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Highly Enantioselective Synthesis of Chiral #Lactams by Rh-Catalyzed Asymmetric Hydrogenation Qiwei Lang, Guoxian Gu, Yaoti Cheng, Qin Yin, and Xumu Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00827 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 23, 2018
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ACS Catalysis
Highly Enantioselective Synthesis of Chiral γ-Lactams by RhCatalyzed Asymmetric Hydrogenation Qiwei Lang,† Guoxian Gu,† Yaoti Cheng,† Qin Yin,*,†,‡ and Xumu Zhang*,† †
Department of Chemistry, Southern University of Science and Technology, Shenzhen 518000, People’s Republic of China Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen 518000, People’s Republic of China ‡
ABSTRACT: A Rh/bisphosphine-thiourea (ZhaoPhos) catalytic system has been identified for the straightforward asymmetric synthesis of chiral γ-lactams. A variety of NH free α,β-unsaturated lactams bearing a β-aryl or -alkyl substituent were smoothly hydrogenated to provide the desired γ-lactams in up to 99% yield and 99% ee. This methodology provides a highly practical pathway to synthesize chiral γ-lactams or γ-amino acids. Key words: asymmetric hydrogenation, bisphosphine-thiourea, α,β-unsaturated lactams, chiral γ-lactams, γ-amino acids
INTRODUCTION Chiral γ-lactams have attracted increasing attention due to their ubiquitous existence in bilogical compounds,1 especially clinic therapeutics, such as the antidepressant drug (R)Rolipram2 and antiepilepsy drug Brivaracetam.3 Additionally, their ring-opened products, γ-amino acids, are key intermediates to access a variety of natural products and drug molecules,4 such as Baclofen5 and Pregabalin6 (upper, Figure 1). Tremendous efforts have thus been devoted to the preparation of chiral γ-lactams.7,8 Rh-catalyzed asymmetric conjugate addition of organometallic reagents to α,β-unsaturated lactams represents a straightforward and efficient strategy.8 Seminal works from He,8a Lin8b,8c and others8d,8e have made this method reliable to obtain chiral γ-lactams. It is noteworthy that a protecting group, either electron-donating or -withdrawing, on the N atom of the substrates is critical and generally no reaction would occur for free lactam substrates. An additional deprotection operation is requisite to liberate more versatile NH amide or amine products (lower left, Figure 1). Alternatively, we envisaged β-substituted chiral γ-lactams could be directly synthesized via asymmetric hydrogenation of β-substituted α,β-unsaturated lactams (lower right, Figure 1). In comparison with well-established asymmetric hydrogenation of α,βunsaturated lactones,9 maleinimides10 and maleic anhydrides,11 hydrogenation of structurally similar but more inert α,βunsaturated lactams remains a formidable task and no asymmetric manner has been developed so far.12,13 Herein, we reported an unprecedented Rh-catalyzed asymmetric hydrogenation of β-substituted α,β-unsaturated lactams, which could successfully tolerate free NH amide group. Recently we have reported a novel ligand, ZhaoPhos, comprising a chiral ferrocenylbisphosphine scaffold and a tunable thiourea moiety.14 This ligand has found broad application in Rh-catalyzed asymmetric hydrogenation of a variety of substrates, such as substituted nitro-olefins,14,15 unsaturated carboxylic acid derivatives,16 unprotected imines,17 isoquino-
lines,18 and others.19 Considering the possible hydrogenbonding interaction between thiourea subunit in ZhaoPhos and the amide group of the substrate, we envisioned that ZhaoPhos could also be applied in the Rh-catalyzed hydrogenation of α,β-unsaturated lactams.
Figure 1. Selected biologically active molecules with a chiral γ-lactam or γ-amino acid unit (upper) and methods toward them (lower).
RESULTS AND DISCUSSION We initiated our study by investigating the performance of different combinations of rhodium precatalysts and various chiral bisphosphine ligands with the standard substrate NH lactam 1a under 60 atm of H2. While several commercially available ligands, such as Binap, SegPhos, DuanPhos and JosiPhos (Figure 2), exhibited very limited activities in the hydrogenation reactions catalyzed by Rh-ligand complexes, we were pleased to observe that ZhaoPhos displayed encouraging outcome in terms of conversion and enantiocontrol (entry 5 vs entries 1-4, Table 1). Further evaluation of rhodium source
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(entries 6 and 7) unveiled Rh(NBD)2BF4 as the best precatalyst, affording the desired product 2a in 91% yield and 96% ee (entry 6). Attempts to use other metal precatalysts, [Ir(COD)Cl]2 and [RuCl2(p-cymene)]2 in combination with ZhaoPhos did not bring satisfactory results (entries 8 and 9).
lectivities were generally obtained, the conversion is highly dependent on the solvent. Overall, dichloromethane afforded the best results in terms of conversion and enantioselectivity (entries 1-7, Table 2). Interestingly, 99% ee of 2a could be achieved in THF at 25 oC albeit with moderate conversion of 1a. Attempts to enhance the reaction conversion in THF by elevating the reaction temperature proved to be fruitful (entries 7-9). The optimal conversion of 1a in THF (88%) was finally achieved when the reaction was carried out at 60 oC, however, with slightly decreased enantioselectivity of 2a (96% ee, entry 9). On the other hand, full conversion was fortunately obtained in the case of CH2Cl2 when the temperature was increased to 35 oC, without sacrificing the reaction enantiocontrol (entry 10). When we reduced the hydrogenation pressure from 60 to 40 atm (entry 11), the conversion decreased to 86%. Given both the yield and enantioselectivity of 2a, the combination of Rh(NBD)2BF4/ZhaoPhos as catalyst, CH2Cl2 as solvent, 60 atm of H2 at 35 oC is the optimal conditions.
Figure 2. Tested chiral bisphosphine ligands for the asymmetric hydrogenation of 1a.
Table 2. Optimization of reaction conditions for the asymmetric hydrogenation of 1aa
Table 1. Screening of ligands and metal precatalysts for the asymmetric hydrogenation of 1aa
Entry Conv.
Ee
(%)b
(%)c
S-Binap
0
-
Rh(COD)2BF4
S-SegPhos
30
2
3
Rh(COD)2BF4
SC,RPDuanPhos
44
4
Rh(COD)2BF4
R-JosiPhos
5
Rh(COD)2BF4
6
Entry
Precatalyst
Ligand
1
Rh(COD)2BF4
2
Temp.
Conv.
Ee
(ºC)
(%)b
(%)c
Solvent
1
CH2Cl2
25
91
96
2
ClCH2CH2Cl
25
21
87
3
i
PrOH
25
14
84
4
MeOH
25
33
94
3
5
CF3CH2OH
25
35
93
19
1
6
1,4-dioxane
25
43
75
ZhaoPhos
74
87
7
THF
25
65
99
Rh(NBD)2BF4
ZhaoPhos
91
96
8
THF
35
84
98
7
[Rh(COD)Cl]2
ZhaoPhos
56
76
9
THF
60
88
96
8
[Ir(COD)Cl]2
ZhaoPhos
9
-78
10
CH2Cl2
35
>99
96
d
CH2Cl2
35
86
96
9
[RuCl2(pcymene)]2
11 ZhaoPhos
0
-
a
Unless otherwise noted, all reactions were carried out with a [Rh(NBD)2BF4]/ligand/1a (0.1 mmol) ratio of 1 : 1.1 : 100 in 1.0 mL of CH2Cl2 under hydrogen (60 atm) for 48 h. bDetermined by 1H NMR. cDetermined by HPLC analysis using a chiral stationary phase. The absolute configuration of 2a was determined as R by comparing the optical rotation data with literature.7j To further improve the conversion and enantiocontrol, solvent and temperature effects were surveyed. While good enantiose-
a
Unless otherwise noted, all reactions were carried out with a [Rh(NBD)2BF4]/ligand/1a (0.1 mmol) ratio of 1 : 1.1 : 100 in 1.0 mL of solvent under hydrogen (60 atm) for 48 h. bDetermined by 1H NMR. cDetermined by HPLC analysis using a chiral stationary phase. dThe reaction was carried out under 40 atm of hydrogen pressure. To gain insight into this catalytic system, two other chiral ligands L1–L2 were tested and the results are summarized in Table 3. Interestingly, the N-methylated ligand L1 exhibited comparable activity and enantiocontrol (entry 2, 96% ee).
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ACS Catalysis However, the ligand L2 without the thiourea group displayed very low activity and led to nearly no asymmetric induction (entry 3). A combination of L2 and thiourea completely shut down the reaction (entry 4). In addition, non-chiral ligand dppf gave similar reactivity as L2 (entry 5). These results suggest the importance of hydrogen bonds in this reaction, and the thiourea motif not only efficiently activates the carbonyl group through hydrogen bonding interaction but also works as a directing group.
91% ee. Substrates with a benzyl (1v) or more bulky isopropyl (1w) group were also transformed smoothly to their corresponding products (95 % ee for 2v, 96% ee for 2w). Substrate 1x with an alkyl group on the α position furnished the desired product 2x in excellent yield albeit with very poor enantioselectivity (6% ee). Table 4. Substrate scopea,b,c R
Table 3. Evaluation of structure-activity relationship of ZhaoPhosa
O
N X
R
Rh(NBD)2BF4 (1 mol%) ZhaoPhos (1.1 mol%) H2 (60 atm), CH2Cl2, 35 C, 48 h
* N X
1
O
2 Me
O
N H
O N H 2b 95%, 95% ee
2a 98%, 96% ee
OMe
MeO
F
O N H 2c 94%, 96% ee
O N H 2d 86%, 95% ee F3C
Cl
MeO N H b
c
Entry
Ligand
Conv. (%)
Ee (%)
1
ZhaoPhos
>99
96
2
L1
>99
96
3
L2
22
2
4
L2 + thiouread
0
-
5
dppf
20
0
O
2e 99%, 92% ee
N H
O N H 2f 99%, 96% ee
O N H 2g 98%, 96% ee MeO
S
O N H
O
2i 99%, 97% ee Me
O
2j 97%, 97% ee MeO
a
All reactions were carried out with a [Rh(NBD)2BF4]/ligand/ 1a (0.1 mmol) ratio of 1 : 1.1 : 100 in 1.0 mL of CH2Cl2 under hydrogen (60 atm) for 48 h. bDetermined by 1H NMR. c Determined by HPLC analysis using a chiral stationary phase. d L2/thiourea = 1:1, thiourea refers to thiocarbamide. With optimal conditions in hand, we then turned our attention to the substrate scope. A wide range of β-aryl substituted α,βunsaturated lactams were investigated, and in general, hydrogenation proceeded smoothly to afford the desired chiral γlactams with free NH in excellent yield as well as enantiomeric excess (Table 4). Both electron-donating (Me or MeO as in 2b-2e) and electron-withdrawing (F, Cl or CF3 as in 2f-2h) substituents on the aryl ring at different positions were well tolerated, and the reactivity and enantioselectivity were excellent throughout. Lactam 1j possessing a thienyl substitute was also compatible, affording the desired product 2j in 97% yield and 97% ee. It is noteworthy that substrate 1k with a disubstituted aryl group on the β position proceeded smoothly to provide 2k, an antidepressant drug (Rolipram), in 98% yield and 96% ee. Besides, a series of aryl substituted α,β-unsaturated lactams (1l-1s) with various substituents (Boc, Me, tBu, Bn, Ph or PMP) on N atom were also investigated. To our delight, all reactions furnished the desired products (2l-2s) in excellent yields and enantiomeric excesses, regardless of the electronic or steric property of the substituents on N atom. Gratifyingly, unsaturated lactam 1t with an alkyl group on the β position is also a suitable substrate, giving the desired γ-lactam 2t, a precursor to the antiepilepsy drug Brivaracetam, in 97% yield and
O N Boc
O N Boc
N H
2k 98%, 96% ee
N Me
O
O N Boc 2l 98%, 99% ee
O
2m 95%, 99% ee
2n 85%, 98% ee
2o 98%, 95% ee
O N Bn 2q 98%, 96% ee
O N Ph 2r 98%, 97% ee
O N PMP 2s 99%, 96% ee
n-C7H15
O N H 2h 97%, 96% ee
N But
O
2p 98%, 97% ee
O N PMP 2t 97%, 91% ee Me
Ph *
O N PMP 2u 99%, 93% ee
O N PMP 2v 99%, 95% ee
O N PMP 2w 98%, 96% ee
N Ph
O
2x 97%, 6% ee
a
Unless otherwise noted, all reactions were carried out with a [Rh(NBD)2BF4]/ligand/1a (0.1 mmol) ratio of 1 : 1.1 : 100 in 1.0 mL of solvent under hydrogen (60 atm) for 48 h. bIsolated yield. cDetermined by HPLC analysis using a chiral stationary phase. The configuration of 2a, 2g, 2k, 2l was determined as R by comparing the optical rotation data with literatures.7j,8b PMP = p-methoxyphenyl. To demonstrate the synthetic utility of this method, a gramscale transformation of 1k was conducted under standard conditions. Satisfyingly, the hydrogenated product 2k was obtained in 94% yield and 96% ee. Similarly, scalable hydrogenations of 1g and 1e were performed to furnish 2g and 2e re-
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spectively in excellent yields and ee (Scheme 1). The former 2g was subsequently treated with 6 N HCl, affording GABA receptor agonist (R)-Baclofen hydrochloride (3),5,8b and the latter 2e underwent hydride reduction with LiAlH4 and subsequent reductive amination, furnishing chiral pyrrolidine 4 (76% yield for 2 steps), which is known to have high affinity for an α-2-adrenoceptor subtype.20
We are grateful to Southern University of Science and Technology, Shenzhen Science and Technology Innovation Committee (JCYJ20170817110055425 and JCYJ20170817104350391) for financial support. We greatly acknowledge Professor Tiezheng Jia (Southern University of Science and Technology) for polishing the paper.
Scheme 1. Practical synthetic transformations
(1) (a) Martelli, G.; Monsignori, A.; Orena, M.; Rinaldi S. Recent Advances in Chemistry of γ-Lactams: Part II. Functionalization by CC or CHeteroatom Bond Formation. Curr. Org. Chem. 2014, 18, 1539-1585. (b) Guo, L.-N.; Pei, Y.-H.; Chen, G.; Cong, H.; Liu, J.-C. Two new compounds from Dictamnus dasycarpus. J. Asian Nat. Prod. Res. 2012, 14, 105-110. (c) Enz, A.; Feuerbach, D.; Frederiksen, M. U.; Gentsch, C.; Hurth, K.; Müller, W.; Nozulak, J.; Roy, B. L. Gamma-lactams–A novel scaffold for highly potent and selective α7 nicotinic acetylcholine receptor agonists. Bioorg. Med. Chem. Lett. 2009, 19, 1287-1291. (2) (a) Zhu, J.; Mix, E.; Winblad, B. The Antidepressant and Antiinflammatory Effects of Rolipram in the Central Nervous System. CNS Drug Rev. 2001, 7, 387-398. (b) Garcia, A. L. L.; Carpes, M. J. S.; de Oca, A. C. B. M.; dos Santos, M. A. G.; Santana, C. C.; Correia, C. R. D. Synthesis of 4-Aryl-2-pyrrolidones and β-Aryl-γ-aminobutyric Acid (GABA) Analogues by Heck Arylation of 3-Pyrrolines with Arenediazonium Tetrafluoroborates. Synthesis of (±)-Rolipram on a Multigram Scale and Chromatographic Resolution by Semipreparative Chiral Simulated Moving Bed Chromatography. J. Org. Chem. 2005, 70, 1050-1053. (3) Rogawski, M. A. Diverse mechanisms of antiepileptic drugs in the development pipeline. Epilepsy Res. 2006, 69, 273-294. (4) (a) Ordóñez, M.; Cativiela, C.; Romero-Estudillo I. An update on the stereoselective synthesis of γ-amino acids. Tetrahedron: Asymmetry 2016, 27, 999-1055. (b) Ordóñez, M.; Cativiela, C. Stereoselective synthesis of γ-amino acids. Tetrahedron: Asymmetry 2007, 18, 3-99. (c) Trabocchi, A.; Guarna, F.; Guarna, A. γ- and δ-Amino Acids: Synthetic Strategies and Relevant Applications. Curr. Org. Chem. 2005, 9, 1127-1153. (5) (a) Ramesh, P.; Suman, D.; Reddy, K. S. N. Asymmetric Synthetic Strategies of (R)-(–)-Baclofen: An Antispastic Drug. Synthesis 2018, 50, 211-226. (b) Bowery, N. G.; Hill, D. R.; Hudson, A. L.; Doble, A.; Middlemiss, D. N.; Shaw, J.; Turnbull, M. (–)Baclofen decreases neurotransmitter release in the mammalian CNS by an action at s novel GABA receptor. Nature 1980, 283, 92-94. (6) (a) Belliotti, T. R.; Capiris, T.; Ekhato, I. V.; Kinsora, J. J.; Field, M. J.; Heffner, T. G.; Meltzer, L. T.; Schwarz, J. B.; Taylor, C. P.; Thorpe, A. J.; Vartanian, M. G.; Wise, L. D.; Ti, Z.-S.; Weber, M. L.; Wustrow, D. J. Structure-Activity Relationships of Pregabalin and Analogues That Target the α2-δ Protein. J. Med. Chem. 2005, 48, 2294-2307. (b) Silverman, R. B. From Basic Science to Blockbuster Drug: The Discovery of Lyrica. Angew. Chem., Int. Ed. 2008, 47, 3500-3504. (7) For recent reviews, see: (a) Rivas, F.; Ling, T. Advances toward the Synthesis of Functionalized γ-Lactams. Org. Prep. Proced. Int. 2016, 48, 254-295. (b) Ye, L.-W.; Shu, C.; Gagosz, F. Recent progress towards transition metal-catalyzed synthesis of γ-lactams. Org. Bio. Chem. 2014, 12, 1833-1845. For selected examples, see: (c) Corey, E. J.; Zhang, F. Y. Enantioselective Michael Addition of Nitromethane to α,β-Enones Catalyzed by Chiral Quaternary Ammonium Salts. A Simple Synthesis of (R)-Baclofen. Org. Lett. 2000, 2, 4257-4259. (d) Thakur, V. V.; Nikalje, M. D.; Sudalai, A. Enantioselective synthesis of (R)-(−)-baclofen via Ru(II)–BINAP catalyzed asymmetric hydrogenation. Tetrahedron: Asymmetry 2003, 14, 581586. (e) Craig, D.; Hyland, C. J. T.; Ward, S. E. Stereoselective γlactam synthesis via palladium-catalysed intramolecular allylation. Chem. Commun. 2005, 3439-3441. (f) Okino, T.; Hoashi, Y.; Furukawa, T.; Xu, X.; Takemoto, Y. Enantio- and Diastereoselective Michael Reaction of 1,3-Dicarbonyl Compounds to Nitroolefins Catalyzed by a Bifunctional Thiourea. J. Am. Chem. Soc. 2005, 127, 119125. (g) Wee, A. G. H.; Duncan, S. C.; Fan, G. J. Intramolecular asymmetric C–H insertion of N-arylalkyl, N-
(1) Gram-scale synthesis of (R)-Rolipram MeO
MeO standard conditions
O
O
94%, 96% ee O N H (R)-Rolipram (2k)
O N H 1k (1.04g, 3.8 mmol) (2) Synthesis of (R)-Baclofen Cl
Cl
Cl 6 N HCl
standard conditions
reflux
97%, 95% ee O N H 1g (232 mg, 1.2 mmol)
N H 2g
O
O NH2 HCl
HO
(R)-Baclofen (3)
(3) Synthesis of -2-adrenoceptor antagonist
1) LiAlH 4, THF
standard conditions 99%, 92% ee MeO
MeO N H
O
1e (227 mg, 1.2 mmol)
N H 2e
MeO 2) ArCHO, MeOH NaBH3CN N 76% for 2 steps Ar Ar = 3,4-(OCH2)2-C6H3 -2-adrenoceptor antagonist (4) O
SUMMARY In summary, we have identified a straightforward asymmetric hydrogenation strategy towards synthetically useful chiral γlactams from β-substituted α,β-unsaturated lactams catalyzed by Rh(NBD)2BF4/ZhaoPhos. Of particular interest is the impressive performance of ZhaoPhos against other commercially available ligands in this method, especially for unsaturated lactams with free NH. The asymmetric hydrogenation strategy reported herein represents a direct synthetic pathway to prepare chiral γ-lactams with high enantiomeric excess. In addition, scalable and facile syntheses of drug molecules such as Rolipram, Baclofen and α-2-adrenoceptor antagonist 4 highlighted the practicality of this methodology.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected];
[email protected] Notes The authors declare no competing financial interest.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Synthesis and characterization of detailed experimental procedures (PDF)
ACKNOWLEDGMENT
REFERENCES
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6 ACS Paragon Plus Environment
