Chemistry
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Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones
Chapters
Summary February 7th, 2019
Enantiomerically enriched bispiro[γ-butyrolactone-pyrrolidin-4,4'-pyrazolone] skeletons are asymmetrically synthesized through a simple organocatalytic 1,3-dipolar cycloaddition reaction.
Transcript
This one, three bipolar cycloaddition provides easy access to highly functionalized bispirocyclic compounds. Such compounds contain important scaffolds that have been found in many natural products that exhibit important biological activities. We can apply this organocatalytic cycloaddition to prepare or compress bispirocyclic scaffolds that contains two chiral spiral centers in only one step.
This reaction avoids high yields and excellent stereoselectivity. It is critical to ensure that all the reagents and solvents are dry before starting the reaction. Also, the cycloaddition reaction should be carried out under an Argon or Nitrogen atmosphere.
Demonstrating the procedure will be Yaping Cheng, a grad student from my laboratory. To prepare the pyrazolone add 45 milliliters of glacial acetic acid to a 250 milliliter round bottom flask containing a magnetic stir bar. Stir the solution at room temperature while adding one equivalent each of hydrazine and ethyl acetoacetate.
Then, equip the flask with a reflux condenser. Heat the reaction flask to 120 degrees Celsius in an oil bath, while stirring, for three hours. After cooling the flask to ambient temperature remove the magnetic stir bar, using a stir bar retriever.
Concentrate the reaction mixture using a rotary evaporator at 60 degrees Celsius. Next, add 20 milliliters of deionized water to the reaction flask and transfer the solution to a separatory funnel. Extract the aqueous layer three times with 30 milliliters of ethyl acetate.
Combine the organic layers in the separatory funnel, and wash them two times with 50 milliliters of brine. Dry the combined organic layers over anhydrous sodium sulfate for one hour. Then, remove the sodium sulfate by gravity filtration.
Remove the solvent on a rotary evaporator, under reduced pressure at 35 degrees Celsius to isolate the pyrazolone. To prepare the alpha-arylidiene pyrazolinone, add one equivalent of pyrazolone, one equivalent of benzaldehyde, 0.6 equivalents of magnesium oxide and a magnetic stir bar to an oven-dried 100 milliliter round bottomed flask under Nitrogen atmosphere. Using an airtight syringe, add 40 milliliters of anhydrous acetonitrile and equip the reaction flask with a reflux condenser.
Heat the reaction flask to 120 degrees Celsius in an oil bath while stirring for 12 hours. Monitor the progress of the reaction by thin-layer chromatography, or TLC, using a two-to-one mixture of petroleum ether and ethyl acetate as the eluent. After the complete consumption of pyrazolone, cool the reaction flask to room temperature.
Then, filter off the magnesium oxide using a celite plug. Remove the excess acetonitrile by using a rotary evaporator under reduced pressure at 35 degrees Celsius. Purify the residue by silica gel column chromatography.
eluting with petroleum ether and ethyl acetate to afford the crude product. Add the crude product to a 100 milliliter Erlenmeyer flask, equipped with a magnetic stir bar and add a minimum volume of 95 percent ethanol. Place the flask on a hot plate and bring it to a gentle boil with stirring until the entire solid is just dissolved.
Then, remove the flask from the hot plate and cool it slowly without any agitation to form the alpha-arylidiene product as pure crystals. To prepare the alpha-imino gamma-lactone, add one equivalent each of alpha-imino-gamma-butyrolactone hydrobromide, magnesium sulfate, triethylamine, and a magnetic stir bar to an oven-dried 100 milliliter round bottom flask under Nitrogen atmosphere. Using an airtight syringe, add 36 milliliters of anhydrous dichloromethane to the reaction flask and stir the reaction mixture at room temperature for one hour.
Then, add 1.1 equivalent of the desired thiophene-two-formaldehyde to the solution and stir for another 12 hours. Monitor the progress of the reaction by TLC using a four-to-one mixture of petroleum ether and ethyl acetate as the eluent until complete consumption of the lactone species has occurred. Then, filter off the reaction using filter paper with a pour size of 30 to 50 microns.
Next, add five milliliters of deionized water to the resulting mixture and separate the organic layer from the aqueous phase. Extract the aqueous phase two times with 30 milliliters of dichloromethane. Combine the organic layers in the separatory funnel and wash them two times with 50 milliliters of brine.
Dry the combined organic layers over anhydrous sodium sulfate for one hour. Remove the sodium sulfate by gravity filtration. Then, remove the solvent on rotary evaporator under reduced pressure at 35 degrees Celsius.
Add one equivalent of alpha-arylidiene pyrazolinone 1.2 equivalents alpha-imino-gamma-lactone and a magnetic stir bar to an oven-dried 15 milliliter round bottomed flask under a nitrogen atmosphere. Using an airtight syringe add 10 milliliters of anhydrous ethyl ether to the reaction flask. Then add 0.1 equivalent of the bifunctional squaramide catalyst to the solution and stir the reaction mixture at 40 degrees Celsius.
Monitor the progress of the reaction by TLC. Using a four-to-one mixture of petroleum ether and ethyl acetate as the eluent. After the reaction is complete, concentrate the mixture using a rotary evaporator at 40 degrees Celsius.
Purefy the residue by a silica gel column chromatography eluting with a four-to-one mixture of petroleum ether and ethyl acetate to afford the final product. Characterize the final product by proton and carbon NMR spectra using a 400 megahertz NMR spectrometer. Determine the enantiomeric excess of the product using a chiral HPLC column.
The representative synthetic process of the bifunctional squaramide catalyst is shown here. The screening of different organo-catalysts resulted in C5 with excellent stereoselectivity, and the highest yield. Further optimization of the solvent suggested that ethyl ether was preferable in this synthetic process.
A variety of substituents of two spirocyclization synthons with different functional groups were tested successfully using the optimized model reaction conditions, affording the desired bispirocycles in good to excellent yields and stereoselectivity. The structure of bispirocyclic products was confirmed by proton and carbon NMR spectroscopy. Representative HPLC chromatograms have the racemic and chiral compound 3E are shown here.
X-ray crystallography of compound 3E revealed the absolute configuration as 5S, 6R, 7R, 13R. The single crystal structure of 3E is shown here. Make sure all of the reagents and solvents are dry before starting the reaction.
Also, the reaction should be performed in an Argon or Nitrogen atmosphere. With this drug-like bispirocyclic production here, we can investigate their biological activities in order to explore their potential impact on protein and cellular levels and their applications in drug discovery. With new tactics, researchers are able to explore the possible biological functions of these bispirocyclic compounds, their influences on proteomic and cellular activities and the cross-bonding mechanisms.
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