Dynamic Kinetic Resolution of 3-Aryl-4-pentenoic Acids - ACS

Apr 14, 2016 - The first example of dynamic kinetic resolution (DKR) of chiral unsaturated carboxylic acids is described. The application of tandem ...
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Dynamic Kinetic Resolution of 3-aryl-4-Pentenoic Acids Dominik Koszelewski, Anna Brodzka, Anna ##d#o, Daniel Paprocki, Damian Trzepizur, Malgorzata Zysk, and Ryszard Ostaszewski ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00271 • Publication Date (Web): 14 Apr 2016 Downloaded from http://pubs.acs.org on April 15, 2016

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Dynamic Kinetic Resolution of 3-aryl-4-pentenoic Acids

Dominik Koszelewski, Anna Brodzka, Anna Żądło, Daniel Paprocki, Damian Trzepizur, Małgorzata Zysk and Ryszard Ostaszewski* Institute of Organic Chemistry Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw (Poland) KEYWORDS dynamic kinetic resolution, unsaturated carboxylic acids, racemization, biotransformations, rhodium catalyst

ABSTRACT: : The first example of dynamic kinetic resolution (DKR) of the chiral unsaturated carboxylic acids is described. The application of tandem metal-enzyme DKR is a powerful tool for the manufacture of high-value chemical commodities. This new protocol of kinetic resolution based on irreversible enzymatic esterification of 3-aryl-4-pentenoic acids with orthoesters was introduced to obtain optically active unsaturated carboxylic acids. This procedure was combined with metal catalyzed racemization of the target substrate providing optically pure (S)-enantiomer of ethyl 3-(phenyl)pent-4-enoate with very high isolated yield (98%). A substantial influence of organic co-solvent, and metal catalyst on conversion and enantioselectivity of the enzymatic dynamic kinetic resolution was noted.

INTRODUCTION The synthesis of enantiomerically pure compounds is of great importance for pharmaceutical industry.1 Large amount of medicines are chiral, non-racemic compounds which biological activity is highly dependent on their absolute configuration of the stereogenic centers. The application of a racemic mixture as a medicine is undesirable because it generally requires a higher dose to cause the wanted biological response. Additionally, the presence of the other enantiomer may have adverse side effects. Thus, the demand for methods for the synthesis of chiral non-racemic substances has increased rapidly in response to these commercial considerations. One of the most convenient way to obtain enantiomerically pure compounds is still based on kinetic resolution (KR).2 Kinetic resolution requires a non-racemic catalyst or reagent to promote a selective reaction with one of the enantiomers, providing enantioenriched unreacted substrate and enantioenriched product. In this methodology the major limitation is that the maximum theoretical yield is 50%.3 If the racemization can occur concurrently with the kinetic resolution, known as dynamic kinetic resolution (DKR), then theoretically 100% of racemic mixture can be converted to one enantiomer. During the last decade DKR has significantly evolved and become an important area in organic synthesis. Several examples of efficient dynamic enzymatic resolution have been reported and recently the use of transition metals for substrate racemization has attracted some interest.4 However, this methodology has successfully been applied to resolution the racemic mixture of alcohol and amine derivatives.5,6 Among others optically active 3-aryl-4-pentenoic acid is an important substrate for the synthesis of numerous medicaments like Rolipram®,7 an anti-inflammatory drug and one of the family of γ-aminobutyric acid (GABA) derivatives as well as Citrocard®,8 Lioresal®,9 Femoxetine®,10 or Paroxetine®10 another drugs belonging to the GABA family.11 Optically active 3-phenyl-4-pentenoic acid is the key intermediate in the synthesis of LG 121071 a modulator of androgen receptors and Neurokinin NK1/NK2 antagonists.12 The (S)-3-aryl-4pentenoic acid derivatives were also used for the synthesis of antiproliferation lactones (HL-60 cells inhibitors),10e,13 antimi-

crobial agents, and also antitumor antibiotic methylenolactocin (Figure 1).14

Figure 1. Pharmaceutical relevant compounds derived from 3aryl-4-pentenoic acids.

In all cases, the beneficial or adverse effects of these compounds depend on their absolute configuration. In the view of the medicinal importance, synthetic studies of these compounds have attracted considerable interest. Several chemical procedures have been explored to provide the routes to them. Two common approaches include lengthy asymmetric syntheses using chiral auxiliaries15 as well as microwave assisted molybdenum-catalyzed allylic alkylations.16 The enzymatic method is available as well and comprises the nitrile hydratase/amidase-containing microbial- catalyzed kinetic resolution of racemic nitriles.17 All of these chemical and enzymatic methods have several disadvantages. The whole-cell preparation catalyzed the hydration of a nitrile followed by the amide hydrolysis catalyzed by the amidase, have become effective methods for the production of carboxylic acids in aqueous environment. However, starting from a racemic mixture, kinetic resolution is always limited to a maximum yield of 50% of the enantiopure material, additionally isolation of products from aqueous solution is usually tedious and time consuming.

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Thus, it seems significant to find a method to overcome these limitations. New methods providing enantiomerically pure compounds based on tandem metal-enzyme dynamic kinetic resolution are still of high interest due to the concepts of asymmetric synthesis. RESULTS AND DISSCUSION In continuation with our current studies on dynamic kinetic resolution of chiral carboxylic acids, we recently reported on the dynamic kinetic resolution of 3-hydroxy-3-(aryl)propanoic acids. In this reaction a rhodium catalyst racemizes β-hydroxy acid during enzymatic esterification with orthoesters.18a,18b Previously, we have successfully developed the kinetic resolution of 3-phenyl-4-pentenoic acid.8a,10a Unfortunately, established methodology is limited to the maximum yield of 50%. To overcome this inconvenience, we have envisioned to introduce the racemization of the unreacted enantiomer to set up a dynamic kinetic resolution (DKR). The classical and commonly applied tandem metal-enzyme DKR is based on sec-alcohol racemization and enzymatic acylation of preferred enantiomer.19 In the case of alcohols the racemization predominantly occurs through hydrogen transfer process.20 To the best of our knowledge there is not any literature report of tandem metalenzyme DKR of carboxylic acids possessing double bond at γ position. Drueckhammer and co-workers have extensively studied racemization of thioesters of α-phenylpropionate which after enzymatic hydrolysis provided enantiomerically enriched α-substituted carboxylic acids.21 Due to well recognized fact that transition metal catalysts interact reversible with activated and non-activated carboncarbon double bounds4 we investigated the racemization of 8a,10a (S)-3-phenyl-4-pentenoic acid ((S)-1a) ([α]ଶ଴ ஽ = −19.3) with different transition metal catalysts (Table 1). We applied several commercially available catalysts which are commonly used in DKR - rhodium(II) acetate dimer (A),22 dichloro(pcymene)-ruthenium(II) dimer (B),23 and tetrakis(triphenylphosphine) palladium(0) (C) (Scheme 1).24 Scheme 1. Racemization catalysts

In all cases catalyst racemization of (S)-1a occurred. The best results were obtained with rhodium acetate (A) (Table 1). This metal complex has been successfully used previously for the racemization of β-hydroxy acids in tert-butyl methyl ether (TBME) at 40 °C.18a This procedure was initially used for the racemization of ((S)-1a). Whereas, racemization occurred, the conversion was low which made it necessary to modify our protocol (Table 1, entry 1). Elevated temperature to 50 °C as well as addition of base enhanced only a bit racemization rates

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(Table 1, entries 2 and 3). Taking into the consideration various factors which may impact the racemization efficiency two different organic solvents were tested. In case of applying cyclohexane adverse effect on racemization rate has been observed (Table 1, entry 4). The use of toluene as the solvent led to substantial racemization rate at 40 °C (Table 1, entry 5). Table 1. Racemization of (S)-3-phenylpent-4-enoic acid ((S)-1a)a entry

metal catalyst

solvent

T (°C)

ee (%)c

1

A: Rh(II)

TBME

40

35

2

A: Rh(II)

TBME

50

21

3

A: Rh(II)b

TBME

50

24

4

A: Rh(II)

cyclohexane

50

37

5

A: Rh(II)

toluene

40

26

6

A: Rh(II)

toluene

50

12

7

A: Rh(II)

toluene

60

rac

8

B: Ru(II)b

toluene

60

24

9

C: Pd(0)

toluene

60

50

a

Reaction conditions: All the reactions were run for 24 h, 0.1 mmol of (S)-1a, solvent (3.0 mL), metal catalyst (10 mol%). btBuOK (4 mol%). cDetermined by HPLC on a Chiralcel OD-H column.

Detailed temperature studies show that full racemization of (S)-1a can be achieved within 24 hours in toluene at 60 °C (Table 1, entries 6 and 7). Also two other metal complexes catalyze racemization but with relevantly lower rates (Table 1, entries 8 and 9). Putative mechanism of studied racemization can be analogous to already known mechanism reported for palladium catalysts. Palladium complexes such as C catalyze racemization of allylic esters under conditions compatible with use of lipases via π-allyl complex formation.5a, 25 Several additional experiments were performed to obtain more information about the mechanism of the racemization process. It is well recognized that the rhodium(II) acetate readily exchange the bridging acetate groups what may explain high efficiency of Rh(II) catalyst in racemization of the acid 1a exclusively what can be followed by Uv-Vis spectroscopy.18c The absorption spectra of acid 1a and ester 2b were recorded using the same concentration after 2 hours of agitation at ambient temperature in DCM. In case of formed rhodium complex with 3-phenyl-4pentenoic acid (1a) decreasing in intensity together with new band at 308 nm was observed (Figure 2). The same exchange with rhodium acetate ligands does not occur for ester 2a what manifests in overlapping of Uv-Vis spectras. Previous study using 1H NMR spectroscopy have showed that chiral dirhodium Mosher’s acid complex Rh2(MTPA)4 obtained upon exchange the acetate groups with Mosher’s acid interacts with carbon - carbon double bonds (C=C). Detailed investigation of several compounds possessing two C=C reveals that such interaction is much stronger with terminal C=C bonds.26 This very efficient ligand exchange between dirhodium tetraacetate and carboxylic acid (1a) manifests also by forming immediately a greenish solution in CDCl3 during NMR probe preparation (photographs are given in supplementary data). Analogous behavior of rhodium (II) was observed by Cotton and Falvello.27 Authors reported the strong binding of caryophyllene molecules with tetrakis(trifluoroacetato)dirothium

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(II) through the endo double bonds. Similar phenomena has not been observed for ester 2a where both spectra with and without rhodium catalyst overlap each other. Ester (S)-2a does not racemize in the presence of rhodium(II) complex. Additional useful data were elicited from the racemization of 3phenyl-4-pentenoic acid (1a) performed under isotopic exchange conditions in the presence of deuterium oxide. The detailed analysis of isotope patterns for acid 1a shown on Figure S1 (supplementary data) and corresponding ester 2a (not shown) indicate only slight incorporation of deuterium into acid 1a. This observation may suggests that the racemization of acid 1a is not assisted by π-allyl complex.

OEt OEt Ph OEt I OEt

OEt EtO

OEt

Ph

III

OEt IV

O

Application of 1,1-diethoxyethane (V) and diethyl carbonate (VI) led to racemic product 2a with very low yield (Table 2, entries 5, 6). Slightly better results regarding productivity, up to 57% isolated yield, were obtained with triethyl orthoacetate (II), triethyl orthoformate (III) and tetraethyl orthocarbonate (VII) (Table 2, entries 2,3 and 7). Finally, the highest enantiomeric excess was observed in case of donors possessing aromatic moieties, triethyl orthobenzoate (I) and benzyl diethyl acetate (IV) providing enantiomerically pure ester (>99% ee) (S)-2a with 47% isolated yield (Table 2, entries 1 and 4). Table 2. Kinetic resolution of rac-3-phenylpent-4-enoic acid (1a) with different alkoxy groupa entry

donor

yield (%)b

ee (%)c

1

I

47

>99 (S)

2

II

57

69 (S)

3

III

13

68 (S)

4

IV

32

81 (S)

5

V

3

nd

6

VI

9

nd

7

VII

23

29 (S)

a

Reaction conditions: 0.2 mmol of 1a, triethyl orthobenzoate 2 equiv., toluene (5.0 mL), Novozym 435 (10 mg), 72 h. bIsolated yield. cDetermined by HPLC on a Chiralcel OD-H column.

Next, the series of experiments with different rac-3-aryl-4pentenoic acid derivatives 1b-g were performed providing corresponding enantiomerically enriched esters 2b-c and 2e-g (Scheme 3). Scheme 3. Enzymatic kinetic resolution of acid rac-1a-g

CO2 H

triethyl orthobenzoate Novozym 435

CO2 Et

toluene, 60 oC

R

R rac -1a-g

Reactions were performed for 72 hours in toluene in 0.2 mmol scale at 60 °C using seven different alkoxy donors I-VII (Figure 3).

OEt

OEt OEt OEt EtO OEt EtO OEt V VI VII Figure 3. Alkoxy donors used in EKR of rac-1a catalyzed by Novozym 435.

Figure 2. Absorption spectra of free and rhodium complex with acid 1a and ester 2a recorded in DCM.

Collected data enriched by the literature reports on olefin isomerization and hydrogenation catalyzed by rhodium catalysts28 may indicate that the racemization process of studied acid 1a consists of two individuals; first activation of catalyst by ligand exchanges followed by double bond isomerization. This conclusion was supported by the fact that the racemization has not been observed in case of optically active ester (S)2a. The immobilized lipase B from Candida antarctica (Novozym 435) was employed for the kinetic resolution of racemic 3-phenyl-4-pentenoic acid (1a) (Scheme 2). Scheme 2. Enzymatic kinetic resolution of rac-1a

OEt OEt OEt II

(S)-2a-g

The substrates 1b-g were synthetized according to the literature procedure in Johnson-Claisen rearrangement of corresponding allylic alcohols and followed by basic hydrolysis.29 Obtained results applying Novozym 435 as biocatalyst indicate that there are specific substrate requirements (Table 3). Table 3. Kinetic resolution of rac-3-aryl-4-pentenoic acids (1a-g)a entry

R

product

yield (%)b

ee (%)c

1

H

2a

47

>99 (S)

2

MeO

2b

21

48 (S)

3

Me

2c

0

nd

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4

3,4-OCH2O

2d

0

nd

5

Cl

2e

41

92 (S)

6

F

2f

45

97 (S)

7

NO2

2g

48

16

a

Reaction conditions: 0.2 mmol of 1, triethyl orthobenzoate 2 equiv., toluene (5.0 mL), Novozym 435 (10 mg). bIsolated yield. c Determined by the chiral HPLC.

The results indicated that the most suitable substrates for the enzymatic reactions are these with small attached groups at para position 1a, 1e and 1f, which offer the best compromise between conversion and selectivity (Table 3, entries 1, 5 and 6). In case of derivative 1g with strong electron withdrawing nitro group corresponding ester 2g was obtained with 48% yield but with very low enantiomeric excess (Table 3, entry 7). Moreover, the size of substituent attached to the para position of the phenyl ring cannot exceed the size of the active pocket in the enzyme and only in case of substrate 1b corresponding ester 2b was obtained with moderate yield and low enantiomeric excess (Table 3, entries 2-4). The next step of our studies was to combine the racemization with the enzymatic transformation in one-pot procedure (Scheme 4). Scheme 3. Dynamic kinetic resolution of acid rac-3-aryl-4pentenoic acid derivatives catalyzed by Novozym 435

Further experiments were assigned with Novozym 435 and rhodium(II) acetate in toluene at 60 °C for 120 hours (Table 4). Table 4. Dynamic kinetic resolution of racemic rac-3-aryl4-pentenoic acid derivatives catalyzed by Novozym 435a entry

R

product

yield (%)b

ee (%)c

1

H

2a

98

>99 (S)

2

MeO

2b

37

43 (S)

3

Cl

2e

78

90 (S)

4

F

2f

92

95 (S)

5

NO2

2g

98

10

a

Reaction conditions: 0.2 mmol of 1, triethyl orthobenzoate 3 equiv., toluene (5.0 mL), Novozym 435 (10 mg) and metal catalyst (10 mol%). bIsolated yield. cDetermined by the chiral HPLC.

Finally, we applied this protocol on a preparative scale. Thus, 176 mg (1 mmol) of racemic 3-phenyl-4-pentenoic acid (1a) was transformed into optically pure ester (S)-2a within six days, and with 97% yield of isolated product. To show the applicability of this concept, other unsaturated acids 1e and 1f were subjected to DKR with Novozym 435 leading to corresponding esters (S)-2e and (S)-2f with 76% and 90% yield and enantiomeric excess values identical with those obtained for the initial DKR (Table 4, entries 3 and 4). Optically active ethyl 3-(phenyl)pent-4-enoate (2a) is a precursor of two medicines, Femoxetine® and Citrocard®.8,10

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As it can be noticed, appropriate combination of enzyme, transition metal catalyst and organic solvent is required for successful enzymatic DKR procedure. Further studies under optimization and amplification of herein proposed protocol are still underway. In spite of described limitations proposed approach diversifies already knew methods and lead to optically pure products with very high yield. CONCLUSIONS In summary, we have featured the generality of the previously established protocol18a and shown high potential in the synthesis of enantiomerically enriched pharmacologically relevant unsaturated carboxylic acids. Nowadays, enzymatic dynamic kinetic resolution was limited to compound possess hydroxy or amino functional groups. Here we have shown that our protocol works efficiently for compounds possessing carbon-carbon double bond and phenyl group in allylic position without mentioned functional groups. Under optimized conditions in situ racemization of the substrate carboxylic acid by rhodium catalyst and enzymatic esterification with a specifically selected alkoxy donor resulted in full transformation of the racemic carboxylic acid in to enantiomerically pure ester 2a with very high isolated yield. It was demonstrated that the high enantioselectivity of the enzyme is compatible with the rhodium catalyzed racemization of various para-substituted 3aryl-4-pentenoic acid derivatives. The protocol reported here should be a useful complement to known methods. Moreover, the easy recovery of both catalyst makes this process suitable for up scaling. EXPERIMENTAL SECTION General Considerations. All the chemicals were obtained from commercial sources and the solvents were of analytical grade. Immobilized lipase B from Candida antarctica (Novozym 435) was purchased from Novo Nordisk. Column chromatography was performed on Merck silica gel 60/230-400 mesh. Enzymatic reactions were performed in a vortex (Heidolph Promax 1020) equipped with incubator (Heidolph Inkubator 1000). To prove the ability of the established protocol each reaction was repeated at least three times. General procedure for enzymatic kinetic resolution. To the solution of acid 1a-g (0.2 mmol) in toluene (5 mL), triethyl orthobenzoate or other alkoxy group donor (3 equiv.) and enzyme (10 mg) were added in 10 mL screwed vial. The reaction mixture was stirred for 72 hours at 60 °C. After cooling, crude product was purified by column chromatography (ethyl acetate/hexanes). The 1H NMR data were in accordance with those recorded for racemates. General procedure for dynamic enzymatic kinetic resolution. To the solution of acid 1a-g (0.2 mmol) in toluene (5 mL), triethyl orthobenzoate (3 equiv.), enzyme (10 mg) and metal catalyst (10 mol%) were added in 10 mL vial. The reaction mixture was stirring for 120 hours at 60 °C. After cooling, crude product was purified by column chromatography (ethyl acetate/hexanes).

ASSOCIATED CONTENT Supporting Information. Supporting Information Available: Experimental details, 1H, 13C NMR spectra, and HPLC traces. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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Corresponding Author * [email protected]

ACKNOWLEDGMENT This work was supported by the Polish National Science Centre project No. 2013/11/B/ST5/02199. We gratefully acknowledge Przemysław Sendys for his assistance in mass spectroscopy measurements. This work was dedicated to prof.Janusz Jurczak on the occasion in his 75th birthday

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