Preparation of Enantiopure Non-Activated Aziridines and Synthesis of Biemamide B, D, and epiallo-Isomuscarine

For the last 30 years, Professor Hyun-Joon Ha’s group has studied the aziridine chemistry and developed the synthesis of nitrogen-containing valuable molecules in an asymmetric manner. In this visual experiment, we show here the synthesis of enantiopure aziridine-2 carboxylate and its application for the synthesis of biemamide B, D, and epiallo-isomuscarine alkaloids. This method shows the preparation of both enantiopure L-methanol ester of 2R and 2S aziridine-2 carboxylate on a large scale via selective crystallization process.

In this visual experiment, we showed the making of enantiopure aziridine-2 carboxylate and its application for the synthesis of biemamide B, D, and epiallo-isomuscarine alkaloids. The protocol in this paper is an easy method to prepare non-activated aziridines on a large scale, which includes simple steps like crystallization, filtration, and column chromatographic process. Begin by adding 2, 3-dibromopropane L-methanol ester 1A and a magnetic stirring bar into an oven-dried, 250-milliliter, two-neck, round-bottom flask under nitrogen atmosphere.

Use an airtight syringe to add 60 milliliters anhydrous acetonitrile to the reaction flask. Then, cool the reaction mixture at zero degrees Celsius on an ice bath and stir the reaction mixture for five minutes. Add potassium carbonate into the reaction mixture at the same temperature and keep stirring for 30 minutes.

Next, add 2R-phenylethylamine in a drop-wise manner at room temperature and allow the reaction mixture to stir for 12 hours. Then, monitor the progress of the reaction with thin-layer chromatography by using 9:1 volume-by-volume of hexane ethyl acetate as an eluent. Once the reaction is complete, filter the mixture over a filter paper and add water into the organic filtrate.

Extract the organic layer with diethyl ether two times using a separating funnel. After drying the combined organic extracts over 7.5 grams of anhydrous sodium sulfate, concentrate the organic extract in vacuo using a rotary evaporator to get a crude mixture of diastereomeric chiral aziridine containing both isomers. Next, add 8.7 grams of the crude mixture of chiral aziridine L-methanol ester 1 derivative in 70 milliliters of methanol to an oven-dried, 250-milliliter, single-neck, round-bottom flask.

Then, warm the reaction mixture up to 70 degrees Celsius using a hot water bath and cool the reaction mixture at minus 10 degrees Celsius until solid crystals form. Filter the solid compound over a filter paper to obtain 2.2 grams of R, 1R, 2S, 5R, 2-isopropyl 5-methyl-cyclohexyl, 1R1 phenylethyl aziridine-2 carboxylate ester and concentrate the filtrate solution again in vacuo using a rotary evaporator. After dissolving the remaining reaction mixture in 50 millimeters of ethanol, re-crystallize at minus 10 degrees Celsius to obtain 1.2 grams of R, 1R, 2S, 5R 2-isopropyl 5-methyl-cyclohexyl 1R1 phenylethyl aziridine-2 carboxylate and use the other alcohol-ethanol in the same way as methanol at this time.

Then filter the re-crystallized product over a filter paper and concentrate the remaining crude 5.3 grams of filtrate solution completely in vacuo using a rotary evaporator. Then, add 50 milliliters of a pentane hydrocarbon solvent while keeping the remaining reaction solution at minus 15 degrees Celsius and a solid compound of nearly 1.9 grams of S, 1R, 2S, 5R, 2-isopropyl 5-methocyclohexyl 1-phenylethyl aziridine-2 carboxylate ester is obtained. Once the crystals are ready, concentrate the solution again in vacuo using a rotary evaporator and dissolve it in 30 milliliters of a pentane hydrocarbon solvent.

Re-crystallize again at minus 15 degrees Celsius to obtain 8 grams of S, 1R, 2S, 5R 2-isopropyl, 5-methocyclohexyl 1R1 phenylethyl aziridine-2 carboxylate ester two prime. Add R, 1R, 2S, 5R, 2-isopropyl 5-methocyclohexyl 1R1 phenylethyl aziridine-2 carboxylate and a magnetic stir bar into an oven-dried, 25-milliliter, two-neck, round-bottom flask under nitrogen atmosphere. Add 1.8 milliliters of ethanol to the reaction flask using an airtight syringe and stir at room temperature.

Then, add potassium carbonate and allow it to stir at room temperature for two days and monitor the progress of the reaction using thin-layer chromatography. After the reaction is completed, filter the mixture over filter paper and add water into the organic filtrate. Using a separating funnel, extract the organic layer with dichloromethane twice.

After drying the combined organic extracts over three grams of anhydrous sodium sulfate, use a rotary evaporator to concentrate in vacuo. Next, purify the crude product by normal-phase column chromatography on silica gel to afford a pure product of R-ethyl-1 R1 phenylethyl aziridine-2 carboxylate. To synthesize azide-opened compound, transfer 500 milligrams of chiral S-ethyl 1R1 phenylethyl aziridine-2 carboxylate and a magnetic stir bar into an oven-dried, 50-milliliter, two-neck, round-bottom flask under open atmosphere.

Then, add 50%aqueous ethanol to the reaction mixture. After cooling the reaction mixture at zero degrees Celsius, drop-wise add concentrated sulfuric acid to maintain pH 4.0 and stir for five minutes. Next, add sodium azide while stirring the reaction mixture for 10 minutes at the same temperature, and then warm at room temperature.

Then, add aluminum chloride hexahydrate as a catalyst at room temperature and allow to stir for an additional three hours. Monitor the progress of the reaction using thin-layer chromatography and quench the reaction mixture with two portions of 20 milliliters of saturated sodium bicarbonate. Then, filter the crude mixture over a celite pad with ethanol and concentrate the reaction mixture in vacuo using a rotary evaporator.

Extract the organic layer dichloromethane two times using a separating funnel. Dry the combined organic layer over five grams of anhydrous sodium sulfate for five minutes. Concentrate the crude organic layer in vacuo using a rotary evaporator to afford a crude azide product.

Purify the crude product with normal-phase column chromatography on silica gel by eluting with 40%ethyl acetate or hexane to afford 490 milligrams of azide-opened compound as a viscous liquid. 2, 3-dibromopropane L-methyl ester and 1R-phenylethylamine lead to the formation of diastereomeric mixture of R, 1R, 2S, 5R, 2-isopropyl 5-methocyclohexyl 1R1 phenylethyl aziridine-2 carboylate and S, 1R, 2S, 5R 2-isopropyl 5-methocyclohexyl 1R1 phenylethyl aziridine-2 carboxylate. Methanol gives rise to enantiopure forms of R, 1R, 2S, 5R, 2-isopropyl methocyclohexyl 1R1 phenylethyl aziridine-2 carboxylate, and N-pentane gives rise to S, 1R, 2S, 5R, 2-isopropyl 5-methocyclohexyl 1R1 phenylethyl aziridine-2 carboxylate.

Transesterification using either magnesium or potassium carbonate with alcohols such as methanol and ethanol yielded corresponding alkyl ester of chiral aziridine-2 carboxylates Regio-selective ring opening of ethyl 2S aziridine-2 carboxylate via azide nucleophile by sequential reactions leads to the total synthesis of biemamide B and biemamide D.Stereo-selective Mukaiyama aldol reaction of chiral aziridine-2 carboxyl aldehyde and silyl enol ether yield an adduct and its regio-and stereo-selective transformations yielded minus epiallo-isomuscarine. We succeeded in the preparation of both an enantiopure methyl and ester of 2R and 2S aziridine-2 carboxylates on a large scale via selective crystallization process. After making enantiopure aziridine-2 carboxylates, here we showed its application for the total synthesis of biemamide B, D, and epiallo-isomuscarine alkaloids.