Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones

Summary

Enantiomerically enriched bispiro[γ-butyrolactone-pyrrolidin-4,4'-pyrazolone] skeletons are asymmetrically synthesized through a simple organocatalytic 1,3-dipolar cycloaddition reaction.

Abstract

Bispirocyclic scaffolds are one of the important structural subunits in many natural products that exhibit diverse and attractive biological activities. Recently, we have developed an efficient organocatalytic strategy, which provides facile access to a variety of enantiomerically enriched bispiro[γ-butyrolactone-pyrrolidin-4,4'-pyrazolone] skeletons. In this paper, we demonstrate a detailed protocol for the asymmetric synthesis of drug-like bispirocyclic compounds with two spirocyclic carbon centers via an organocatyltic 1,3-dipolar cycloaddition reaction. Spirocyclization synthons α-imino γ-lactones and alkylidene pyrazolones are prepared first, which are then subjected to a cycloaddition reaction in the presence of a bifunctional squaramide organocatalyst to afford the desired bispirocycles in high yields and excellent stereoselectivities. Chiral high-performance liquid chromatography (HPLC) is carried out to determine the enantiomeric purity of the products, and the d.r. value is examined by proton nuclear magnetic resonance (¹H NMR). The absolute configuration of the product is assigned according to an X-ray crystallographic analysis. This synthetic strategy allows scientists to prepare a diversity of bispirocyclic scaffolds in high yields and excellent diastereo- and enantioselectivities.

Introduction

Chiral spirocyclic compounds found prevalent in natural products, chiral ligands and organometallic complexes have emerged as attractive synthetic targets due to their structural complexity and biological activity^1,2,3. Specifically, bispirocyclic scaffolds, featured by three rings with two rigid spirocenters, are structural subunits in many natural products with important biological activities^4,5. Consequently, the construction of compounds with stereocontrolled, optically pure bispirocyclic skeletons has drawn great attention over the last few decades. A large number of spirocyclic compounds and their derivatives have been synthesized successfully through organometallic approaches and organocatalytic approaches, for example, asymmetric cycloadditions such as 1,3-dipolar cycloadditions and Diels-Alder reactions^6,7,8. However, these molecules are mostly monospirocyclic structures, while bispirocyclic structures are less reported on and limited to the construction of indole-based bispirocycles.

In order to obtain more structurally diverse bispirocyclic compounds, the versatility of cycloaddition synthons for the asymmetric construction of spirocyclic centers has been explored^9,10,11. Especially with bifunctional squaramide organocatalysts, azomethine ylide^12,13,14, such as α-imino γ-lactones, and dipolarophiles, such as alkylidene pyrazolones^15,16,17, are able to undergo a simple 1,3-dipolar cycloaddition to construct bispirocyclic skeletons with multiple stereocenters, making them the perfect spirocyclization synthons (Figure 1). After the optimization of the structure of organocatalyst and reaction solvent, this cycloaddition process efficiently affords the desired product with high yields and excellent enantio- and diastereoselectivity. Moreover, this reaction exhibits a relatively high structural tolerance on a broad scope of cycloaddition synthons with diverse functional groups¹⁸. This new method provides an efficient access to a variety of highly functionalized drug-like compounds with two quaternary spirocenters via a simple organocatalytic cycloaddition, shining lights on its application in the structural diversity-oriented synthesis of this intriguing class of compounds.

Protocol

CAUTION: Please consult all relevant material safety data sheets (MSDS) before use. Chemicals and solvents used were of reagent grade and were used without further purification. All reactions involving air or moisture-sensitive reagents or intermediates were performed under an argon atmosphere.

1. Preparation of α-Arylidiene Pyrazolinone Species

1. Preparation of pyrazolones
   1. Add 40 mL of glacial acetic acid to a 250 mL round-bottom flask from a graduated cylinder at room temperature. Stir the solution while adding hydrazine (1 equivalent, 1.58 mol/L) and methyl acetoacetate (1 equivalent, 1.58 mol/L). Equip the flask with a reflux condenser.
      NOTE: This concentration is used because a lower concentration leads to a slower reaction rate.
   2. Heat the reaction flask to 120 °C in an oil bath while stirring for 3 h. After cooling the reaction flask down to ambient temperature, remove the magnetic stir bar, using a stir bar retriever. Concentrate the reaction mixture, using a rotary evaporator at 60 °C. Avoid spilling the reaction mixture because of negative pressure.
   3. Add 20 mL of deionized water to the reaction flask and transfer the solution into a separatory funnel. Extract the aqueous layer 3x with ethyl acetate (30 mL). Combine the organic layers in the separatory funnel and wash them 2x with brine (50 mL).
   4. Dry the combined organic layers over anhydrous sodium sulfate for 1 h and, then, remove the sodium sulfate by gravity filtration.
   5. Remove the solvent on a rotary evaporator at reduced pressure and at 35 °C.
   6. After removing all the solvent, apply the pyrazolone species when performing section 4.
2. Preparation of α-Arylidiene Pyrazolinones
   1. Add pyrazolone (1 equivalent, 0.49 mol/L), benzaldehyde (1 equivalent, 0.49 mol/L), magnesium oxide (0.5 g, 0.6 equivalent), and a magnetic stir bar into an oven-dried 100 mL round-bottom flask under N₂ atmosphere.
   2. Add anhydrous acetonitrile (40 mL) to the reaction flask, using an airtight syringe, and then, equip the flask with a reflux condenser. Heat the reaction flask to 120 °C in an oil bath while stirring for 12 h.
   3. Monitor the progress of the reaction by thin layer chromatography (TLC), using petroleum ether:ethyl acetate (2:1 [v/v], retention factor R_f = 0.86) as an eluent.
   4. After the complete consumption of pyrazolone, cool the reaction flask down to room temperature. Filter off the magnesium oxide through a Celite plug.
   5. Remove the excess acetonitrile by using a rotary evaporator under reduced pressure and at 35 °C. Purify the residue by column chromatography on silica gel eluting with petroleum ether:ethyl acetate (10:1 to 8:1 [v/v]) to provide the crude product.
   6. Add the crude product into a 100 mL Erlenmeyer flask equipped with a magnetic stir bar, and then, add a minimum volume of 95% ethanol. Place the flask on a hot plate and bring it to a gentle boil until the entire solid is just dissolved. Take the flask off the hot plate and cool it slowly without any agitation.
      NOTE: When the mixture is cooled to room temperature, the corresponding α-arylidiene pyrazolinone is formed as pure crystals.

2. Synthesis of α-imino γ-lactones species

1. Add α-amino-γ-butyrolactone hydrobromide (1 equivalent, 0.41 mol/L), magnesium sulfate (1 equivalent, 0.41 mol/L), triethylamine (1 equivalent, 0.41 mol/L), and a magnetic stir bar into an oven-dried 100 mL round-bottom flask under N₂ atmosphere.
2. Add 36 mL of anhydrous dichloromethane to the reaction flask, using an airtight syringe. Stir the reaction mixture at room temperature for 1 h. Add the corresponding thiophene-2-carbaldehyde (1.1 equivalent, 0.45 mol/L) to the solution and stir for another 12 h.
3. Monitor the progress of the reaction by TLC, using petroleum ether:ethyl acetate (4:1 [v/v]) as an eluent until the complete consumption of the lactone species has occurred, and then, filter off the reaction mixture, using a filter paper with a pore size of 30−50 μm.
4. Add 5 mL of deionized water to the resulting mixture and separate the organic layer from the aqueous phase. Extract the aqueous phase 2x with dichloromethane (30 mL). Combine the organic layers in the separatory funnel and wash them 2x with brine (50 mL).
5. Dry the combined organic layers over anhydrous sodium sulfate for 1 h, and then, remove the sodium sulfate by gravity filtration. Remove the solvent on a rotary evaporator at reduced pressure and at 35 °C.
6. After removing all the solvent, apply the α-Imino γ-lactones species when performing section 4.

3. Synthesis of bifunctional squaramide catalyst C5¹⁹

NOTE: For the synthesis of organocatalysts 5C, see Figure 2.

1. Preparation of 3-((3,5-bis(trifluoromethyl)phenyl)amino)-4-methoxycyclobut-3-ene-1,2-dione (compound 1)
   1. Add 3,4-dimethoxycyclobut-3-ene-1,2-dione (1 equivalent, 0.63 mol/L), 3,5-bis(trifluoromethyl)aniline (1.1 equivalent, 0.69 mol/L), 20 mL of methanol, and a magnetic stir bar into an oven-dried 100 mL round-bottom flask under N₂ atmosphere.
   2. Stir the mixture at room temperature for 48 h. The formation of yellow precipitate is an indication that the reaction is taking place.
   3. Filter the reaction solution through a funnel fitted with filter paper and wash the solid product 3x with methanol (15 mL). Dry the yellow solid in vacuo overnight to afford the final products as yellow solid.
2. Synthesis of catalyst C5
   1. Add 3-((3,5-bis(trifluoromethyl)phenyl)amino)-4-methoxycyclobut-3-ene-1,2-dione (compound 1; 1 equivalent, 0.2 mol/L) and (S)-(6-methoxyquinolin-4-yl)((1S,2R,4S,5R)-5-vinylquinuclidin-2-yl)methanamine (compound 2; 1 equivalent, 0.2 mol/L) and a magnetic stir bar into a 25 mL round-bottom flask under N₂ atmosphere.
   2. Add anhydrous dichloromethane (5 mL), using an airtight syringe. Stir the mixture at room temperature for 48 h.
   3. Monitor the progress of the reaction by TLC, using dichloromethane:methanol (10:1 [v/v], R_f = 0.49) as an eluent. After the reaction is completed, concentrate the reaction mixture, using a rotary evaporator at 40 °C.
   4. Purify the residue by column chromatography on silica gel eluting with dichloromethane:methanol (20:1 [v/v]) to provide the desired product.

4. Asymmetric synthesis of bispirocyclic compounds

1. Dry a 50 mL round-bottom reaction flask containing a magnetic stir bar. Remove the flask from the oven and cool it to room temperature by blowing on it with inert gas before use.
2. Add α-arylidiene pyrazolinone (1 mmol, 1 equivalent, 0.1 mol/L) and α-imino γ-lactones (1.2 mmol, 1.2 equivalent, 0.12 mol/L) to the 50 mL round-bottom flask under N₂ atmosphere.
3. Add anhydrous ethyl ether (10 mL) to the reaction flask, using an airtight syringe. Then, add the corresponding organocatalyst (0.1 equivalent, 0.01 mol/L) to the solution and stir the reaction mixture at 40 °C.
4. Monitor the progress of the reaction by TLC, using petroleum ether:ethyl acetate (4:1 [v/v], R_f = 0.51) as an eluent.
   NOTE: The spots of the starting materials and products were visualized using a hand-held 254 nm UV lamp.
5. After the reaction was completed, concentrate the reaction mixture, using a rotary evaporator at 40 °C.
6. Purify the residue by column chromatography on silica gel eluting with petroleum ether:ethyl acetate (4:1 [v/v]) to provide the final product.
7. Characterize the final product by ¹H and ¹³C NMR spectra, using a 400 MHz NMR spectrometer. Determine the ee values of the product, using a chiral column.

Results

Various hydrogen-bond donor bifunctional organocatalysts were examined in the presence of organocatalysts in dichloromethane (DCM) at 25 °C (Table 1). The representative synthetic process of organocatalysts is shown in Figure 1. The screening of different organocatalysts (Table 1, entries 1-6) resulted in C5 with excellent stereoselectivity (94% ee, >20:1 d.r., entry 5) and the best yield (85% yield). A further optim...

Discussion

The successful preparation of bispiro[γ-butyrolactone-pyrrolidin-4,4'-pyrazolone] skeletons is dependent on a number of factors.

The key step of this one-step asymmetric cycloaddition process is the synergistical activation of the α-arylidiene pyrazolinone 1a and cyclic imino ester 2a by the bifunctional squaramide catalyst. It is achieved by the formation of multiple intermolecular hydrogen bonds between catalyst as a hydrogen-bond donor and two...

Materials

Name	Company	Catalog Number	Comments
Acetonitrile, anhydrous, 99.9%	Innochem (China)	A0080
α-amino-γ-butyrolactone hydrobromide, 98%	Alfa Aesar	B23148
3,5-bis(trifluoromethyl)aniline, 98+%	Adamas	48611B
Dichloromethane, 99.5%	Greagent	G81014H
3,4-dimethoxycyclobut-3-ene-1,2-dione, 98+%	Leyan (China)	1062550
Ethanol, 99.5%	Greagent	G73537B
Ethyl acetate, 99.5%	Greagent	G23272L
Ethyl ether,anhydrous,99.5%	Greagent	G69159B
Ethyl 3-oxobutanoate, 98%	TCI	A0649
4-fluorobenzaldehyde, 98%	Innochem (China)	A24295
Glacial acetic acid, 99.5%	Greagent	G73562B
Magnesium oxide, 99+%	Alfa Aesar	44733
Magnesium sulfate, 98%	Greagent	G80872C
Methanol, 99.5%	Greagent	G75851A
Petroleum ether	Greagent	G84208D
Phenylhydrazine, 98%	Innochem (China)	A57671
(S)-(6-methoxyquinolin-4-yl)((1S,2R,4S,5R)-5-vinylquinuclidin-2-yl)methanamine	DAICEL Group	111240
Sodium sulfate,anhydrous,99%	Greagent	G82667A
Thiophene-2-carbaldehyde, 98%	J & K scientific (China)	124605
Triethylamine, 99%	J & k scientific (China)	432915