This work was supported by the National Research Foundation of Korea (NRF-2012M3A7B4049645 and HUFS Research Fund (2018).

1. Synthesis of (6R)-1-[(R)-1-Phenylethyl)-1-Azoniabicyclo[4.1.0]Heptane Tosylate (4)
<ol>
	<li>Synthesis of 4-(R)-[1-(R)-1-phenylethyl)aziridin-2-yl]butyl 4-methylbenzenesulfonate (2)
	<ol>
		<li>Add 100 mg of 4-[(R)-1-(R)-1-phenylethyl)aziridin-2-yl]butan-1-ol (1)12 (0.46 mmol, 1.0 equiv), 140 &#181;L of triethylamine (Et3N, 1.0 mmol, 2.2 equiv), and a magnetic stir-bar into an oven-dried 25 mL two-neck round-bott...

<ol>
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The authors declare no competing financial interests.

Piperidine and azepane are two most abundant azaheterocycles in many life-saving drugs and antibiotics including various biologically active natural products16. In order to access both enantiopure piperidine (5) and azepane (6) with diverse substituents, we develped an efficient synthetic method through the formation of 1-azoniabicyclo[4.1.0]heptane tosylate from entiopure 2-(4-hydroxybutyl)aziridne followed by regiospecific nucleophilic attack either at the bridg...

Among three membered cyclic compounds, aziridine has similar ring strain energy as cyclopropane and oxirane to afford various nitrogen-containing cyclic and acyclic compounds via ring opening1,2,3. However, the characteristics and reactivity of aziridine depend on the substituent of ring nitrogen. Aziridine with an electron-withdrawing group at the ring nitrogen4, is called &#34;activated aziridine&#34;, which is activated to react with the incoming nucleophile without any additional activating reagent. On the other hand, &#34;non-activated aziridine&#34; with electron-donating substituent at nitrogen is quite stable and inert to the nucleophiles, unless it is activated as an aziridinium ion as Ia (Figure 1a)5,6,7. The ring opening of a non-activated aziridine depends on various factors such as the substituents at C2 and C3 carbon of aziridine, the electrophile to activate the aziridine ring and the incoming nucleophile. The isolation and characterization of an aziridinium ion is not possible due to its high reactivity towards ring-opening reaction by nucleophiles, but its formation and characteristics were observed spectroscopically with a non-nucleophilic counter anion5,8,9,10. The regio- and stereoselective ring-opening reaction of aziridinium ion by a suitable nucleophile yields nitrogen-containing acyclic valuable molecules (Pi and Pii)5,6,7,8,9,10.
Similarly, a bicyclic aziridinium ion (Ib) is possibly generated via the removal of the leaving group by the nucleophilic attack of ring nitrogen of aziridine in intramolecular fashion (Figure 1b). Then, this intermediate undergoes ring-expansion with the incoming nucleophile via the release of ring strain. The formation and stability of bicyclic aziridinium ion are dependent on many factors such as the substituents, the size of the ring, and solvent medium9. The regio- and stereoselectivity of the aziridine ring-expansion is a pivotal aspect of its synthetic utility, which depends on the nature of the substituents in the starting substrate and the characteristics of applied nucleophile.
In our early study, we succeeded to prepare 1-azoniabicyclo [3.1.0]hexane tosylate Ib (n = 1) whose subsequent ring-expansion resulted in the formation of a pyrrolidine and a piperidine (Piii and Piv, n = 1, Figure 1)8. As a part of our continuing study on the bicyclic aziridinium ion chemistry, we describes herein the formation of 1-azoniabicyclo [4.1.0]heptane tosylate Ib (n = 2) as a representative example. This was prepared from 2-(4-toluensulfonyloxybutyl)aziridine and its ring-expansion was trigged by a nucleophile to afford valuable piperidine and azepane (Pi and Pii, n = 2, Figure 1) with diverse substituents around the ring11. The ring-expansion of enantiopure aziridine 4-[(R)-1-(R)-1-phenylethyl)aziridin-2-yl]butan-1-ol (1) resulted in the asymmetric synthesis of substituted azaheterocycles which are applicable to build biologically active molecules with piperidine and azepane skeleton. This synthetic protocol has been applied for various compounds ranging from simple 2-cyanomethylpiperidine 5f, 2-acetyloxymethylpiperidine 5h and 3-hydroxyazepane 6j to more complex molecules including natural products such as fagomine (9), febrifugine analogue (12) and balanol (15) in optically pure forms11.

<table border="0" cellpadding="0" cellspacing="0" width="1213">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Thin Layer Chromatography (TLC)</td>
			<td style="width:260px;">Merck</td>
			<td style="width:207px;">100390</td>
			<td style="width:183px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">UV light</td>
			<td style="width:260px;">Sigma-Aldrich</td>
			<td>Z169625-1EA</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Bruker AVANCE III HD (400 MHz) spectrometer</td>
			<td style="width:260px;">Bruker</td>
			<td style="width:207px;">NA</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">JASCO P-2000</td>
			<td style="width:260px;">JASCO</td>
			<td style="width:207px;">P-2000</td>
			<td>For optical rotation</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">High resolution mass spectra/ MALDI-TOF/TOF Mass Spectrometry</td>
			<td style="width:260px;">AB SCIEX</td>
			<td style="width:207px;">4800 Plus&#160;</td>
			<td>High resolution mass spectra</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">(2R)-1-[(1R)-1-Phenylethyl]-2-aziridinecarboxylic acid (&#8211;)-menthol ester, 98%</td>
			<td style="width:260px;">Sigma-Aldrich</td>
			<td style="width:207px;">57,054-0</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">(2S)-1-[(1R)-1-Phenylethyl]-2-aziridinecarboxylic acid (&#8211;)-menthol ester</td>
			<td style="width:260px;">Sigma-Aldrich</td>
			<td style="width:207px;">57,051-6</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Triethylethylamine</td>
			<td style="width:260px;">DAEJUNG</td>
			<td style="width:207px;">8556-4400-1L</td>
			<td>CAS No: 121-44-8</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Dichloromethane</td>
			<td style="width:260px;">SAMCHUN</td>
			<td style="width:207px;">M0822-18L</td>
			<td>CAS No: 75-09-2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">p-Toluenesulfonic anhydride</td>
			<td style="width:260px;">Sigma-Aldrich</td>
			<td style="width:207px;">259764-25G</td>
			<td>CAS No: 4124-41-8</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">n-Hexane</td>
			<td style="width:260px;">SAMCHUN</td>
			<td style="width:207px;">H0114-18L</td>
			<td>CAS No: 110-54-3</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Ethyl acetate</td>
			<td style="width:260px;">SAMCHUN</td>
			<td style="width:207px;">E0191-18L</td>
			<td>CAS No: 141-78-6</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Sodium sulfate</td>
			<td style="width:260px;">SAMCHUN</td>
			<td style="width:207px;">S1011-1kg</td>
			<td>CAS No: 7757-82-6</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Acetonitrile-d3</td>
			<td style="width:260px;">Cambridge Isotope Laboratories, Inc</td>
			<td style="width:207px;">15G-744-25g</td>
			<td>CAS No: 2206-26-0</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Acetonitrile</td>
			<td style="width:260px;">SAMCHUN</td>
			<td style="width:207px;">A0127-18L</td>
			<td>CAS No: 75-05-8</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">1,4-Dioxane</td>
			<td style="width:260px;">SAMCHUN</td>
			<td style="width:207px;">D0654-1kg</td>
			<td>CAS No: 123-91-1</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Sodium hydroxide</td>
			<td style="width:260px;">DUKSAN</td>
			<td style="width:207px;">A31226-1kg</td>
			<td>CAS No: 1310-73-2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Sodium acetate</td>
			<td style="width:260px;">Alfa Aesar</td>
			<td style="width:207px;">11554-250g</td>
			<td>CAS No: 127-09-3</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Lithium aluminum hydride</td>
			<td style="width:260px;">TCI</td>
			<td style="width:207px;">L0203-100g</td>
			<td>CAS No: 16853-85-3</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Tetrahydrofuran</td>
			<td style="width:260px;">SAMCHUN</td>
			<td style="width:207px;">T0148-18L</td>
			<td>CAS No: 109-99-9</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Sodium azide</td>
			<td style="width:260px;">D.S.P</td>
			<td style="width:207px;">703301-500g</td>
			<td>CAS No: 26628-22-8</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Cesium fluoride</td>
			<td style="width:260px;">aldrich</td>
			<td style="width:207px;">18951-0250-25g</td>
			<td>CAS No: 13400-13-0</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Tetrabutylammonium bromide</td>
			<td style="width:260px;">aldrich</td>
			<td style="width:207px;">426288-25g</td>
			<td>CAS No: 1643-19-2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Sodium iodide</td>
			<td style="width:260px;">aldrich</td>
			<td style="width:207px;">383112-100g</td>
			<td>CAS No: 7681-82-5</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Sodium cyanide</td>
			<td style="width:260px;">Acros Organics</td>
			<td style="width:207px;">424301000-100g</td>
			<td>CAS No: 143-33-9</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Sodium thiocyanate</td>
			<td style="width:260px;">aldrich</td>
			<td style="width:207px;">467871-250g</td>
			<td>CAS No: 540-72-7</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Sodium methoxide</td>
			<td style="width:260px;">aldrich</td>
			<td style="width:207px;">156256-1L</td>
			<td>CAS No: 124-41-4</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Benzylamine</td>
			<td style="width:260px;">Alfa Aesar</td>
			<td style="width:207px;">A10997-1000g</td>
			<td>CAS No: 100-46-9</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Phenol</td>
			<td style="width:260px;">TCI</td>
			<td style="width:207px;">P1610-500g</td>
			<td>CAS No: 108-95-2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Sodium benzoate</td>
			<td style="width:260px;">Alfa Aesar</td>
			<td style="width:207px;">A15946-250g</td>
			<td>CAS No: 532-32-1</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:564px;">Chloroform-d</td>
			<td style="width:260px;">Cambridge Isotope Laboratories, Inc</td>
			<td style="width:207px;">DLM-7TB-100S/16H-239, 100g</td>
			<td>CAS No: 865-49-6</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Dimethyl sulfoxide-d6</td>
			<td style="width:260px;">Cambridge Isotope Laboratories, Inc</td>
			<td style="width:207px;">DLM-10-25, 25g</td>
			<td>CAS No: 2206-27-1</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Methanol</td>
			<td style="width:260px;">SAMCHUN</td>
			<td style="width:207px;">M0585-18L</td>
			<td>CAS No: 67-56-1</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Ninhydrin</td>
			<td style="width:260px;">Alfa Aesar</td>
			<td style="width:207px;">A10409-250g</td>
			<td>CAS No: 485-47-2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">Phosphomolybdic acid hydrate</td>
			<td style="width:260px;">TCI</td>
			<td style="width:207px;">P1910-100g</td>
			<td>CAS No: 51429-74-4</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:564px;">p-Anisaldehyde</td>
			<td style="width:260px;">aldrich</td>
			<td style="width:207px;">A88107-5g</td>
			<td>CAS No: 123-11-5</td>
		</tr>
	</tbody>
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preparation of stable bicyclic aziridinium ions and their ring-opening for the synthesis of azaheterocycles

Bicyclic aziridinium ions were generated by the removal of an appropriate leaving group through internal nucleophilic attack by nitrogen atom in the aziridine ring. The utility of bicyclic aziridinium ions, specifically 1-azoniabicyclo[3.1.0]hexane and 1-azoniabicyclo[4.1.0]heptane tosylate highlighted in the aziridine ring openings by the nucleophile with the release of the ring strain to yield the corresponding ring-expanded azaheterocycles such as pyrrolidine, piperidine and azepane with diverse substituents on the ring in regio- and stereospecific manner. Herein, we report a simple and convenient method for the preparation of the stable 1-azabicyclo[4.1.0]heptane tosylate followed by selective ring opening via a nucleophilic attack either at the bridge or at the bridgehead carbon to yield piperidine and azepane rings, respectively. This synthetic strategy allowed us to prepare biologically active natural products containing piperidine and azepane motif including sedamine, allosedamine, fagomine and balanol in highly efficient manner.

The reaction of 4-[(R)-1-(R)-1-phenylethyl)aziridin-2-yl]butan-1-ol (1)12 with p-toluenesulfonic anhydride and triethylamine in CH2Cl2 at room temperature for 1.0 h yielded the corresponding 2-(4-tosyloxybutyl)aziridine 2 in 96% yield11. 1H NMR (400 MHz) spectrum of compound 2 in CD3CN at different time intervals shows th...

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Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles

Bicyclic aziridinium ions such as 1-azoniabicyclo[4.1.0]heptane tosylate were generated from 2-[4-tolenesulfonyloxybutyl]aziridine, which was utilized for the preparation of substituted piperidines and azepanes via regio- and stereospecific ring-expansion with various nucleophiles. This highly efficient protocol allowed us to prepare diverse azaheterocycles including natural products such as fagomine, febrifugine analogue and balanol.

Bicyclic aziridinium ions were generated by the removal of an appropriate leaving group through internal nucleophilic attack by nitrogen atom in the ...

This method can help to answer key questions in the field of bicyclic aziridinium ion chemistry for the synthesis of various various azaheterocycles including bipyridine and azepine ring systems. The main advantage of this technique is that the synthesis of kai-dal bipyridines and azepine core present in various natural products is achieved in a straight-forward manner. First, add 100 milligrams of the alcohol, 140 microliters of triethylamine, and a magnetic stir bar into an oven dried 25 milliliter two-neck round bottom flask under nitrogen atmosphere.Using an airtight syringe, add 5 milliliters of anhydrous dichloromethane to the reaction flask. Cool the reaction mixture to 0 degrees Celsius using an ice water bath and stir the mixture for 5 minutes. Next, add 164 milligrams of p-toluenesulfonic anhydride to the reaction mixture and stir for another 45 minutes.After stirring for 45 minutes, warm the reaction mixture to room temperature and stir for 30 minutes. Monitor the reaction progress by thin layer chromatography using hexanes and ethyl acetate as the eluant. After the complete consumption of the starting alcohol, quench the reaction mixture with 5 milliliters of water.Then extract the heterogeneous mixture three times with 15 milliliters of dichloromethane. Dry the combined organic layer over anhydrous sodium sulfate for 10 minutes. After filtration, concentrate the filtrate in vacuo using a rotary evaporator.Purify the crude product by flash-column chromatography using a gradient of hexanes and ethyl acetate to afford the tosylate in 96%yield as a viscous liquid. Now, transfer 5 milligrams of the tosylate in an NMR tube and add 300 microliters of deuterated acetonitrile. Set aside this solution at room temperature for 24 hours to attain complete conversion to the azoniabicyclo.Monitor the conversion by NMR at different time points. Add 160 milligrams of the previously prepared tosylate and a magnetic stir bar into an oven-dried 25 milliliter round-bottom flask. Using an airtight syringe, add 4 milliliters of anhydrous acetonitrile to the reaction flask.Add three equivalents of a suitable nucleophile to the reaction mixture and stir for 8 to 15 hours. Once the reaction is complete, quench the mixture with 5 milliliters of water. Then extract three times with 15 milliliters of dichloromethane.Dry the combined organic layer over anhydrous sodium sulfate. After filtration, concentrate the filtrate in vacuo using a rotary evaporator. Purify the crude product by column chromatography using a gradient of hexanes and ethyl acetate to afford the pure ring expanded products.Transfer 100 milligrams of the aziridinyl tosylate and a magnetic stir bar into an oven dried 25 milliliter round-bottom flask fitted with a reflux condenser. Using a syringe, add 4 milliliters of 1, 4-dioxane to the reaction flask. Add 0.4 milliliters of 2 molar sodium hydroxide solution to the reaction mixture and heat to reflux for 2 hours.Once the reaction is complete, quench the mixture with 4 milliliters of water. Then extract the reaction mixture four times with 15 milliliters of dichloromethane. Dry the combined organic layer over anhydrous sodium sulfate.After filtration, concentrate the filtrate in vacuo using a rotary evaporator. Purify the crude product by column chromatography using a gradient of hexanes and ethyl acetate to afford the pure hydroxy-azepane in a 97%yield. Transfer 220 milligrams of the aziridinyl alcohol, 180 microliters of triethylamine, and a magnetic stir bar into an oven-dried 25 milliliter two-neck round-bottom flask under nitrogen atmosphere.Using an airtight syringe, add 4 milliliters of anhydrous dichloromethane to the reaction flask. Cool the reaction mixture to 0 degrees Celsius, and stir for 5 minutes. Following this, add 200 milligrams of p-toluenesulfonic anhydride to the reaction mixture, and stir for another 45 minutes.Then warm the reaction mixture to room temperature, and stir for 30 minutes. Monitor the reaction progress by thin layer chromatography using hexanes and ethyl acetate as the eluant. After the complete consumption of the alcohol, quench the reaction mixture with 5 milliliters of water.Then extract three times with 15 milliliters of dichloromethane. Dry the combined organic layer over anhydrous sodium sulfate. After filtration, concentrate the filtrate in vacuo using a rotary evaporator.Now, transfer the crude tosylate and a magnetic stir bar into an oven-dried 25 milliliter round-bottom flask. Using an airtight syringe, add 4 milliliters of anhydrous acetonitrile to the reaction flask. Next, add 125 milligrams of sodium acetate to the reaction mixture and stir for 12 hours.Once the reaction is complete, quench the mixture with 5 milliliters of water. Then extract three times with 15 milliliters of dichloromethane. Dry the combined organic layer over anhydrous sodium sulfate.After filtration, concentrate the filtrate in vacuo using a rotary evaporator. Finally, purify the crude product by column chromatography using a gradient of hexanes and ethyl acetate to afford the pure substituted properidine in 83%yield. The proton NMR spectrum of tosylate-2 at different time intervals shows the conversion to azoniabicyclo.A quartet at 4.07 PPM in the NMR spectrum of ring expanded product 5a corresponds to the CH proton of phenylethyl group. A similar quartet at 3.81 PPM was observed for ring expanded product 6a. The aziridine ring expansion of cyanide, thiocyanide, and acetate nucleophiles afforded properidine rings, while azide, hydroxide, and amine nucleophiles yielded azepane rings.Synthesis of natural azasugar d-fagomine and its three epimer was achieved by ring expansion of aziridinyl-alcohol-7, followed by removal of the protecting groups to afford the product in 94%yield. Benzyl protected alcohol-10 was treated with p-toluenesulfonic anhydride and triethylamine followed by nucleophilic ring opening resulting in the ring-expanded cyanomethyl-properidine-11 in 90%yield as a single isomer. The ring expansion of alcohol-13 followed by reaction with sodium azide yielded hydroxy-azapane-14 which was used for the azepane core of Balanol by one-pot benzyl deprotection and azide reduction under catalytic hydrogenation.Once mastered, this technique for aziridine ring expansion can be done in six to eight hours if it is performed properly. While attempting this procedure, it is important to remember to use para-toluenesulfonic anhydride, because it is more sensitive to moisture. Following this procedure, other methods like use of para-toluenesulfonic chloride instead of para-toluenesulfonic anhydride can be used in order to explore the eco-friendly nature of the pro-gen protocol.After its development, this technique paved the way for the researchers in the field of synthetic organic chemistry to explore the environment of aziridinium ion in azaheterocycles and natural product synthesis. After watching this video, you should have a good understanding of how to use bicyclic aziridinium ion chemistry for the synthesis of biologically-relevant molecules having an azaheterocycles including a variety of natural products. Don't forget that working with toxic reagents such as sodium cyanide used in the synthesis as a nucleophile can be extremely hazardous, and extreme safety precautions should always be taken while performing this procedure.

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8.3K Views.  North Eastern Regional Institute of Science and Technology. This method can help to answer key questions in the field of bicyclic aziridinium ion chemistry for the synthesis of various various azaheterocycles including bipyridine and azepine ring systems. The main advantage of this technique is that the synthesis of kai-dal bipyridines and azepine core present in various natural products is achieved in a straight-forward manner. First, add 100 milligrams of the alcohol, 140 microliters of triethylamine, and a magnetic stir bar into an oven dried 25 mi...