Enantioselective Reduction of Prochiral Ketones Employing Sprouted Pisum sativa as Biocatalyst

Abstract

Sprouted green peas have been used for the first time as biocatalysts for enantioselective reduction of prochiral ketones. The reactions are highly enantioselective to furnish chiral alcohols in good yields. The sprouted peas as biocatalysts are a cheap and easy way for generating some interesting chiral alcohols. This process is efficient and convenient to produce chiral secondary alcohols in water.

Biocatalysts are very useful for the preparation of chiral drugs, fragrances, and pheromones. In recent years, the application of enzymes as biocatalysts in organic synthesis is quite well known. [¹] Asymmetric reduction of prochiral ketones is an essential transformation in organic synthesis owing to the presence of secondary alcohols in biologically active natural products. In recent years, secondary chiral alcohols have received great commercial importance because of their numerous applications. [²] They are being used as drug intermediates in pharmaceuticals and as toxophore in agrochemicals. These chiral synthons can be produced by economically viable and eco-friendly manner by using biocatalysts such as lipase, horse liver dehydrogenase, yeast (Geotricum candidum), etc. [³] Recently, plants have been considered as suitable biochemical systems for the biotransformation of organic xenobiotics such as monoterpenes, aliphatic ketones, and aromatic ketones. [4] [5] Both immobilized plant cell cultures [6] and whole plant tissues have been utilized for the stereoselective reduction of ketones. [7] [8] Recently, vegetables such as Daucas carota root and green grams have been used as biocatalysts for the enantioselective reduction of prochiral ketones to furnish chiral secondary alcohols. [9] [¹0] However, in some cases, no reduction was observed with β-keto esters and also require large excess of biomass to achieve acceptable yields.

In this report, we describe an enzymatic approach for the preparation of chiral secondary alcohols in highly enantioselective manner by means of reduction of prochiral ketones using sprouted Pisum sativa in water.

Initially, we attempted the reduction of acetophenone (1) with sprouted green peas in water. The reduction occurred at room temperature to furnish (S)-1-phenethyl alcohol (2a) in 72% yield with 98% ee (Scheme  [¹] ). The bioreduction of the keto group was performed using dried green peas obtained from super market. Similarly, various substituted acetophenones were converted into their chiral secondary alcohols with high degree of enantioselectivity (Table  [¹] , entries b-g). The corresponding S-alcohols were obtained in all cases, with enantiomeric excesses ranging from 91-98%. The effects of ring substituents on the reduction of substituted acetophenones by P. sativa are quite general. With aryl methyl ketones, it was observed that electron-donating substituents slowed the reaction, highlighting the sensitivity of bioreduction kinetics to electronic effects, which is in accordance with results reported in the literature. [9] When compared to aryl methyl ketones, cyclic and acyclic ketones gave the products relatively in low yields and with moderate enantioselectivity (Table  [¹] , entries h-k).

Next, we examined the reduction of 2-aryltetrahydro-2H-pyran-4-one. The reduction of (±)-2-(4-chlorophenyl)tetrahydro-2H-pyran-4-one with green peas in water gave (2S,4S)-2-(4-chlorophenyl)tetrahydropyranol and (2R,4S)-2-(4-chlorophenyl)tetrahydropyranol in a 1:1 ratio (Scheme  [²] ).

The stereoisomers could be easily separated by column chromatography. The enantiomeric excesses of both the cis- and trans-tetrahydropyranols were determined by HPLC using chiral columns and were higher than 87%. The structures of the products were established by ^¹H NMR, IR, and mass spectrometry, and also by comparison with authentic samples. [¹¹] The absolute stereochemistry of (2S,4S)- and (2R,4S)-2-aryltetrahydropyranols (Scheme  [²] ) was established by comparison with authentic samples. [¹¹] The use of green peas in water offers several advantages compared to Baker’s yeast, such as low cost, ease of availability of the biocatalyst, simple workup and product recovery. The reductions in water gave the best results. No specific match/mismatch effect from the adjacent stereocenter was observed, as can be seen from the reduction of (±)-2-aryltetrahydropyran-4-one (Table  [¹] , entries l and m). At 100% conversion, equal amounts of enantiopure tetrahydropyranols 3l and 4l were isolated in 70% yield. Reduction occurs from the re-face only of the prochiral ketone, independently of the configuration of the 2-aryl-substituted stereocenter. The stereochemical course of many bioreductions of ketones may be predicted from a simple model, which is generally referred to as Prelog’s rule. [¹²] This empirical model, originally designed for the reduction of ketones by the fungus Curvularia falcata­ implies that the outcome is mainly dependent on the steric requirements of the substrate (Figure  [¹] ).

Table 1 Reduction of Prochiral Ketones to Chiral Alcohols Using Germinated Green Peas in Water (continued)

Entry	Prochiral ketone	Producta	Time (h)	[α]D ²5 (CHCl3)b	Yield (%)c	ee (%)d	Configuration
a			24	-29.0	72	98	S
b			72	-10.2	60	93	S
c			32	-17.2	55	94	S
d			72	-23.2	63	91	S
e			48	-27.7	71	95	S
f			48	-39.5	65	91	S
g			48	-12.6	68	92	S
h			72	+14.1	45	89	1R,2S
i			58	+21.4	42	88	S
j			72	+15.7	40	87	S
k			72	-12.8	38	80	R
l			24	-21.4	35	91	2S,4S
				+15.7	35	93	2R,4S
m			24	-21.2	33	87	2S,4S
				+18.6	32	92	2R,4S
a All products were characterized by IR and NMR spectroscopy and mass spectrometry. b c = 1.0. c Yield refers to pure products after chromatography. d Enantiomeric excesses of alcohols were determined using chiral HPLC.

The observed enantioselectivity is in accordance with Prelog’s rule. The presence of several competing oxidoreductases with opposite stereoselectivities in the plant tissues may be the origin of the stereochemical outcome with these substrates. Hence, this biocatalytic approach is found to be suitable for the preparation of a wide range of chiral secondary alcohols. The results are summarized in Table  [¹] . Finally; the recovered peas were used three times with gradual decrease in conversion. For example, treatment of 1 mmol of acetophenone with 25 grams of sprouted seeds of green peas for 24 hours gave 72, 65, 57% yields over three cycles. After three cycles, the recovered biomass was used as manure.

In summary, the enantioselective reduction of a series of prochiral ketones was achieved using sprouted green peas in aqueous buffer pH 7.0, which gave the corresponding S-alcohols with ee varying from 80-98%. The low cost and the easy availability of the biocatalyst besides the experimental simplicity suggest the possible use of the present method for large scale preparations of important optically pure secondary alcohols.

Melting points were recorded on a Buchi R-535 apparatus and are uncorrected. IR spectra were recorded on a Perkin-Elmer FT-IR 240-c spectrophotometer using KBr optics. NMR spectra were recorded on a Gemini-200 spectrometer (200 MHz for ^¹H NMR, 50 MHz for ^¹³C NMR) in CDCl₃ using TMS as an internal standard. Mass spectra were recorded on a Finnigan MAT 1020 mass spectrometer operating at 70 eV. Column chromatography was performed using F Merck 100-200 mesh silica gel. The enantiomeric excess of the product was determined using a Shimadzu HPLC system equipped with a chiral HPLC column (Chiralcel OD) and a UV detector (254 nm). A solvent system of n-hexane and isopropanol (8:2) at a flow rate of 1.0 mL/min was used. Optical rotations were measured on a Perkin-Elmer model 341 digital polarimeter. The absolute configuration of the products was determined by optical rotation/chiral HPLC and was compared with literature values. [8a]

Preparation of Sprouted Green Peas

Dried green peas (10 g) obtained from a local market were soaked in H₂O (75 mL) for 7-8 h. The resulting soaked wet seeds were tightly wrapped with porous cloth for 24 h at r.t. to obtain sprouts. The gradated seeds (25 g of wet sprouted seeds) were used for the bioreduction.

Bioreduction of Prochiral Ketones with Sprouted Peas; General Procedure

The reduction was initiated by adding sprouted seeds of green peas (25 g) to the prochiral ketone (1 mmol) dissolved in H₂O (50 mL). The reaction mixture was incubated in an orbital shaker at 25 ˚C. On optimal conversion, the reaction was stopped and the mixture was filtered off and the seeds were washed with H₂O (3 ×). The filtrates were extracted with EtOAc and the combined organic layers were dried (Na₂SO₄), filtered, and evaporated. The crude products were purified by flash chromatography to furnish the corresponding chiral secondary alcohols. The reduction was also carried out on a 10 gram scale under similar conditions.

(S)-1-Phenylethanol (2a)

[α]_D ^²5 -29.0 (c 1.0, MeOH).

IR (neat): 3359, 3030, 2974, 2927, 1493, 1451, 1369, 1203, 1010, 760, 699, 539 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 7.39-7.15 (m, 5 H), 4.80 (q, J = 6.6 Hz, 1 H), 2.77 (br s, 1 H, OH), 1.41 (d, J = 6.6 Hz, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 24.9, 69.7, 125.1, 126.9, 128.09, 145.7.

MS: m/z = 123 (M⁺ + H).

( S )-4-(1-Hydroxyethyl)phenol (2b)

[α]_D ^²5 -10.2 (c 1.0, EtOH).

IR (neat): 3354, 2972, 2926, 1457, 1370, 1284, 1126, 1076, 896, 760, 729, 458 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 7.87 (d, J = 8.2 Hz, 2 H), 6.89 (d, J = 8.2 Hz, 2 H), 4.83 (q, J = 6.6 Hz, 1 H), 1.95 (br s, 1 H, OH), 1.48 (d, J = 6.6 Hz, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 24.6, 70.3, 115.4, 126.9, 131.1, 155.4.

MS: m/z = 138 (M⁺).

( S )-1-(4-Chlorophenyl)ethanol (2c)

[α]_D ^²5 -17.2 (c 1.0, CHCl₃).

IR (neat): 3349, 2975, 2926, 1574, 1431, 1201, 1076, 786, 697 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 7.86 (d, J = 8.0 Hz, 2 H), 7.21 (d, J = 8.0 Hz, 2 H), 4.87 (q, J = 6.6 Hz, 1 H), 1.46 (d, J = 6.6 Hz, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 25.0, 69.5, 123.4, 125.5, 127.3, 129.6, 134.1, 147.7.

MS: m/z = 156 (M⁺).

( S )-1-(4-Methoxyphenyl)ethanol (2d)

[α]_D ^²5 -23.2 (c 1.0, CHCl₃).

IR (neat): 3411, 2925, 2854, 1512, 1245, 1176, 1034, 832, 760 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 7.25 (d, J = 8.6 Hz, 2 H), 6.80 (d, J = 8.6 Hz, 2 H), 4.80 (q, J = 6.4 Hz, 1 H), 3.77 (s, 3 H), 1.95 (br s, 1 H, OH), 1.45 (d, J = 6.4 Hz, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 137, 126, 113, 69, 55, 24.

MS: m/z = 151 (M - H⁺).

( S )-1-(4-Nitrophenyl)ethanol (2e)

[α]_D ^²5 -27.7 (c 1.0, CHCl₃).

IR (neat): 3450, 2925, 2858, 1460, 1248, 1044, 758, 674 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 8.10 (d, J = 8.3 Hz, 2 H), 7.48 (d, J = 8.3 Hz, 2 H), 4.95 (q, J = 6.7 Hz, 1 H), 2.92 (br s, 1 H, OH), 1.47 (d, J = 6.7 Hz, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 25.3, 69.3, 123.6, 126.0, 147.0, 153.1.

MS: m/z = 190 (M⁺ + Na).

( S )-1-( p -Tolyl)ethanol (2f)

[α]_D ^²5 -39.5 (c 1.0, CHCl₃).

IR (neat): 3352, 2974, 2925, 1574, 1428, 1200, 1076, 785, 696 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 7.48 (d, J = 8.8 Hz, 2 H), 7.12 (d, J = 8.3 Hz, 2 H), 5.08 (q, J = 5.8 Hz, 1 H), 2.33 (s, 3 H), 2.05 (br s, 1 H, OH), 1.44 (d, J = 5.8 Hz, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 25.0, 29.6, 69.5, 123.4, 125.5, 127.3, 129.6, 134.1, 147.7.

MS: m/z = 136 (M⁺).

( S )-1-(4-Bromophenyl)ethanol (2g)

[α]_D ^²5 -12.6 (c 1.0, CHCl₃).

IR (neat): 3357, 2973, 1489, 1081, 1009, 824, 534 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 7.45 (d, J = 8.3 Hz, 2 H), 7.20 (d, J = 8.3 Hz, 2 H), 4.80 (q, J = 6.0 Hz, 1 H), 2.50 (br s, 1 H, OH), 1.50 (d, J = 6.0 Hz, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 145, 131, 127, 121, 70, 25.

MS: m/z = 223 (M - H + Na⁺).

Methyl (1 R ,2 S )-2-Hydroxycyclopentanecarboxylate (2h)

[α]_D ^²5 +14.1 (c 0.5, CHCl₃).

IR (neat): 3435, 2924, 2854, 1629, 1460, 1265, 758 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 4.09 (t, J = 6.0 Hz, 1 H), 3.86 (s, 3 H), 2.36-2.19 (m, 1 H), 2.16-2.02 (m, 2 H), 1.96-1.64 (m, 5 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 21.9, 27.1, 34.1, 51.8, 52.5, 96.1, 175.3.

MS: m/z = 167 (M⁺ + Na).

( S )-2,3-Dihydro-1 H -inden-1-ol (2i)

[α]_D ^²5 +15.7 (c 1.0, CHCl₃).

IR (neat): 3271, 2927, 1480, 1341, 1197, 1035, 738 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 7.36-7.26 (m, 1 H), 7.23-7.12 (m, 3 H), 5.10 (dd, J = 11.7, 5.8 Hz, 1 H), 3.17 (br s, 1 H, OH), 2.98-2.87 (dd, J = 3.6, 2.2 Hz, 1 H), 2.79-2.62 (dd, J = 7.3, 7.3 Hz, 1 H), 2.43-2.25 (m, 1 H), 1.91-1.74 (m, 1 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 29.6, 35.5, 75.9, 124.1, 124.6, 126.4, 127.9, 143.0, 144.8.

MS: m/z = 157 (M⁺ + Na).

( S )-1,2,3,4-Tetrahydronaphthalen-1-ol (2j)

[α]_D ^²5 +21.4 (c 1.0, CHCl₃).

IR (neat): 3435, 2924, 2853, 1458, 1260, 1090, 1027, 801, 738 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 7.35 (t, J = 8.5 Hz, 1 H), 7.15-7.05 (m, 2 H), 7.05 (t, J = 8.5 Hz, 1 H), 4.68 (dd, J = 4.5, 8.3 Hz, 1 H), 2.90-2.60 (m, 2 H), 2.15-1.60 (m, 4 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 158, 156, 149, 147, 146, 87, 80, 52, 49, 38.

MS: m/z = 149 (M + H⁺).

( R )-Nonan-2-ol (2k)

[α]_D ^²5 = 12.8 (c 1.0, CHCl₃).

IR (neat): 3451, 2927, 2857, 1163, 770 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 3.76 (m, 1 H), 1.47-1.21 (m, 15 H), 0.88 (d, J = 7.3 Hz, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 13.8, 22.5, 23.1, 29.1, 29.5, 31.7, 39.1, 60.2, 67.7.

MS: m/z = 167 (M⁺ + Na).

(2 S ,4 S )-2-(4-Chlorophenyl)tetrahydro-2 H -pyran-4-ol (3l)

[α]_D ^²5 -21.4, (c 1.0, CHCl₃).

IR (neat): 3380, 2952, 2921, 2858, 1515, 1448, 1371, 1284, 1125, 1068, 1044, 985, 813 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 7.28 (d, J = 8.2 Hz, 2 H), 7.23 (d, J = 8.2 Hz, 2 H), 4.24 (dd, J = 2.2, 11.7 Hz, 1 H), 4.14 (dd, J = 4.4, 11.7 Hz, 1 H), 3.87-3.78 (m, 1 H), 3.50 (td, J = 4.4, 12.4, 24.2 Hz, 1 H), 2.12 (m, 1 H), 2.05-1.89 (m, 2 H), 1.68-1.45 (m, 2 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 32.8, 40.6, 62.8, 64.2, 73.8, 125.8, 127.3, 128.3, 142.6.

MS: m/z = 213 (M⁺ + H).

(2 R ,4 S )-2-(4-Chlorophenyl)tetrahydro-2 H -pyran-4-ol (4l)

[α]_D ^²5 +15.7 (c 1.0, CHCl₃).

IR (neat): 3382, 2940, 2842, 1489, 1448, 1409, 1364, 1301, 1249, 1163, 1142, 1085, 1050, 987, 691, 885, 823, 717, 689, 594 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 7.29 (d, J = 8.2 Hz, 2 H), 7.23 (d, J = 8.2 Hz, 2 H), 4.72 (dd, J = 3.9, 10.4 Hz, 1 H), 4.34-4.22 (m, 1 H), 4.02 (td, J = 2.6, 11.7, 23.4 Hz, 1 H), 3.89 (td, J = 2.6, 11.7, 23.4 Hz, 1 H), 2.11-1.48 (m, 5 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 32.8, 40.6, 62.8, 64.2, 73.8, 125.8, 127.3, 128.3, 142.6.

MS: m/z = 213 (M⁺ + H).

(2 S ,4 S )-2-(Naphthalen-2-yl)tetrahydro-2 H -pyran-4-ol (3m)

[α]_D ^²5 -21.2 (c 1.0, CHCl₃).

IR (neat): 3433, 3028, 2908, 2842, 1457, 1342, 1208, 1081, 1012, 812, 748, 698, 477 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 8.04 (d, J = 9.2 Hz, 1 H), 7.80 (dd, J = 1.8, 7.1 Hz, 1 H), 7.71 (d, J = 8.3 Hz, 1 H), 7.60 (d, J = 6.4 Hz, 1 H), 7.50-7.37 (m, 3 H), 5.54 (dd, J = 1.8, 11.5 Hz, 1 H), 4.37 (m, 1 H), 4.20 (td, J = 2.2, 11.7, 24.3 Hz, 1 H), 4.02 (dd, J = 5.6, 12.0 Hz, 1 H), 2.23-1.98 (m, 2 H), 1.91 (td, J = 2.6, 11.5 Hz, 1 H), 1.75-1.63 (m, 2 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 35.6, 42.2, 66.5, 68.5, 75.3, 123.0, 123.1, 125.42, 125.46, 125.9, 128.1, 128.8, 130.3, 133.7, 137.2.

MS: m/z = 252 (M⁺ + H + Na).

(2 R ,4 S )-2-(Naphthalen-2-yl)tetrahydro-2 H -pyran-4-ol (4m)

[α]_D ^²5 +18.6 (c 1.0, CHCl₃).

IR (neat): 3428, 3056, 2923, 2850, 1465, 1361, 1245, 1081, 819, 748, 477 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 7.96 (d, J = 3.4 Hz, 1 H), 7.78 (dd, J = 15.6, 7.8 Hz, 2 H), 7.60 (d, J = 6.5 Hz, 1 H), 7.54-7.37 (m, 3 H), 4.98 (d, J = 11.7 Hz, 1 H), 4.25 (dd, J = 11.7, 3.9 Hz, 1 H), 4.11-3.91 (m, 1 H), 3.69 (td, J = 13.0, 2.6 Hz, 1 H), 2.36 (dt, J = 13.0, 2.6 Hz, 1 H), 2.12-1.92 (m, 2 H), 1.76-1.53 (m, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 35.1, 42.9, 66.1, 67.8, 78.1, 122.3, 123.9, 124.2, 125.5, 125.7, 126.7, 127.3, 127.7, 129.2, 134.3.

MS: m/z = 252 (M⁺ + H + Na).

Acknowledgment

Ch.S. thanks Director of IICT for financial assistance.

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  Search in Google Scholar
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  Search in Google Scholar
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