Name: preparation of contiguous bisaziridines for regioselective ring-opening reactions
Uploaded: 2022-07-28T14:30:00
Description: Watch this Scientific Journal Video about Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions at JoVE.com

This research was supported by the Korea Basic Science Institute (National Research Facilities and Equipment Center) grant funded by the Ministry of Education (2022R1A6C101A751). This work was also supported by the National Research Foundation of Korea (NRF) grants (2020R1A2C1007102 and 2021R1A5A6002803).

The details of all the synthesized products (1-5), including the structure, full NMR spectra, optical purity, and HRMS-MALDI data, are provided in Supplementary File 1.
1. Synthesis of 3-(aziridin-2-yl)acryl aldehyde (1a)
<ol>
	<li>Flame dry a 50 mL round-bottomed flask equipped with a stirrer bar and a septum under vacuum conditions. Cool it to room temperature while filling it with argon gas.</li>
	<li>Add anhydrous toluene (19 mL) and (<e...

The formation of an inseparable mixture of diastereomers has occasionally been observed during the course of organocatalytic aziridination of chiral 3-[1-(1-phenylethyl)aziridin-2-yl)]acrylaldehyde, when N-Boc-O-tosyl or N-Ts-O-tosyl hydroxylamine was used as the nitrogen source. Further, the yield of contiguous bisaziridine product decreased when the amount of diaryl silyl ether prolinol as catalyst was increased from 7 mol% to 20 mol%47,<sup class="...

Rational design of small organic molecules with diverse reactive sites that precisely control product selectivity is a key goal in modern organic synthesis and green chemistry1,2,3,4,5,6,7,8. To achieve this goal, we were interested in the modular synthesis of aziridines. Aziridines are of interest to most organic chemists, owing to their structurally important framework9 with an electronically diverse set of N-substituents that can lead to regioselective ring-opening reactions with multiple nucleophiles10,11,12,13,14,15,16,17,18,19, and varied pharmacological activities such as antitumor, antimicrobial, and antibacterial properties. Despite the advances in aziridine chemistry, non-activated aziridine and activated aziridine have independent syntheses and ring-opening reactions in the literature20.
Therefore, we aimed to synthesize contiguous bisaziridines comprising both the non-activated and activated aziridines. These contiguous bisaziridines can be used to systematically rationalize a chemoselective ring-opening pattern based on the following electronic properties of the two different aziridines and their reactivity to nucleophiles20,21,22,23,24: a) activated aziridines, in which the electron-withdrawing substituents conjugatively stabilize the negative charge on the nitrogen, readily react with multiple nucleophiles to allow ring-opened products; b) non-activated aziridines, in which the nitrogen is bound to the electron-donating substituents, are considerably inert toward nucleophiles; hence, a pre-activation step with a suitable activator (mainly Br&#248;nsted or Lewis acids) is required to afford the ring-opened products in high yields20,21,25,26.
The present study describes the rational design of contiguous bisaziridines as chiral building blocks via transition-metal-free organocatalysis and the synthesis of diverse nitrogen-rich molecules utilizing predictive modeling tools for ring-opening reactions of bisaziridines. This study aims to stimulate the advancement of practical methods for the construction of nitrogen-enriched bioactive compounds and natural products and the polymerization of aziridines.

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	<tbody>
		<tr>
			<td>(R)-(+)-&#945;,&#945;-Diphenyl-2-pyrrolidinemethanol trimethylsilyl ether</td>
			<td>Sigma-Aldrich</td>
			<td>677191</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>(R)-1-((R)-1-phenylethyl)aziridine-2-carbaldehyde</td>
			<td>Imagene Co.,Ltd.</td>
			<td></td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>(S)-(&#8211;)-&#945;,&#945;-Diphenyl-2-pyrrolidinemethanol trimethylsilyl ether</td>
			<td>Sigma-Aldrich</td>
			<td>677183</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>(S)-2-(diphenyl((trim ethylsilyl)oxy)methyl)pyrrolidine</td>
			<td>Sigma-Aldrich</td>
			<td>677183</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>(Triphenylphosphoranylidene) acetaldehyde</td>
			<td>Sigma-Aldrich</td>
			<td>280933</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>1,2-Dichloroethane</td>
			<td>Sigma-Aldrich</td>
			<td>284505</td>
			<td>solvent</td>
		</tr>
		<tr>
			<td>AB Sciex 4800 Plus MALDI TOFTM (2,5-dihydroxybenzoic acid (DHB) matrix</td>
			<td>Sciex</td>
			<td></td>
			<td>High resolution mass spectra</td>
		</tr>
		<tr>
			<td>Acetic acid</td>
			<td>Sigma-Aldrich</td>
			<td>A6283</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Ammonium chloride</td>
			<td>Sigma-Aldrich</td>
			<td>254134</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>aniline</td>
			<td>Sigma-Aldrich</td>
			<td>132934</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Autopol III digital polarimeter</td>
			<td>Rudolph Research Analytical</td>
			<td></td>
			<td>polarimeter</td>
		</tr>
		<tr>
			<td>AVANCE III HD (400 MHz) spectrometer</td>
			<td>Bruker</td>
			<td></td>
			<td>NMR spectrometer</td>
		</tr>
		<tr>
			<td>Bruker Ascend 500 (500 MHz)</td>
			<td>Bruker</td>
			<td></td>
			<td>NMR spectrometer</td>
		</tr>
		<tr>
			<td>Celite 535</td>
			<td>Sigma-Aldrich</td>
			<td>22138</td>
			<td>For Celite pad</td>
		</tr>
		<tr>
			<td>Dichloromethane</td>
			<td>Sigma-Aldrich</td>
			<td>270997</td>
			<td>solvent</td>
		</tr>
		<tr>
			<td>Di-tert-butyl dicarbonate</td>
			<td>Sigma-Aldrich</td>
			<td>361941</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Ethyl Acetate</td>
			<td>Sigma-Aldrich</td>
			<td>270989</td>
			<td>solvent</td>
		</tr>
		<tr>
			<td>Ethyl nitroacetate</td>
			<td>Sigma-Aldrich</td>
			<td>192333</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Imidazole</td>
			<td>Sigma-Aldrich</td>
			<td>I2399</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>INOVA 400WB (400 MHz)</td>
			<td>Varian</td>
			<td></td>
			<td>NMR spectrometer</td>
		</tr>
		<tr>
			<td>JMS-700</td>
			<td>JEOL</td>
			<td></td>
			<td>High resolution mass spectra</td>
		</tr>
		<tr>
			<td>Methanol</td>
			<td>Sigma-Aldrich</td>
			<td>322415</td>
			<td>solvent</td>
		</tr>
		<tr>
			<td>N-Boc-O-tosylhydroxylamine</td>
			<td>Sigma-Aldrich</td>
			<td>775037</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>P-2000</td>
			<td>JASCO</td>
			<td></td>
			<td>polarimeter</td>
		</tr>
		<tr>
			<td>Palladium hydroxide on carbon</td>
			<td>Sigma-Aldrich</td>
			<td>212911</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Phenyl-1H-tetrazole-5-thiol</td>
			<td>TCI</td>
			<td>P0640</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Silica gel</td>
			<td>Sigma-Aldrich</td>
			<td>227196</td>
			<td>For flash clromatography</td>
		</tr>
		<tr>
			<td>Silica gel on TLC plates</td>
			<td>Merck</td>
			<td>60768</td>
			<td>TLC plate</td>
		</tr>
		<tr>
			<td>Sodium acetate</td>
			<td>Sigma-Aldrich</td>
			<td>S8750</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Sodium azide</td>
			<td>Sigma-Aldrich</td>
			<td>S2002</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Sodium borohydride</td>
			<td>Sigma-Aldrich</td>
			<td>452882</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Sodium carbonate</td>
			<td>Sigma-Aldrich</td>
			<td>S2127</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>tert-Butyldimethylsilyl chloride</td>
			<td>Sigma-Aldrich</td>
			<td>190500</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Tetrahydrofuran</td>
			<td>Sigma-Aldrich</td>
			<td>401757</td>
			<td>solvent</td>
		</tr>
		<tr>
			<td>Toluene</td>
			<td>Sigma-Aldrich</td>
			<td>244511</td>
			<td>solvent</td>
		</tr>
		<tr>
			<td>Zinc bromide</td>
			<td>Sigma-Aldrich</td>
			<td>230022</td>
			<td>reagent</td>
		</tr>
		<tr>
			<td>Zinc chloride</td>
			<td>Sigma-Aldrich</td>
			<td>429430</td>
			<td>reagent</td>
		</tr>
	</tbody>
</table>

preparation of contiguous bisaziridines for regioselective ring-opening reactions

Aziridines, a class of reactive organic molecules containing a three-membered ring, are important synthons for the synthesis of a large variety of functionalized nitrogen-containing target compounds through the regiocontrolled ring-opening of C-substituted aziridines. Despite the tremendous progress in aziridine synthesis over the past decade, accessing contiguous bisaziridines efficiently remains difficult. Therefore, we were interested in synthesizing contiguous bisaziridines bearing an electronically diverse set of N-substituents beyond the single aziridine backbone for regioselective ring-opening reactions with diverse nucleophiles. In this study, chiral contiguous bisaziridines were prepared by organocatalytic asymmetric aziridination of chiral (E)-3-((S)-1-((R)-1-phenylethyl)aziridin-2-yl)acrylaldehyde with N-Ts-O-tosyl or N-Boc-O-tosyl hydroxylamine as the nitrogen source in the presence of (2S)-[diphenyl(trimethylsilyloxy)methyl]pyrrolidine as a chiral organocatalyst. Also demonstrated here are representative examples of regioselective ring-opening reactions of contiguous bisaziridines with a variety of nucleophiles such as sulfur, nitrogen, carbon, and oxygen, and the application of contiguous bisaziridines to the synthesis of multi-substituted chiral pyrrolidines by Pd-catalyzed hydrogenation.

To investigate the achievability of preparing a contiguous bisaziridine, (E)-3-((S)-1-((R)-1-phenylethyl)aziridin-2-yl)acrylaldehyde (1a) was first synthesized as a model substrate according to the procedure mentioned in step 1 (Figure 1)28.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/64019/64019fig01.jpg" /> 
Figu...

Watch this Scientific Journal Video about Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions at JoVE.com

Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions

Contiguous bisaziridines containing non-activated and activated aziridines were synthesized by asymmetric organocatalytic aziridinations and then subjected to chemoselective ring-opening reactions under acidic or basic conditions. The non-activated aziridine ring opens with less reactive nucleophiles under acidic conditions, whereas the activated aziridine ring opens with more reactive nucleophiles under basic conditions.

Aziridines, a class of reactive organic molecules containing a three-membered ring, are important synthons for the synthesis of a large variety of ...

The significance of our protocol is rational design of contiguous bisaziridine containing both activated and non-activated aziridines, and subject it to regioselective ring-opening reactions with diverse nucleophiles. The advantage of this technique is that it allows the synthesis of nitrogen-rich molecules using predictive modeling tools for selective ring openings. The current protocol can be used to develop practical methods for the synthesis of nitrogen-enriched bioactive compounds and natural products.To begin, flame dry a 50-milliliter round-bottomed flask with a stirrer bar and a septum under vacuum conditions. After cooling it to room temperature, fill it with argon gas. Then add anhydrous methanol and aldehyde to the flask, and stir the solution for a minute.Next, add sodium borohydride to the stirred solution, and stir the reaction mixture at zero degrees Celsius for one hour. Monitor the reaction progress by TLC, using ethyl acetate and hexanes as an eluent. After one hour, quench the reaction mixture with distilled water, and extract with ethyl acetate in a separatory funnel.Dry the combined organic layer over anhydrous sodium sulfate, filter, and concentrate in vacuo. Purify the crude residue by silica gel flash chromatography with ethyl acetate and hexanes as the eluent, to isolate pure products as a yellow liquid. Afterward, confirm the product by NMR and polarimeter measurements.First, flame-dry a five-milliliter round-bottomed flask with a stirrer bar and a septum under vacuum conditions. Then cool it to room temperature while filling it with argon gas. Next, add the previously synthesized bisaziridine and acetic acid to the flask.Stir the mixture at room temperature for five hours, and monitor the reaction progress by TLC, using ethyl acetate and hexanes as an eluent. After five hours, remove the acetic acid in vacuo, and purify the crude residue by silica gel flash chromatography with ethyl acetate and hexanes as the eluent, to isolate the pure product as a yellow liquid. Confirm the product by NMR and polarimeter measurements.The proton NMR spectrum revealed a peak at 1.42 ppm, corresponding to the alcohol hydrogen in the ethyl alcohol adjacent to the aziridine, indicating the reduction of an aldehyde into ethyl alcohol. Moreover, the peaks at 4.00 and 3.54 ppm represent the methylene hydrogens in ethyl alcohol. The peak observed at 2.13 ppm corresponds to the methyl hydrogens of acetate, whereas peaks at 4.43 and 4.15 ppm correspond to the methylene hydrogens adjacent to acetate formed after the aziridine ring-opening reaction.Reduction involving NaBH4 have found to be exothermic, and thus, maintain the reaction temperature at zero degrees Celsius. Also, care is required during the quenching of NaBH4 by H2O. We believe that this technique will stimulate further research toward the development of practical methods for the construction of nitrogen-enriched complex molecules having biological activities.

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