The facile and stereoselective synthesis of pyrrolidine β-amino acids via copper(I)-catalyzed asymmetric 1,3-dipolar cycloaddition

Abstract

A highly efficient copper(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides with β-phthaliminoacrylate esters was developed under mild conditions, providing the desired pyrrolidine β-amino acid derivatives in high yields (up to 98%) with excellent diastereo- and enantioselectivities (dr >20 : 1, ee up to >99%). Subsequent deprotection and hydrogenolysis of the corresponding cycloadducts could deliver highly functionalized and biologically important free pyrrolidine β-amino acids in a straightforward and efficient pathway.

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Conformationally constrained heterocyclic β-amino acids and their derivatives are key elements of peptides and many bioactive products.¹ As an interesting class of five-membered N-heterocyclic compounds, pyrrolidine β-amino acids have received considerable attention among synthetic and medicinal chemists in the past few decades because of their important biological activities.^2–4 For instance, β-aminopyrrolidinecarboxylic acid (APC) together with β-aminocyclopentanecarboxylic acid (ACPC) have been utilized as building blocks for the construction of β-peptides that possess antimicrobial activities,² while A-87380 and A-192558 are influenza neuraminidase inhibitors.³ In addition, a series of compounds containing a pyrrolidine β-amino acid framework have been evaluated as DPP-IV inhibitors,^4a TACE inhibitors^4b and factor Xa inhibitors (Fig. 1).^4c Due to their potential application in chemical biology and drug discovery, numerous synthetic methods have been developed for the synthesis of functionalized pyrrolidine β-amino acids, especially in an enantiomerically pure form.^1,5 However, the majority of synthetic routes suffered from tedious synthetic steps through chiral auxiliary-based asymmetric synthesis, and a narrow substrate scope, thus greatly limiting their further application in a drug discovery scenario. Therefore, the development of efficient catalytically asymmetric access to functionalized pyrrolidine β-amino acids is still highly desirable.

Fig. 1 Selected examples of biologically active pyrrolidine β-amino acids.

On the other hand, the catalytic asymmetric 1,3-dipolar cycloaddition of azomethine ylides with activated dipolarophiles has become one of the most powerful tools for preparing enantioenriched polysubstituted pyrrolidines.^6,7 Over the past decade, much effort has been devoted to the development of novel catalytic systems, and the exploration of a variety of new dipolarophiles. However, unlike the commonly utilized activated alkenes^7,8 such as acrylates,^7b,8a–c enones,^7c–f,8d,e maleates,^8f–j fumarates,^8k maleimides,^8l,m nitroalkenes,^8n–t vinyl sulfones^8u,v and α-heteroatom substituted alkenes,^8w–z β-heteroatom anchored electron-deficient alkenes have rarely been explored.⁹ In connection with our continuing efforts in developing catalytic asymmetric transformations of azomethine ylides,^8x–z,10 we have recently reported a rapid access to 3,4-diaminopyrrolidines via a newly designed 4-iodo-DHIPOH ligand/Cu(I) catalyzed 1,3-dipolar cycloaddition of azomethine ylides with β-phthalimidonitroethene (Scheme 1a).^10d Inspired by this work, we further envisioned whether pyrrolidine β-amino acids could be produced in a concise manner via asymmetric 1,3-dipolar cycloaddition of azomethine ylides with β-amino substituted acrylates serving as new dipolarophiles. Nevertheless, β-amino substituted acrylates as dipolarophiles could suffer from low reactivity due to the electron-donating nitrogen substituent at the β-position, thus making the catalytic asymmetric cycloaddition process more challenging. To address this issue, a phthalimide group for amino protection could be employed to reduce its electron-donating properties and thus enhance the reaction activity. Herein, we report the first example of Cu(I)-catalyzed 1,3-dipolar cycloaddition of azomethine ylides with β-phthaliminoacrylate esters¹¹ to generate pyrrolidine β-amino esters in high yields with excellent diastereo- and enantioselectivities, which can be easily converted into highly functionalized free pyrrolidine β-amino acids (Scheme 1b).

Scheme 1 Catalytic asymmetric 1,3-dipolar cycloaddition of azomethine ylides with β-heteroatom alkenes.

Initially, imino ester 1a and β-phthaliminoacrylate methyl ester 2a were chosen as model substrates for probing this reaction (Table 1). We commenced our investigation using CuBF₄ (10 mol%)/N,O-ligands L1 or L2 (11 mol%) as the catalyst system, in the presence of 20 mol% of Et₃N as the base in CH₂Cl₂ at rt, given that these conditions had proven to be very effective in our previous 1,3-dipolar cycloaddition.^10d However, the expected cycloaddition did not occur under these reaction conditions (Table 1, entries 1 and 2). Pleasingly, when AgOAc was used instead of CuBF₄, changing 20 mol% of Et₃N into two equivalents of K₂CO₃ in THF, the reaction took place after 24 h and gave the desired cycloadduct 3aa in low yield (30%) with excellent diastereoselectivity (dr >20 : 1) and moderate enantioselectivity (51% ee) (Table 1, entry 3). Encouraged by this result, we decided to investigate the effect of chiral ligands including N,O-ligand L2 and planar-chiral ferrocene P,N-ligands (Phosferrox) L3–L6 (Table 1, entries 4–8). To our delight, all the tested ligands promoted this reaction smoothly within 2 h and Phosferrox ligand L5 exhibited the optimal reaction outcome, delivering the corresponding cycloadduct 3aa in 93% yield and 86% ee (Table 1, entry 7). Further switching the metal source from Ag(I) back to Cu(I) had a remarkable improvement in enantioselectivity (Table 1, entries 9 and 10), with CuBF₄ giving the optimal results (96% yield, 98% ee) (Table 1, entry 9). The effect of solvent was then examined, and THF showed the best result (Table 1, entries 11 and 12). Furthermore, lowering the temperature to 0 °C afforded 3aa in 96% yield with extremely high enantioselectivity (>99% ee) (Table 1, entry 13). Finally, when a lower catalyst loading (5 mol%) was employed at this temperature (0 °C), 3aa was obtained with almost the same excellent outcome (95% yield, >99% ee), which was identified as the optimal reaction conditions (Table 1, entry 14).

Table 1 Optimization for the catalytic asymmetric 1,3-dipolar cycloaddition of azomethine ylide 1a with β-phthaliminoacrylate methyl ester 2a ^a

Entry	Metal/L	Base	Solvent	Yield^b (%)	ee^c (%)
a Unless otherwise stated, reactions were performed with 1a (0.15 mmol), 2a (0.10 mmol) in 1 mL of solvent (C = 0.1 M). CuBF4 = Cu(CH3CN)4BF4, CuPF6 = Cu(CH3CN)4PF6. b Isolated yield. c >20 : 1 dr was determined by crude ^1H NMR spectroscopy, and ee was determined by chiral HPLC analysis. d In 24 h. e In 2 h. f In 3 h. g T = 0 °C. h 5 mol% cat, 0.2 mmol scale.
1^d	CuBF4/L1	Et3N	CH2Cl2	Trace
2^d	CuBF4/L2	Et3N	CH2Cl2	Trace
3^d	AgOAc/L1	K2CO3	THF	30	−51
4^e	AgOAc/L2	K2CO3	THF	89	−67
5^e	AgOAc/L3	K2CO3	THF	91	75
6^e	AgOAc/L4	K2CO3	THF	88	64
7^e	AgOAc/L5	K2CO3	THF	93	86
8^e	AgOAc/L6	K2CO3	THF	90	83
9^e	CuBF4/L5	K2CO3	THF	96	98
10^e	CuPF6/L5	K2CO3	THF	91	93
11^d	CuBF4/L5	K2CO3	CH2Cl2	70	98
12^d	CuBF4/L5	K2CO3	Et2O	81	97
13^f^,^g	CuBF4/L5	K2CO3	THF	96	>99
14^f^,^g^,^h	CuBF4/L5	K2CO3	THF	95	>99

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With the optimized reaction conditions in hand, the generality and substrate scope of this process were examined, and the results are shown in Table 2. To begin with, the ester groups of β-phthaliminoacrylate esters (2a–2d) were varied (Me, Et, ^tBu, Bn), giving the corresponding products (3aa–3ad) in high yields (91–96%) with high diastereoselectivities (dr >20 : 1) and excellent enantioselectivities (99 to >99% ee) (Table 2, entries 1–4). By changing the ester groups of imino esters from methyl to ethyl and tert-butyl groups (1b–1c) led to a decrease in enantioselectivity of the desired cycloadducts (3ba–3ca) (96% ee and 92% ee, respectively) (Table 2, entries 5 and 6 versus entry 1). Then, a variety of imino esters (1d–1m) derived from aryl aldehydes bearing electron-deficient (Table 2, entries 7–10), electron-rich (Table 2, entries 12–15), and electron-neutral substituents (Table 2, entries 10 and 16) on the aryl rings reacted smoothly with β-phthaliminoacrylate methyl ester (2a), affording the corresponding products (3da–3ma) in high yields (92–98%) with high diastereoselectivities (dr >20 : 1) and excellent enantioselectivities (94 to >99% ee). Additionally, the heteroaryl imino esters (1n–1o) derived from 2-thenaldehyde and 2-furylaldehyde worked efficiently in this transformation, resulting in the desired cycloadducts (3na–3oa) in >99% ee and 98% ee, respectively (Table 2, entries 17 and 18). Notably, less reactive aliphatic-substituted imino ester 1p was also tolerated in this reaction and the desired adduct 3pa was obtained in good yield (87%) with excellent diastereoselectivity (dr >20 : 1) and enantioselectivity (98% ee) in the presence of a stronger base Cs₂CO₃ (Table 2, entry 19).

Table 2 Substrate scope of the catalytic asymmetric 1,3-dipolar cycloaddition of azomethine ylide 1 with β-phthaliminoacrylate esters 2 ^a

Entry	R^1/R^2 (1)	R^4 (2)	Yield^b (%)	ee^c (%)
a All reactions were performed with 1 (0.30 mmol), and 2 (0.20 mmol) in 2 mL of THF. b Isolated yield. c >20 : 1 dr was determined by crude ^1H NMR spectroscopy, and ee was determined by chiral HPLC analysis. d Cs2CO3 was used instead of K2CO3 and the reaction completed in 3 h, but no product was obtained under standard reaction conditions after 12 h.
1	p-ClC6H4/Me (1a)	Me (2a)	95 (3aa)	>99
2	p-ClC6H4/Me (1a)	Et (2b)	91 (3ab)	99
3	p-ClC6H4/Me (1a)	^tBu (2c)	94 (3ac)	>99
4	p-ClC6H4/Me (1a)	Bn (2d)	96 (3ad)	>99
5	p-ClC6H4/Et (1b)	Me (2a)	91 (3ba)	96
6	p-ClC6H4/^tBu (1c)	Me (2a)	92 (3ca)	92
7	o-ClC6H4/Me (1d)	Me (2a)	96 (3da)	>99
8	m-ClC6H4/Me (1e)	Me (2a)	92 (3ea)	95
9	p-BrC6H4/Me (1f)	Me (2a)	98 (3fa)	99
10	p-CNC6H4/Me (1g)	Me (2a)	97 (3ga)	>99
11	Ph/Me (1h)	Me (2a)	96 (3ha)	>99
12	o-MeC6H4/Me (1i)	Me (2a)	95 (3ia)	>99
13	m-MeC6H4/Me (1j)	Me (2a)	93 (3ja)	94
14	p-MeC6H4/Me (1k)	Me (2a)	95 (3ka)	99
15	p-MeOC6H4/Me (1l)	Me (2a)	97 (3la)	>99
16	2-Naphthyl/Me (1m)	Me (2a)	94 (3ma)	98
17	2-Thienyl/Me (1n)	Me (2a)	93 (3na)	>99
18	2-Furyl/Me (1o)	Me (2a)	81 (3oa)	98
19^d	Cy/Me (1p)	Me (2a)	87 (3pa)	98

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Remarkably, α-methyl imino ester 4 derived from alanine was also examined for the construction of pyrrolidine β-amino ester bearing one quaternary and three contiguous tertiary stereogenic centers. Due to the lower reactivity, we made some additional optimization for this cyclization process, including the use of Cs₂CO₃ instead of K₂CO₃ as the base and the rise of the temperature from 0 °C to rt. The reaction proceeded smoothly, furnishing the corresponding pyrrolidine 5a in 83% yield, >20 : 1 dr, and >99% ee (Scheme 2).

Scheme 2 Asymmetric 1,3-dipolar cycloaddition of imino ester 4 derived from alanine with β-phthaliminoacrylate methyl ester 2a.

To further demonstrate the synthetic utility of this process, the asymmetric 1,3-dipolar cycloaddition between 1a and 2a was carried out on a gram scale with lower catalyst loading (1 mol%), giving 3aa in 99% yield with >20 : 1 dr and >99% ee. The absolute configuration of 3aa was unequivocally determined to be (2S,3S,4R,5R) by single crystal X-ray diffraction of the corresponding N-tosylated amide 6 (Scheme 3).¹² The further transformation of the corresponding cycloadducts by employing 3ad as an example into free pyrrolidine β-amino acids was demonstrated as follows. The phthalimide group of 3ad was removed in the presence of ethylenediamine to afford the free amine 7 in good yield (83%) without loss of the ee value, which was then subjected to a catalytic hydrogenation condition to deliver optically pure free pyrrolidine β-amino acid 8 in 96% yield (Scheme 4).

Scheme 3 Gram scale synthesis and the absolute configuration determination of the cycloadduct derivative 6.

Scheme 4 Deprotection and hydrogenolysis of cycloadduct 3ad to pyrrolidine β-amino acid 8.

Conclusions

In summary, we have developed a facile and stereoselective synthesis of a series of structurally novel and biologically important pyrrolidine β-amino acid derivatives via catalytic asymmetric 1,3-dipolar cycloaddition of azomethine ylides with β-phthaliminoacrylate esters. The Cu(I)/Ph-Phosferrox catalytic system exhibited excellent efficiency even in 1 mol% catalyst loading, providing the corresponding cycloadducts in high yields (up to 98%) with excellent stereoselectivity control (dr >20 : 1, ee up to >99%). Importantly, highly functionalized free pyrrolidine β-amino acids could be easily obtained from the cycloadducts by simple deprotection and hydrogenolysis, showing great potential in organic synthesis and drug discovery. Further application of this protocol in drug discovery is still underway in our laboratory.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (no. 21372074 and 21572053).

Notes and references

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12. CCDC 1495843 (6) contains the supplementary crystallographic data for this paper.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1495843. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qo00512h

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