Enantioselective reduction of heterocyclic ketones with low level of asymmetry using carrots

Article abstract of DOI:10.1080/10242422.2021.1879795

A whole spectrum of biocatalysts for asymmetric reduction of prochiral ketones is well known including the Daucus carota root. However, this type of reaction is still challenging when pro-chiral ketones present low level of asymmetry, like heterocyclic ketones. In this work, 4,5-dihydro-3(2H)-thiophenone (1), 2-methyltetrahydrofuran-3-one (2), N-Boc-3-pyrrolidinone (3), 1-Z-3-pyrrolidinone (4) and 1-benzyl-3-pyrrolidinone (5) were studied in order to obtain the respective enantioselective heterocyclic secondary alcohols. Except for 5, the corresponding alcohols were obtained in high values of conversion and with high selectivity. In order to circumvent the low isolated yield of the corresponding chiral alcohol from 2, we observed that the use of carrots in the absence of water is feasible. Addition of co-solvents was needed to the water-insoluble ketones 3 and 4. Comparatively, baker’s yeast was used for bio reductions of 1, 3 and 4. And in terms of conversion, selectivity and work-up, the use of carrots were a more efficient biocatalyst, as well as a viable method for obtaining 5-member heterocyclic secondary alcohols.

Full text of DOI:10.1080/10242422.2021.1879795

Biocatalysis and Biotransformation
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Enantioselective reduction of heterocyclic ketones
with low level of asymmetry using carrots
Naira Vieira Machado & Álvaro Takeo Omori
To cite this article: Naira Vieira Machado & Álvaro Takeo Omori (2021): Enantioselective
reduction of heterocyclic ketones with low level of asymmetry using carrots, Biocatalysis and
Biotransformation, DOI: 10.1080/10242422.2021.1879795
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BIOCATALYSIS AND BIOTRANSFORMATION
https://doi.org/10.1080/10242422.2021.1879795
RESEARCH ARTICLE
Enantioselective reduction of heterocyclic ketones with low level of
asymmetry using carrots
ꢀ
Naira Vieira Machado and Alvaro Takeo Omori
Centro de Ci ^e ncias Naturais e Humanas, Universidade Federal do ABC, Santo Andr ꢀe , Brazil
ABSTRACT
ARTICLE HISTORY
A whole spectrum of biocatalysts for asymmetric reduction of prochiral ketones is well known
including the Daucus carota root. However, this type of reaction is still challenging when pro-
chiral ketones present low level of asymmetry, like heterocyclic ketones. In this work, 4,5-dihy-
dro-3(2H)-thiophenone (1), 2-methyltetrahydrofuran-3-one (2), N-Boc-3-pyrrolidinone (3), 1-Z-3-
pyrrolidinone (4) and 1-benzyl-3-pyrrolidinone (5) were studied in order to obtain the respective
enantioselective heterocyclic secondary alcohols. Except for 5, the corresponding alcohols were
obtained in high values of conversion and with high selectivity. In order to circumvent the low
isolated yield of the corresponding chiral alcohol from 2, we observed that the use of carrots in
the absence of water is feasible. Addition of co-solvents was needed to the water-insoluble
ketones 3 and 4. Comparatively, baker’s yeast was used for bio reductions of 1, 3 and 4. And in
terms of conversion, selectivity and work-up, the use of carrots were a more efficient biocatalyst,
as well as a viable method for obtaining 5-member heterocyclic secondary alcohols.
Received 12 July 2020
Revised 20 December 2020
Accepted 18 January 2021
KEYWORDS
Carrot; bioreduction;
heterocyclic ketones; chiral
heterocyclic alcohols
Introduction
So far, Cossy reported the bioreduction of 6-mem-
bered amino ketones using D. carota (Lacheretz et al.
Biocatalysis is part of a green chemist’s toolkit
2
009). However, the use of the root to obtain chiral
(Erythropel et al. 2018; Sheldon et al. 2020) and has
heterocyclic alcohols containing oxygen or sulphur is
unknown. Chiral heterocyclic alcohols are building-
blocks for several bioactive compounds such as sulo-
penem (Liang et al. 2010), thiarabine (Bianchi et al.
2017) and ibrutinib (Chen et al. 2019). The asymmetric
synthesis of such compounds by non-enzymatic meth-
ods are believed to present low selectivity due to the
stereoeletronic differences between the a and a’-car-
bonyl groups (Sun et al. 2016).
great potential to also be part of a circular chemist’s
kit (Keijer et al. 2019). Currently, advanced protein
engineering protocols offer enzymes for almost any
synthetic organic reaction (Fasan et al. 2019).
Nevertheless, many organic chemists are still discour-
aged to use and handle enzymes for organic synthesis.
Even so, there are useful and expeditious biocatalytic
techniques that are still attractive. For instance, the
use of carrot bits in enantioselective reduction of pro-
chiral ketones is extremely simple and replicable
Among the non-enzymatic methods available for
enantioselective ketone reduction, the use of boranes
in the presence of a chiral oxazoboralidine catalyst
(Blanchard and de Weghe, 2006). Carrots (Daucus car-
ota) are extensively explored because they are easily
available and the aseptic conditions are not necessary.
Several applications of this renewable biocatalyst are
available. From total synthesis (Omori et al. 2015) to
one-pot chemoenzymatic methodology (Oliveira et al.
(CBS reaction) and Noyori (Ru-based) catalysts are the
most known (Cho 2009; Wang et al. 2008). However,
the CBS reaction requires severe “dry” conditions and
the use of highly reactive BH . Also, CBS and Ru cata-
2013) or even for the synthesis of high value biomass
3
lysts are expensive.
derivatives (Omori et al. 2020). Although several sub-
strates are susceptible to reduction, this reaction is still
a major challenge for pro-chiral ketones that present
low level of asymmetry, such as heterocyclic ketones.
In this contribution, we report a bioreduction of 5-
membered heterocyclic ketones using carrots, a green
and inexpensive biocatalyst. In order to compare the
ꢀ
Centro de Ciencias Naturais e Humanas, Universidade Federal do ABC, Santo Andre,
^ ꢀ
CONTACT Alvaro Takeo Omori
alvaro.omori@ufabc.edu.br
Sao Paulo 09210-580, Brazil
Supplemental data for this article can be accessed here.
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

2
N. V. MACHADO AND ÁT. OMORI
selectivity and the activity, the use of baker’s yeast
was also tested.
borohydride in methanol (Da Costa and Omori 2017).
Compounds 1a–5a were identified by GC–MS and
used for TLC and GC standards without further
purification.
Materials and methods
General
(RS)-Tetrahydrothiophene-3-ol (1a) CAS [3334-05-2]
(
(
(
(
RS)-2-Methyltetrahydro-3-furanol (2a) CAS [29848-
All reagents (1–5) listed below were purchased from
Sigma–Aldrich and used directly without further
purification.
4
4-0]
RS)-N-(tert-Butoxycarbonyl)-3-hydroxypyrrolidine (3a)
CAS [103057-44-9]
RS)- 3-Hydroxy-1-N-Cbz-pyrrolidine (4a) CAS [95656-
Dihydro-3(2H)-thiophenone (1): CAS [1003-04-9]
8
8-5]
2-Methyltetrahydro-3-furanone (2): CAS [3188-00-9]
RS)-N-Benzyl-3-pyrrolidinol (5a) CAS [775-15-5]
N-Boc-3-pyrrolidinone (3): CAS [101385-93-7]
1
1
-Z-3-Pyrrolidinone (4): CAS [130312-02-6]
-Benzyl-3-pyrrolidinone (5): CAS [775-16-6]
Chiral GC analyses
General conditions: chiral GC Column: Hydrodex
Solvents (reagent grade) were purchased from
b-TBDAc 25 m ꢀ 0.25mm. Carrier gas (H ). Flow:1 mL/
2
Synth (Brazil) and used without further purification for
extraction and chromatographic techniques. Analytical
thin-layer chromatography (TLC) was performed with
aluminium-backed silica plates coated with a 0.25 mm
thickness of silica gel 60 F254 (Merck), exposure to
vanillin or potassium permanganate solution and heat-
ing. Column chromatography separations were fol-
lowed using 35–70mm (240–400 mesh) silica gel
purchased from Sigma-Aldrich. Nuclear magnetic res-
onance spectra were recorded on a Varian 500 spec-
min. Oven in isothermal mode (Table 1).
General procedure for biocatalysed reactions
The plants were washed with water and then cut into
small, thin slices (5 mm). The plants (10 g), water
(60 mL), and the appropriate ketones 1–5 (0.5 mmol)
were added to an Erlenmeyer flask (250 mL). The bio-
transformation was carried out in an orbital shaker
(160 rpm) at room temperature for 3 days. The pro-
1
13
trometer at 500 MHz for H- and 125 MHz for C-NMR.
The H-NMR and
gress of the reaction was monitored by GC analysis
every 24 h. Sampling: 2 mL of sample was extracted
with 1 mL of ethyl acetate in a 15-mL Falcon tube. The
organic layer was then directly analysed by TLC (gen-
erally hexane:ethyl acetate, visualized by UV and
stained with sulphuric vanilin), chiral GC, and GC–MS.
At the end of the reaction, the reaction mixture was
decanted and the aqueous phase was extracted with
ethyl acetate (5 ꢀ 30 mL). The organic phases were
combined and dried with sodium sulphate, filtered
and concentrated under reduced pressure. The crude
product was then purified by flash column
1
13
C-NMR chemical shifts (d) are
reported in ppm relative to CDCl . The coupling con-
3
stants (J) are given in hertz (Hz). Optical rotations
were determined on a JASCO DIP-378 polarimeter
Sodium D line at 589 nm). GC–MS data were acquired
(
on a Varian 4000 ion trap operating at 70 eV. Chiral
GC-FID analyses were recorded on a 450-GC with a
Hydrodex b-TBDAc (Macherey-Nagel, 25 m ꢀ 0.25 mm)
column using H as the carrier gas. The enantiomeric
2
excess of heterocyclic alcohols were calculated accord-
ing to equation: [(E1–E2)/(E1 þ E2)] ꢀ 100, where E1
and E2 are the ratio of enantiomers of each com-
pound as determined by chiral GC analyses.
Table 1. GC parameters and retention times of heterocyclic
alcohols 1a–5a and ketones 1–5.
ꢁ
Temperatures ( C)
Retention time (min.)
Biocatalysts
ꢁ
Injector: 100 C
Detector: 180 C
(1): 10.82
(S)-1a:35.25
(R)-1a:31.05
ꢁ
Healthy orange carrots (D. carota ssp. sativus var. sati-
vus) were obtained from a local market (Brazil).
ꢁ
Oven: 120 C
ꢁ
Injetor: 100 C
(2): 3.15/3.21
trans-(2a): 9.18/9.88
cis-(2a): 11.58/12.05
(3): 8.16
(S)-3a: 13.80
(R)-3a: 13.20
(4):31.79min
(S)-4a:52.07min
(R)-4a:50.00min
ꢁ
Detector: 180 C
ꢁ
Oven: 110 C
ꢁ
Injetor: 180 C
Synthesis of racemic alcohols
ꢁ
Detector: 180 C
ꢁ
Oven: 150 C
Synthesis of racemic alcohols for GC and TLC stand-
ards: The alcohols (±)-1a–5a were prepared by reduc-
tion of the corresponding ketones 1–5 with sodium
ꢁ
Injetor: 220 C
ꢁ
Detector: 240 C
ꢁ
Oven: 180 C

BIOCATALYSIS AND BIOTRANSFORMATION
3
chromatography with silica gel, using a mixture of
hexane and ethyl acetate as the eluent. Isolated prod-
ucts were characterised by NMR. The absolute config-
(RS)-N-Benzyl-3-pyrrolidinol (5a) CAS [775-15-5].
1
RMN H (500 MHz, CDCl ): d(ppm): 7.08-7.36 (m, 5H);
3
4.09-4.17 (m, 1H); 3.91 (s, 2H); 2.85 (ddd, J ¼ 12.2 Hz,
J ¼ 4.8, 1H); 2.65 (dd, J ¼ 10.4 Hz, J ¼ 2.6 Hz, 1H);
uration were inferred by comparison of the [a] values
D
with the literature (Brown et al. 1985, 1986; Lacheretz
et al. 2009; Dai et al. 2015).
2
2
.54 (dd, J ¼ 10.9 Hz, J ¼ 2.4, 1H); 2.36–2.48 (m, 2H);
13
.18–2.25 (m, 1H); 1.80–1.86 (1H, m). RMN
C
(
5
125 MHz, CDCl ): d(ppm): 130.2; 127.9; 70.3; 59.7;
2.0; 44.2; 34.6. The NMR spectral data were accord-
3
(
3S)-Tetrahydrothiophene-3-ol ((S)-1a) CAS [79107-
1
7
5-8]. RMN H (500 MHz, CDCl ): d(ppm): 4.62–4.65
3
ance with those reported in the literature (Nemia
et al. 1983). MS (70 eV, EI): m/z (%) 177 (15), 176 (12),
1
(
m, 1H); 3.02 (dd, J ¼ 3.91 Hz, J ¼ 15.4 Hz, 1H); 2.97
(dd, J ¼ 6.60 Hz, J ¼ 16.6 Hz, 1H); 2.89–2.90 (m, 1H);
57 (8), 132 (32), 91 (100), 65 (20).
2
1
7
.83–2.86 (m, 1H); 2.16–2.20 (m, 1H); 1.87–1.89 (m,
13
H); 1.77 (s, 1H). RMN C (125 MHz, CDCl ): d(ppm):
3
4.6; 39.8; 38.0; 28.0. The NMR spectral data were
Biocatalyzed reactions with cosolvent
accordance with those reported in the literature
25
(
Duguet et al. 2008). ½aꢂ -4.43 (c 0.7, CHCl ), litera-
D
3
Applying the same procedure described above, except
that the substrate was previously dissolved in a micro-
tube containing 500 ll of solvent (isopropanol
or ethanol).
25
D
ture ½aꢂ -14,5 (c 1.0, CHCl ) (Brown et al. 1985). MS
3
(
70 eV, EI): m/z (%)104 (100), 85 (60), 71 (32), 59 (55),
7 (24).
2S,3S)-2-Methyltetrahydrofuran-3-ol
5
(
((2S,3S)-2a)
CAS [1643965-13-2]. RMN H (500 MHz, D O): d(ppm):
1
2
4
6
1
.05 (quintet, J ¼ 3.5 Hz, 1H); 3.98 (td, J ¼ 3.9 Hz, J ¼
.5 Hz, 1H); 3.88–3.92 (m, 2H); 2.20–2.26 (m, 1H);
.84–1.90 (m, 1H); 1.16–1.18 (d, J ¼ 6.5 Hz, 3H). RMN
Solventless biocatalysed reactions
1
mmol (100 mg) of ketone 2 was added over 1 cm
1
3
C (125 MHz, D O): d(ppm): 81.8; 76.2; 66.1; 33.0;
2
thick carrot slices (12 g) in a covered petri dish and
left for 3 days at room temperature. At the end of the
reaction, the carrot slices were washed with ethyl
ether (5 ꢀ 10 mL). The organic phase was dried over
anhydrous sodium sulphate and the ethyl acetate
evaporated under reduced pressure. The crude prod-
uct was analysed by TLC and chiral GC. The crude
product was then purified by flash column chromatog-
17.8. The NMR spectral data were in accordance with
those reported in the literature (Duguet et al. 2008).
2
5
25
½
aꢂ þ7.33 (c 0.2, MeOH), literature ½aꢂD þ11.7 (c
D
2
.5, MeOH) (Dai et al. 2015).
(
3S)-N-(tert-Butoxycarbonyl)-3-hydroxypyrrolidine
1
((S)-3a) CAS [101469-92-5]. RMN H (500 MHz, CDCl ):
3
d(ppm): 4.40–4.43 (m, 1H); 3.46–3.41 (m, 3H);
.31–3.37 (m, 1H); 2.57 (s, 1H) 1.90–1.99 (m, 2H); 1.44
3
13
raphy with silica gel, using a mixture of (Acetone:Et O,
2
(s, 9H). RMN C (125 MHz, CDCl ): d(ppm): 154.8;
3
2
3
:1) as the eluent. The isolated yield of 2a was
0% (30.6 mg).
79.4; 70.9; 54.3; 43.9; 33.9; 28.5. The NMR spectral
data were in accordance with those reported in the
25
literature (Duguet et al. 2008). ½aꢂ þ10.36 (c 0.6,
D
25
CHCl ), literature ½aꢂ þ23.2 (c 0.9, CHCl ) (Lacheretz
3
D
3
et al. 2009). MS (70 eV, EI): m/z (%) 186 (1), 154 (1),
32 (86), 114 (87), 86 (17), 57 (100).
S)-1-Carbobenzoxy-3-pyrrolidinol CAS
Bioreduction of ketones 1, 3 and 4 with
baker’s yeast
1
(
((S)-4a)
100858-32-0]. RMN H (500 MHz, CDCl ): d(ppm):
1
In a 50-mL Erlenmeyer flask, 55 mg of yeast and 67 mg
of sucrose were solubilised in 25 mL of milli-Q water.
After 1 hour stirring in an orbital shaker, 0.3 mmol of
the ketone was added. The progress of the reaction
was monitored by GC analysis every 24 h for 3 days.
Sampling: 2 mL of sample was extracted with 1 mL of
ethyl acetate in a 15-mL Falcon tube. The organic
layer was then directly analysed by TLC (generally hex-
ane:ethyl acetate, visualized by UV and stained with
sulphuric vanilin), chiral GC, and GC–MS.
[
3
7.33–7.38 (m, 5H); 5.13 (s, 2H); 4.44 (s, 1H); 3.40–3.58
13
(
(
m, 4H); 2.50 (s, 1H); 1.93–2.05 (m, 2H). RMN
125 MHz, CDCl ): d(ppm): 154.8; 136.3; 128.0; 127.4;
C
3
127.4; 70.5; 66.3; 54.2; 43.6; 33.5. The NMR spectral
data were accordance with those reported in the lit-
25
erature (Lihammar et al. 2011). ½aꢂ þ14.7 (c 1.0,
D
25
CHCl ), literature ½aꢂ þ20.5 (c 3.7, CHCl ) (Brown
3
D
3
et al. 1986). MS (70 eV, EI): m/z (%) 221 (8), 203 (1),
76 (3), 130 (10), 104 (5), 91 (100), 65 (9).
1

4
N. V. MACHADO AND ÁT. OMORI
Table 2. Reduction of heterocyclic ketones 1–5 with carrots in water.
a
a
b
Substrate
time (days)
conversion (%)
enantiomeric excess (%)
Isolated Yield
Absolute configuration
c
1
2
3
90
98
>99
80
80
80
45%
S
1
d
e
1
2
3
20
85
>99
>99 (57% dr)
>99 (81% dr)
>99 (81% dr)
30%
2S,3S
2
f
1
2
3
90
99
>99
>99
>99
>99
63%
S
3
1
2
3
15
45
80
>99
>99
>99
41%
S
4
1
2
3
0
0
0
–
–
–
–
–
5
a
2
Reaction conditions: carrots 10 g, substrate 0.5 mmol, 60 mL H O, room temperature, 180 rpm. From chromatograms obtained from chiral GC analysis.
c d
b
Assigned after comparison the optical rotation sign with the literature. Using dichloromethane instead of ethyl acetate. dr (diastereomeric ratio).
e
f
Reaction without solvent. Using isopropanol as cosolvent.
Results and discussion
reactivity of compound 5 may be associated with the
solubility of the compounds in water. A brief analysis of
the calculated values of log P (obtained from scifinder.-
cas.org) for the amino ketones (compound 3:
Based on the low stereoelectronic difference between
the a and a’ carbonyl groups, the Reetz group used a
directed evolution approach in alcohol dehydrogen-
ases from Thermoethanolicus brockii to obtain more
enantioselective mutant catalysts (Sun et al. 2016). Our
aim is to verify if the high selectivity presented by the
carrot is maintained with the reduction of ketones 1–5
presented above.
0.385 ±0.5875, compound 4: 0.987 ±0.586 and com-
pound 5: 1.479 ±0.419) suggests a strong relationship
between solubility and conversion rate.
The data in Table 2 also show different values of
enantioselectivities and isolated yield depending on the
substrate. Chiral alcohol (S)-1a showed a moderate ee of
Thus, after obtaining the racemic standards of
80% and 45% isolated yield. The use of dichloromethane
1
a–5a (see Materials and Methods section) 0.5 mmol
was necessary with this compound because the extrac-
tion of the reaction using ethyl acetate furnished lower
isolated yield (29%). While the (R) isomer of 1a is an
important building block for penem-based antibiotics
of ketones 1–5 were placed in contact with carrot sli-
ces (10 g) and water. The reactions were done on an
orbital shaker and monitored for 3 days. At the end of
the reaction, the organic content was extracted and
the product purified and isolated. The analyses pro-
vided the following data summarized in Table 2.
According to the Table 2, the carrot root was able to
reduce enantioselectively all the ketones except for the
ketone 5, which contains a benzyl protecting group. In
terms of conversion values, mainly after 48 and 72 hours,
high values (>99%) were obtained for ketones 1, 2 and
(Liang et al. 2010), the (S) isomer can also be a precursor
of chiral sulfolane carbamates which can act as potent
HIV-1 protease inhibitors (Ghosh and Brindisi 2015). In
comparison to the literature, our results with 1a are as
expected considering the use of a wild-type biocatalyst.
The microorganisms (Penicillium, Aspergillus and
Streptomyces) tested by Konuki and co-workers obtained
(R)-1a with 70–92% ee (Konuki et al. 2014). Liang and
co-workers tested several commercially available KREDs
3. The ketone 4, on the other hand, show a lower con-
version rate. The lower conversion to 4 and the non-

BIOCATALYSIS AND BIOTRANSFORMATION
5
1
00
100
80
60
40
20
0
in the differentiation of the faces of the ketone by the
carrot dehydrogenases (going back to the case of
8
6
4
2
0
0
0
0
0
ketone 1, the differentiation between CH and S atom
2
in b positions led to a lower enantioselectivity rate).
Presumably, there is a facial differentiation by the car-
rot even with identical a and a’ groups. In addition,
the more different (in terms of size and atomicity) the
substituents in the b and b’ positions are, higher
enantioselectivity is achieved.
The 63% of isolated yield of (S)-3a was achieved
only in the presence of isopropanol as cosolvent and
it is a bit lower than that reported by Cossy (76%)
(Lacheretz et al. 2009). Tests using only water gave
Figure 1. Bioreduction of ketones 1, 3 and 4 using carrots and
baker’s yeast (BY) after 3 days of reaction. Shown are conversions
lower isolated yields. However, the substrate/biocata-
lyst/water ratio applied in this presented protocol
(
dashed lines) and enantiomeric excesses of the S-enantiomer
black) (calculated from chromatograms obtained from chiral GC
analysis). Baker’s yeast reaction conditions: 55 mg of yeast,
7 mg of sucrose, 0.3 mmol ketone, 25 mL H O.
(
0.5 mmol/10 g carrot/60 mL H O) is more efficient in
2
(
comparison to Cossy protocol (2 mmol/140 g carrot/
00 mL H O). In the case of (S)-4a, no influence of
6
6
2
2
cosolvent was observed.
(
ketoreductases) with ketone 1 and the ee of 1a ranged
from 81% for (S) enantiomer to 63% for (R) enantiomer
Liang et al. 2010).
Attempts to observe a possible reduction of ketone
5 by addition of surfactants (Da Costa and Omori
2017) or by reacting in organic solvent (Majewska and
Kozłowska 2013) were unsuccessful. Biphasic media
using diisobutylphthalate showed a conversion to the
corresponding alcohol (Xiong et al. 2015). However,
extraction of the product failed in our hands.
In order to compare the efficiency of carrots against
other biocatalytic systems, compounds 1, 3 and 4
were reacted with baker’s yeast in water. The results
are summarized in Figure 1.
The results showed that by using baker’s yeast, the
values of conversion and enantiomeric excess after
3 days of reaction decreased in all ketones tested. In
our case, the conversion values decreased to 65% for
1a, 58% for 3a and no conversion was observed for
4a. As expected, the baker’s yeast afforded the same
enantiomer (S) as carrot did. However, the enantio-
meric excesses also decreased to 32% for (S)-1a and
90% for (S)-3a.
(
The biggest challenge was to obtain the alcohol
(S)-2a in reasonable yield. Initial attempts for the
reduction of the ketone 2 using standard conditions
carrot bits in water) showed almost complete conver-
(
sion to the corresponding alcohol 2a after 3 days of
reaction. Even though the GC analysis showed high
values of conversion and fair enantiomeric excess, the
reaction work-up by liquid–liquid extraction proved to
be ineffective, obtaining isolated yield values up to
1
0%. In order to prove the high solubility of the com-
pound in water, we decided to approach a new proto-
col, carrying out the reaction without water. For this
purpose, 10 g of sliced carrots were placed in a petri
dish and the ketone was added with a microsyringe.
The petri dish was covered to prevent evaporation
and kept for 3 days at room temperature. Carefully
extraction of the product with diethyl ether, followed
by purification furnished the (2S,3S)-2a in 30% isolated
yield. Separation of the diastereomers and enantiom-
ers of 2a by chiral GC was also tricky (Supplementary
Material); however, it is clearly evident that the carrot
showed both good diastereoselectivity and enantiose-
lectivity. To the best of our knowledge, the (2S,3S)-2a
was obtained only by Sharpless asymmetric dihydroxy-
lation (Dai et al. 2015). In this case, the alcohol was
obtained in 63% ee.
Conclusions
We observed that the carrots (D. carota) are efficient
wild-type biocatalysts to synthesise optically enriched
heterocyclic alcohols from the corresponding ketones.
The low substrate concentration is still a concern, but
we demonstrated that this simple and cheap reaction
can be carried out without water. Also, the carrots
presented tolerance with the nature of the a- and
Regarding to the bioreduction of amino ketones 3
and 4, excellent values of ee (99%) were obtained.
This high enantioselectivity might be related with the
presence of bulky protective groups that would assist
0
a -substituents. Additional studies with other ketones
are underway to show the relevance and reproducibil-
ity of this easily available catalyst.

6
N. V. MACHADO AND ÁT. OMORI
Erythropel HC, Zimmerman JB, de Winter TM, Petitjean L,
Melnikov F, Lam CH, Lounsbury AW, Mellor KE, Jankoviꢀc
NZ, Tu Q, et al. 2018. The Green ChemisTREE: 20 years
after taking root with the 12 principles. Green Chem.
Acknowledgements
The authors thank the Multiuser Central Facilities (UFABC)
for the experimental support.
2
0(9):1929–1961.
Fasan R, Kan SBJ, Zhao H. 2019. A continuing career in bio-
catalysis: Frances H. Arnold. ACS Catal. 9(11):9775–9788.
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Disclosure statement
No potential conflict of interest was reported by
the author(s).
2
895–2940.
Keijer T, Bakker V, Slootweg JC. 2019. Circular chemistry to
enable a circular economy. Nat Chem. 11(3):190–195.
Funding
Konuki K, Nagai H, Ito M, Sameshima T. 2014. Process for
producing optically active (r)-tetrahydrothiophene-3-ol
with high optical purity and high purity: bioconversion
and crystallization. Org Process Res Dev. 18(2):310–314.
Lacheretz R, Pardo DG, Cossy J. 2009. Daucus carota medi-
ated-reduction of cyclic 3-oxo-amines. Org Lett. 11(6):
The authors are grateful to the Funda c¸ ~a o de Amparo ꢁa
Pesquisa do Estado de S ~a o Paulo (2017/18007-2) for financial
support.
1
245–1248.
ORCID
Liang J, Mundorff E, Voladri R, Jenne S, Gilson L, Conway A,
Krebber A, Wong J, Huisman G, Truesdell S, et al. 2010.
Highly enantioselective reduction of a small heterocyclic
ketone: biocatalytic reduction of tetrahydrothiophene-3-
one to the corresponding (R)-alcohol. Org Process Res
Dev. 14(1):188–192.
ꢀ
Alvaro Takeo Omori
http://orcid.org/0000-0002-0115-3485
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