ester (1.0 equiv., 0.2 mmol) in an 8 mL microwave reaction vial. The scale of the reaction can be increased to several mmol without any adverse effect on reaction progress.</li>
	<li>Dissolve the azido-nitrate ester in 20% aq. acetone (0.1 M, 2.0 mL). Add pyridine (5.0 equiv., 0.08 mL, 1.0 mmol) to the reaction vessel. Cap the microwave irradiation vial and place the reaction vessel in a microwave reactor cavity.</li>
	<li>Irradiate the ...

<ol>
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B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Strategy+for+the+Solid-Phase+Synthesis+of+Oligosaccharides.">A Strategy for the Solid-Phase Synthesis of Oligosaccharides.</a> Science. 260 (5112), 1307-1309 (1993).</li><li>Demchenko, A. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stereoselective+chemical+1,2-cis+O-glycosylation:+From+'sugar+ray'+to+modern+techniques+of+the+21st+century.">Stereoselective chemical 1,2-cis O-glycosylation: From 'sugar ray' to modern techniques of the 21st century.</a> Synlett. (9), 1225-1240 (2003).</li><li>Fraserreid, B., Wu, Z. F., Udodong, U. E., Ottosson, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Armed-Disarmed+Effects+in+Glycosyl+Donors+-+Rationalization+and+Sidetracking.">Armed-Disarmed Effects in Glycosyl Donors - Rationalization and Sidetracking.</a> J. Org. Chem. 55 (25), 6068-6070 (1990).</li><li>Bohe, L., Crich, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+propos+of+glycosyl+cations+and+the+mechanism+of+chemical+glycosylation;+the+current+state+of+the+art.">A propos of glycosyl cations and the mechanism of chemical glycosylation; the current state of the art.</a> Carbohydr. Res. 403, 48-59 (2015).</li><li>Toshima, K., Tatsuta, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+Progress+in+O-Glycosylation+Methods+and+Its+Application+to+Natural-Products+Synthesis.">Recent Progress in O-Glycosylation Methods and Its Application to Natural-Products Synthesis.</a> Chem. Rev. 93 (4), 1503-1531 (1993).</li><li>Koenigs, W., Knorr, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ueber+einige+Derivate+des+Traubenzuckers+und+der+Galactose.">Ueber einige Derivate des Traubenzuckers und der Galactose.</a> Chem. Ber. 34 (1), 957-981 (1901).</li><li>Mukaiyama, T., Murai, Y., Shoda, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+Efficient+Method+for+Glucosylation+of+Hydroxy+Compounds+Using+Glucopyranosyl+Fluoride.">An Efficient Method for Glucosylation of Hydroxy Compounds Using Glucopyranosyl Fluoride.</a> Chem. Lett. (3), 431-432 (1981).</li><li>Meloncelli, P. J., Martin, A. D., Lowary, T. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Glycosyl+iodides.+History+and+recent+advances.">Glycosyl iodides. History and recent advances.</a> Carbohydrate Research. 344 (9), 1110-1122 (2009).</li><li>Lian, G., Zhang, X., Yu, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thioglycosides+in+carbohydrate+research.">Thioglycosides in carbohydrate research.</a> Carbohydr. Res. 403, 13-22 (2015).</li><li>Schmidt, R. R., Kinzy, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anomeric-Oxygen+Activation+for+Glycoside+Synthesis+-+the+Trichloroacetimidate+Method.">Anomeric-Oxygen Activation for Glycoside Synthesis - the Trichloroacetimidate Method.</a> Advances in Carbohydrate Chemistry and Biochemistry. 50, 21-123 (1994).</li><li>Schmidt, R. R., Toepfer, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Glycosylation+with+highly+reactive+glycosyl+donors:+efficiency+of+the+inverse+procedure.">Glycosylation with highly reactive glycosyl donors: efficiency of the inverse procedure.</a> Tetrahedron Lett. 32 (28), 3353-3356 (1991).</li><li>Keith, D. J., Townsend, S. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct,+microwave-assisted+substitution+of+anomeric+nitrate-esters.">Direct, microwave-assisted substitution of anomeric nitrate-esters.</a> Carbohydr. Res. 442, 20-24 (2017).</li><li>Bukowski, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+and+Conformational+Analysis+of+the+T-Antigen+Disaccharide(B-D-Gal-(1->3)-a-D-GalNAc-OMe).">Synthesis and Conformational Analysis of the T-Antigen Disaccharide(B-D-Gal-(1->3)-a-D-GalNAc-OMe).</a> Eur. J. Org. Chem. 14, 2697-2705 (2001).</li></ol>

The protocol described in this tutorial provides a method to convert nitrate esters to useful, reactive functionality. In a broader sense, employing a microwave reactor to complete specific maneuvers over the course of a carbohydrate synthesis has the potential to make difficult transformations facile and routine. Our goal in this tutorial is to demonstrate how to handle carbohydrates in the context of microwave irradiation.
In the case of the parent reaction, previous efforts to achieve denit...

Due to their ubiquity in molecular biology, carbohydrates have been longstanding targets for chemical synthesis.1,2,3 At the core of any successful synthetic campaign is the correct deployment of glycosylation reactions to build the oligosaccharide chain.4,5,6,7,8,9,10,11,12 Not surprisingly, there are a large number of methods to install glycosidic bonds.13,14 The Koenigs-Knorr method is one of the earliest known procedures and involves coupling a glycosyl chloride or bromide with an alcoholic component, usually under heavy metal (mercury or silver) activation.15 Related glycosyl fluorides were first introduced as donors in 1981 by the Mukaiyama group and have found widespread application due to their increased thermal and chemical stability.16 On the opposite end of the reactivity spectrum are glycosyl iodides, which are far more reactive than the other halides. Increased reactivity is accompanied by increased stereocontrol, particularly when forming &#945;-linked oligosaccharides.17 In addition to &#34;haloglycosides&#34;, thioglycosides have found wide utility, in part, due to their ease of formation, stability to a multitude of reaction conditions, and activation with electrophilic reagents.18
The methods described above focus on converting an anomeric alcohol to a &#34;non-oxygen&#34; containing, latent leaving group that is activated and ultimately displaced by an alcohol from an acceptor molecule. Anomeric oxygen activation as described by the Schmidt school, focuses on converting the C1 oxygen itself, to a leaving group.19 This method is the most powerful and widely used in chemical glycosylation reactions. Trichloroacetimidate donors are readily prepared from a reducing sugar and trichloroacetonitrile in the presence of a base such as potassium carbonate (K2CO3) or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). These species are then activated using Lewis acids.20
Recently, we have reported that 2-azido-1-trichloroacetimidate donors can be directly prepared from glycals. The process involves a two reaction, one-pot procedure from 2-azido-1-nitrate esters.21 This detailed protocol is intended to assist practitioners in successfully completing the transformation in high yield. Of particular interest is the first step of the sequence, which focuses on thermal denitration under microwave- assisted heating. We also hope to provide a visual tutorial on employing microwave reactors in organic synthesis.

<table>
	<tbody>
		<tr>
			<td>230 400 mesh silica gel</td>
			<td>SiliCycle Inc</td>
			<td>R10030B</td>
			<td></td>
		</tr>
		<tr>
			<td>TLC plates</td>
			<td>SiliCycle Inc</td>
			<td>TLG-R10014B-527</td>
			<td></td>
		</tr>
		<tr>
			<td>Ceric ammonium molybdate</td>
			<td>Sigma-Aldrich</td>
			<td>A1343</td>
			<td></td>
		</tr>
		<tr>
			<td>Solvent Still</td>
			<td>Mbraun</td>
			<td>MB-SPS-800</td>
			<td></td>
		</tr>
		<tr>
			<td>Infared spectrometer</td>
			<td>Thermo</td>
			<td>Thermo Electron IR100</td>
			<td></td>
		</tr>
		<tr>
			<td>Nuclear Magnetic Resonance</td>
			<td>Bruker</td>
			<td></td>
			<td>400, 600 MHz</td>
		</tr>
		<tr>
			<td>LC/MS</td>
			<td>Thermo/Dionex</td>
			<td></td>
			<td>Single quad, ESI</td>
		</tr>
		<tr>
			<td>HRMS</td>
			<td>Agilent</td>
			<td>Synapt G2 S HDMS</td>
			<td></td>
		</tr>
		<tr>
			<td>Microwave reactor</td>
			<td>Anton Parr</td>
			<td>Anton Parr G10 Monowave 200</td>
			<td></td>
		</tr>
		<tr>
			<td>DBU</td>
			<td>Sigma-Aldrich</td>
			<td>139009</td>
			<td></td>
		</tr>
		<tr>
			<td>CCl3CN</td>
			<td>Sigma-Aldrich</td>
			<td>T53805</td>
			<td></td>
		</tr>
		<tr>
			<td>Pyridine</td>
			<td>Sigma-Aldrich</td>
			<td>270970</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>Fisher Scientific</td>
			<td>A18-20</td>
			<td>Tech. grade</td>
		</tr>
		<tr>
			<td>Phase separator</td>
			<td>Biotage</td>
			<td>120-1901-A</td>
			<td></td>
		</tr>
		<tr>
			<td>Rotary evaporator</td>
			<td>Buchi</td>
			<td>R-100</td>
			<td></td>
		</tr>
	</tbody>
</table>

one-pot microwave-assisted conversion of anomeric nitrate-esters to trichloroacetimidates

The goal of the following procedure is to provide a demonstration of the one-pot conversion of a 2-azido-1-nitrate-ester to a trichloroacetimidate glycosyl donor. Following azido-nitration of a glycal, the product 2-azido-1-nitrate ester can be hydrolyzed under microwave-assisted irradiation. This transformation is usually achieved using strongly nucleophilic reagents and extended reaction times. Microwave irradiation induces hydrolysis, in the absence of reagents, with short reaction times. Following denitration, the intermediate anomeric alcohol is converted, in the same pot, to the corresponding 2-azido-1-trichloroacetimidate.

The technology described herein was demonstrated on a pool of three 2-azido-1-nitrate esters. In each case the first step of the reaction was complete within 20 minutes.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56610/56610fig2v3.jpg" /> 
Figure 2. Representative example of hydrolysis (1 -&#62;2), and one-pot conversion of 2-azido-1-nitrate ester of <strong...

Watch this Scientific Journal Video about One-pot Microwave-assisted Conversion of Anomeric Nitrate-esters to Trichloroacetimidates at JoVE.com

One-pot Microwave-assisted Conversion of Anomeric Nitrate-esters to Trichloroacetimidates

A 2-azido-1-nitrate-ester can be converted to the corresponding 2-azido-1-trichloroacetimidate in a one-pot procedure. The goal of the manuscript is to demonstrate utility of the microwave reactor in carbohydrate synthesis.

The goal of the following procedure is to provide a demonstration of the one-pot conversion of a 2-azido-1-nitrate-ester to a trichloroacetimidate ...

The overall goal of this experiment is to use a one-pot procedure to synthesize trichloroacetimidates directly from azido nitrate-esters with minimal workup in purification. This method poses clear advantages for the synthesis of trichloroacetimidate donors for use in oligosaccharide synthesis directly from azido nitrate-esters. The main advantage of this technique is that it is faster, cleaner and greener than previously reported methods.To begin the procedure, place 0.2 millimoles of the chosen azido nitrate-ester in an eight milliliter microwave reaction vial. Dissolve the azido nitrate-ester in two milliliters of 20%aqueous acetone. Add to the reaction vial 0.08 milliliters of pyridine and cap the vial.Place the vial in a microwave reactor and ramp the reaction mixture to 120 degrees Celsius over the course of two minutes. Irradiate the mixture at 110 degrees Celsius while stirring for fixed whole time of 10 minutes. Then spot the mixture on a TLC plate, and run the plate with a one to one solution of ethyl acetate and hexanes.Visualize the plate with a ceric ammonium molybdate stain. Verify that the starting material has been completely consumed. Using an airline, evaporate the solvent to a reduced volume.Then take the reaction mixture off of the airline and dilute the mixture with two milliliters of dichloromethane. Create a vortex with the stir plate. Then use a syringe to remove the water layer.Finally, cool the reaction mixture to zero degrees Celsius in an ice water bath. Add to the mixture 0.08 milliliters of DBU, and 0.25 milliliters of trichloroacetonitrile. Remove the mixture from the ice water bath and allow the mixture to warm to room temperature while stirring.Monitor the reaction progress with TLC. Once the anomeric alcohol has been completely consumed, transfer the mixture to a recovery flask. Concentrate the organic layer under vacuum at 30 degrees Celsius to a pale yellow or brown oil.Finally, purify the crude product by silica gel column chromatography in a 1.5 centimeter column with a one to four mixture of ethyl acetate and hexanes as the eluant. Adjust the eluant proportions as needed for optimal separation. Three, two azido one nitrate-esters were converted to trichloroacetimidates using this procedure.In each reaction, the microwave assisted thermal denitration was completed within 20 minutes. An 11 to one to six alpha manol, alpha gluco, beta gluco mixture of compound one was converted to a five to one alpha gluco alpha mano mixture of compound three, an 87%overall yield, an 92%alpha only yield. Compound four was converted to compound six in greater than 95%yield.The hydrolysis of compound four only required 10 minutes of irradiation at 110 degrees Celsius. Conversion of the anomeric alcohol to compound six was achieved in 30 minutes. The overall alpha beta imidate ratio was 34 to one.A one to 14.4 to 27.4 beta gluco alpha mano alpha gluco mixture of compound seven was converted to a one to 1.7 alpha gluco alpha mano mixture of compound nine, in 68%overall yield, and a 66%alpha only yield. A greater excess of DBU was needed to achieve complete conversion to compound seven. This development allows for the physio transformation of azido nitrate-esters directly to trichloroacetimidate donors in a one-pot synthetic method.While attempting this procedure, be sure to check the settings on the rotary evaporator and microwave to ensure that using the proper conditions. Once mastered, this technique can be performed in under three hours if done properly. Following this procedure, anomeric characterization can be performed to answer additional questions about the extent of reaction completion prior to workup.After watching this video, you should have a good understanding of how to synthesize trichloroacetimidate donors directly from the azido nitrate-esters. Remember that working with trichloroacetonitrile can be extremely hazardous. Precautions such as wearing proper PPE should always be taken while following this procedure.

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