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Combining Rh-catalyzed diazocoupling and enzymatic reduction to efficiently synthesize enantioenriched 2-substituted succinate derivatives Yajie Wang, Mark J. Bartlett, Carl A. Denard, John F. Hartwig, and Huimin Zhao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00254 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017
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ACS Catalysis
Combining Rh-catalyzed diazocoupling and enzymatic reduction to efficiently synthesize enantioenriched 2-substituted succinate derivatives Yajie Wang,a Mark J. Bartlett,b Carl A. Denard,a John F. Hartwig*,b and Huimin Zhao*,a,c a
Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 Department of Chemistry, University of California-Berkeley, Berkeley CA 94720 c Departments of Chemistry, Biochemistry, and Bioengineering, Carl R. Woese Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801 b
KEYWORDS: Chemo-enzymatic catalysis, biocatalysis, transition metal catalysis, diazocoupling, alkene reduction, one-pot
ABSTRACT: We report the development of a modular, one-pot, sequential chemoenzymatic system for the formal enantioselective construction of the C-C bond in 2-aryl 1,4-dicarbonyl compounds. This sequence comprises a rhodium-catalyzed diazocoupling that provides >9:1 selectivity for heterocoupling of two diazoesters and a reduction mediated by an ene-reductase (ER), which occurs in up to 99% enantiomeric excess (ee). The high yield and enantioselectivity of this system result from the preferential generation of an (E)-alkene from the diazo coupling reaction and selective reduction of the (E)-alkene in a mixture of (E) and (Z) isomers by the ER. Screening of a panel of ERs revealed that OPR1 from Lycopersicum esculentum catalyzes the reduction of bulky tertbutyl or benzyl esters to afford chiral diesters that are poised for orthogonal reactions at the two distinct ester units of the product. Overall, this work demonstrates the benefit of combining organometallic and enzymatic catalysis to create unusual overall transformations that do not require the isolation and purification of intermediates.
The increasing demand for chiral building blocks, particularly those used for the preparation of biologically active compounds, has motivated the development of novel strategies for enantioselective synthesis. To this end, systems that catalyze enantioselective carbon-carbon bond formation have been studied intensively.1 Biocatalysts are increasingly employed to provide levels of chemo-, regio- and stereoselectivity that are challenging to achieve with a small-molecule catalyst, but applications of biocatalysts for carbon-carbon bond formation are limited.2 The combination of chemical and biological catalysts in one-pot could enable transformations that cannot be achieved by either of the two catalysts alone,2a, 3 and we considered that formal, enantioselective C-C bond formation could be achieved by combining a chemical catalyst that forms a C-C bond and an enzyme that can set a stereogenic center at one of the carbons of the new bond. Asymmetric 2-substituted succinic acid derivatives are versatile building blocks for the preparation of biologically active compounds,4 particularly natural products containing the γ-butyrolactone-unit.5 Rhodium-catalyzed asymmetric hydrogenation of itaconic acid derivatives (Figure 1a) has been widely used to prepare enantiomerically enriched 2subsituted succinic acid derivatives, but these methods are not applicable to the synthesis of 2-aryl substituted succinic acid derivatives.6 Meanwhile, the synthesis of 2-aryl succinic acid derivatives via organocatalytic and transition metal-catalyzed asymmetric hydrogenation of prochiral aryl-substituted fumar-
ic (E) and maleic (Z) acid derivatives remains challenging.7 Only recently did Pfaltz and coworkers report an enantioselective Ir-catalyzed hydrogenation of 2-aryl-substituted fumarates and maleates (Figure 1b).7a However, the hydrogenation reactions were limited to symmetric diesters and required high catalyst loadings and hydrogen pressure. Alternative methods to prepare enantioenriched 2-aryl succinic derivatives include Rh-catalyzed 1,4-additions of arylboronic acids to fumarate derivatives (Figure 1c),8 Cinchona alkaloid-catalyzed parallel kinetic resolution of monosubstituted succinic anhydrides (Figure 1d),9 or enantioselective Nheterocyclic carbene (NHC)-catalyzed β-protonation through the orchestration of three distinct organocatalysts: triazolium salt, thiourea (HDB), and DMAP (Figure 1e).10 However, each reaction has its limitations, such as requiring di-tert-butyl fumarates for high enantioseletivity, forming inseparable mixtures of constitutional isomers from fumarates bearing two different ester functional groups, having a maximum yield of 50%, and forming product with lower ee at higher conversion. Enzymatic C=C bond reduction was once coupled with olefin synthesis to prepare non-chiral products.11 Here we report a two-step, one-pot sequential chemoenzymatic transformation to prepare enantioenriched 2-aryl 1,4-dicarbonyl compounds with good yield and excellent enantioselectivity. This transformation combines a Rh(II)-catalyzed diazo coupling reaction with an enzymatic reduction into one-pot catalytic sequence
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that occurs in greater efficiency than the two individual steps, while avoiding purification of the alkene intermediates (Figure 1). This process constitutes a novel chemoenzymatic carboncarbon bond formation with control of the absolute configuration of the new stereogenic center at one of the two carbons, while addressing limitations of the prior syntheses of 2-aryl succinic acid derivatives.
Figure 1. Synthesis of 2-aryl succinate derivatives using sequential rhodium- and enzyme catalysis (GDH: Glucose dehydrogenase, rt: room temperature).
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unsymmetrical 2-aryl-substituted, alkenes containing carbonyl groups at the 1 and 4 positions of the alkene. Davies and coworkers reported the [Rh2(OPiv)4]-catalyzed cross-coupling of diazo compounds to form 2-aryl-fumarate derivatives with high chemo- and stereoselectivity,1c and we tested reactions catalyzed by both [Rh2(OPiv)4] and the commercially available rhodium(II) octanoate dimer [Rh2(Oct)4]. As shown in Table 1, cross-coupling of aryldiazo compounds 1 with diazoacetate derivatives 2 affords predominantly the dicarbonyl compounds 3a and 3b with only trace amounts (10:1 stereoselectivity favoring (E)-ketone-ester, except entry 8). The reactions catalyzed by [Rh2(Oct)4] occur with yields, chemoselectivity and stereoselectivity that are similar to those of reactions catalyzed by [Rh2(OPiv)4]. With improved methods for preparation of diazo-compounds,12 Rh(II)-catalyzed carbenoid-induced cross-coupling of diazo compounds serves as an attractive convergent method for the synthesis of unsymmetrical alkenes. To achieve the proposed enantioselective chemoenzymatic formation of the C-C bond in 1,4-dicarbonyl compounds, an appropriate enzyme is needed to reduce the 2,3unsaturated 1,4-dicarbonyl compounds resulting from the diazo coupling. Ene-reductases (ERs) have been shown to be efficient, sustainable and cost-effective catalysts for reducing activated α,β-unsaturated ketones, aldehydes, nitroalkenes, carboxylic acids, and carboxylic acid derivatives,2a, 13 and a few enantioselective reductions of 2-substituted fumaric acid derivatives have been reported.13a, 14 However, our efforts were focused on the synthesis and reduction of fumarate derivatives containing two different carbonyl groups with orthogonal reactivity and, therefore, greater synthetic utility than symmetric fumarate derivatives.
To create a modular, enantioselective route to 2-arylfumarate derivatives, we first investigated the synthesis of Table 1. Synthesis of asymmetric 2-aryl-sbustitued dicarbonyl alkenes by Rh-catalyzed cross-coupling of diazo compounds
Entry
Ar
R1
R2
1
Ph
OMe
OEt
[Rh2(Oct)4]
[Rh2(OPiv)4]
3aa
3ba
3aa
3bb
44%
-
67%
-
2
Ph
OMe
OtBu
57%
-
74%
-
3
Ph
OMe
OBn
44%
6%
67%
-
4
Ph
OtBu
OEt
39%
-
N.A.
N.A.
5
4-F-Ph
Me
OEt
64%
7%
57%
5%
6
4-Cl-Ph
Me
OEt
68%
7%
64%
4%
7
3-CF3-Ph
Me
OEt
47%
10%
46%
3%
8
Ph
OMe
4-F-Ph
32%
4%c
20%
3%
a
b
Isolated yield. Yield determined by GC analysis.
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ERs containing flavin mononucleotides (FMN) require NADPH or NADH as co-factors to catalyze asymmetric reduction. In this study, glucose dehydrogenase (GDH) from Bacillus megaterium was used to recycle NADPH continuously by reduction of NADP+ with concomitant conversion of glucose to gluconic acid. We tested the reduction of the 2-aryl fumaric and maleic acid derivatives with a series of ERs originating from different organisms, including Old Yellow Enzymes OYE1, OYE2 and OYE3 from Saccharomyces cerevisiae, OYE4 from Achromobacter sp., 1, 2-oxophytodienoate reductases OPR1 and OPR3 from Lycopersicum esculentum, alkene reductase YersER from Yersinia bercovieri; thermophilic ‘ene’-reductase TOYE from Thermoanaerobacter pseudethanolicus, LacER from Lactobacillus casei, XenA and XenB from Pseudomonas putida, and NADPH dehydrogenase EBP1 from Candida albicans.
distinct esters that allow orthogonal reactivity at each terminus.10 None of the enzymes catalyzed the reduction of the substrate containing a bulky ester group and aryl group on the same alkene carbon (Entry 4). Compared to metal-catalyzed hydrogenation of dimethyl 2-phenylmaleates,7a the enzymatic reduction offers several advantages. The turnover numbers for the enzymatic reduction were seventy times higher than those obtained with current catalysts, and the process occurred with excellent enantioselectivity (>99%) at room temperature, in phosphate buffer under atmospheric pressure. Furthermore, YersER tolerates both water miscible organic solvents, such as DMSO and ethylene glycol, and immiscible organic solvents such as hexane and toluene.13b This tolerance toward organic solvents enhances the potential use of ERs for reactions of less water-soluble substrates.
Most of the ERs catalyzed the reduction of the E isomers (3a) of 2-aryl-substitued, 1,4-dicarbonyl alkenes generated from the cross-coupling reactions of the diazo compounds, but some of them resulted in products with poor ee (Table S1). None of the ERs we tested reduced the trace amount of Z isomers 3b (99%) (Entries 1&2) and the reduction of alkenes containing one ketone and one ester with good to excellent enantioselectivity (85%-99%) (Entry 5-8). OPR1 is the only enzyme we tested that reduced the unsaturated 1,4-dicarbonyl compounds containing bulky ester groups at the carbon β to the aryl group (Entries 2&3) to form enantioenriched 2-aryl succinate derivatives bearing two
Having identified an ER that reduces the substrates of the Rhcatalyzed coupling of diazocompounds, we sought to develop conditions to conduct the two reactions in one pot without purification of the intermediates. After conducting the diazo coupling reaction, we evaporated the DCM, added all reagents and the ER for the reduction step into the crude mixture. The one-pot process formed the enantioenriched succinate derivatives in >70% yield by completely converting the (E)-alkenes, the major product from cross-coupling reaction (Table 3), and leaving the (Z)-alkene unreacted. Similar yield and enantioselectivity were observed with purified samples of the alkene (Table 2), suggesting that the ERs are compatible with the Rhodium(II) catalyst and trace amounts of chemical impurities contained in the crude cross-coupling reaction.
Table 2. Enzymatic reduction of (E)-2-aryl-sbustitued, 1,4-dicarbonyl alkenes by ene-reductases
Entry
Ar
R1
R2
Yielda 4
%ee
Most selective ER
Large scale isolated yieldb
TONe
1
Ph
OMe
OEt
91% 71%f
>99%c
YersER
84%
455 7071f
2
Ph
OMe
OtBu
76%
>99% c
OPR1
65%
650
3
Ph
OMe
OBn
86%
>99% c
OPR1
80%
430
4
Ph
OtBu
OEt
-
-
-
-
-
5
4-F-Ph
Me
OEt
80%
87% c
YersER
75%
375
6
4-Cl-Ph
Me
OEt
85%
85%d
YersER
78%
423
7
3-CF3-Ph
Me
OEt
78%
85% d
YersER
70%
322
>99%
d
OYE2
80%
460
74%
>99%
d
YersER
69%
370
93%
93% d
OPR1
85%
465
8
Ph
OMe
4-F-Ph
87%
a
GC yield. blarge-scale (0.2 mmol) reactions and isolated yield. cee determined by chiral SFC. dee determined by chiral HPLC. eTurn over number (TON) is defined as the number of moles of substrate that a mole of catalyst can convert before becoming inactivated. KPi: sodium phosphate buffer. fReaction with 0.01mol% enzyme loading
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Table 3. Two-step sequential reaction to synthesize enantioenriched 2-aryl-substituted succinate derivatives
%eed
Isolated yield over two stepse
TON
>99%
62%
465
R1
R2
1
Ph
OMe
OEt
2
Ph
OMe
OBn
72%
0%
88%
>99%
54%
440
3
Ph
OMe
4-F-Ph
38%
4%
90%
>99%
26%
450
4
3-CF3-Ph
Me
OEt
56%
4%
82%
89%
37%
410
Entry
a
Yielda
Ar
b
b
c
3a
3b
4
78%
0%
93%
b
Determined by GC analysis. Trace amount of reaction mixture was taken after the diazocoupling reaction and evaluated by GC analysis. cCalculated based on the starting materials for enzymatic reaction. dee determined by chiral HPLC. eCalculated based on the starting materials of the cross-coupling reaction.
Figure 2. Docking of A) 4-ethyl 1-methyl 2- phenylfumarate, B) 4-ethyl 1-methyl 2- phenylmaleate, and C) 4-(tert-butyl) 1-methyl 2phenylfumarate within the active site of OPR1. D) the surface representation of the active site view of OPR1 with 4-(tert-butyl) 1-methyl 2-phenylfumarate docked at active site. In the stick models, substrates shown in green, FMN shown in yellow, protein residues shown in grey.
To provide possible explanations of the origin of the selectivity of ERs for (E)-alkenes over (Z)-alkenes, we conducted computational docking studies. Docking of 4-ethyl 1-
methyl 2-phenylfumarate and 4-ethyl 1-methyl 2phenylmaleate into the active site of OPR1 was conducted using the crystal structure of OPR1 (Figure S1) containing p-
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hydroxybenzoic acid (PHB) bound to the active site. As shown in Figure 2A, the carbonyl oxygen of 4-ethyl 1-methyl 2phenylfumarate is hydrogen bonded to His-187 Nϵ2 and His190 Nδ1 with distances of 3.2 Å and 2.8 Å respectively. The distance between the N(5) of FMNH2 (transferring a hydride) and the carbon of the alkene that is β to the aryl group (βcarbon) of the substrate is 4.0 Å, and the distance between the Tyr 192 (a proton donor) and α-carbon of the substrate is 3.9 Å. The (Z)-alkene 4-ethyl 1-methyl 2-phenylmaleate binds to the same site of OPR1 (Figure 2B). However the distance between N(5) of FMNH2 and the β carbon is more than 5.0 Å. This distance would be too large for the reaction to occur. In addition, the more hindered 4-(tert-butyl) 1-methyl 2phenylfumarate did not dock into most of the ERs (data are not shown), but it did dock into OPR1. As shown in Figure 2C, the distance between the N(5) of FMNH2 and the β-carbon of 4-(tert-butyl) 1-methyl 2-phenylfumarate, and the distance between Tyr 192 and α-carbon of 4-(tert-butyl) 1-methyl 2phenylfumarate are both 4.0 Å. These distances are comparable to those between the proton and hydride donors of OPR1 to 4-ethyl 1-methyl 2-phenylfumarate. A pocket formed by Tyr-78, Lys-79 and Tyr-358 at the entrance of active site accommodates the phenyl ring, while the relatively open active site entrance may tolerate the bulky tert-butyl group (Figure 2D). In summary, we have developed a one-pot, sequential catalytic system for the synthesis of 2-aryl-succinate derivatives by formal asymmetric C-C bond formation created by integrating a transition-metal catalyst with an enzyme, whereby a Rh-catalyzed cross-coupling of carbene units is followed by an ER-catalyzed enantioselective reduction of (E)-2-arylsubstituted dicarbonyl alkenes among a mixture of the E and Z isomers. With this system, 2-aryl-substituted succinate derivatives are generated from two different diazaoesters or ketones and reducing equivalents in high yield and excellent ee without purification of the alkene intermediates or separation of E and Z isomers. Evaluation of a panel of ERs as catalyst led to the identification of OPR1, which reacts with substrates containing bulky tert-butyl esters and produces enantioenriched, chiral unsymmetrical diesters that have great potential as synthetic intermediates.
ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Supplemental tables and figures, and additional information of experimental procedures and methods, characterization data, and NMR spectra of organic products.
AUTHOR INFORMATION Corresponding Author Huimin Zhao:
[email protected] John F. Hartwig:
[email protected] Present Addresses Carl Denard: University of Texas-Austin, Chemistry Department, 2500 Speedway, Austin Texas, 78712 Mark J. Bartlett: Gilead Sciences, Inc., 333 Lakeside Dr., Foster City, CA 94404
Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.
Funding Sources This work was supported by the NSF under the CCI Center for Enabling New Technologies through Catalysis (CENTC) Phase II Renewal, CHE-1205189.
ACKNOWLEDGMENT Prof. Kurt Faber is acknowledged for the gift of plasmid pET21aOPR1, pET21a-OPR3 and pET22a_EBP1. Prof. Uwe T. Bornscheuer is acknowledged for the gift of plasmid pGastonXenA and pGaston-XenB. Prof. Nigel S. Scrutton is acknowledged for the gift of plasmid pET21b_TOYE. Prof. Zhongliu Wu is acknowledged for the gift of plasmid pET28a_OYE4. We also thank Metabolomics Center of UIUC for GC-MS facilities and Dr. Alexander Vladimirovich Ulanov’s suggestions on GC analysis. Part of NMR data was collected in the IGB Core on a 600 MHz NMR funded by NIH grant number S10-RR028833. We thank Dr. Xudong Guan’s assistance with NMR data acquisition.
ABBREVIATIONS ER, ene-reductase; GDH, glucose dehydrogenase, OYE, old yellow enzyme; FMN, flavin mononucleotides; ee, enantiomeric excess; rt, room temperature; GC, gas chromatography; HPLC, high performance liquid chromatography; SFC, supercritical fluid chromatography; PHB, p-hydroxybenzoic acid.
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