Simple Plug-In Synthetic Step for the Synthesis of (?)-Camphor from Renewable Starting Materials

Article abstract of DOI:10.1002/cbic.202100187

Racemic camphor and isoborneol are readily available as industrial side products, whereas (1R)-camphor is available from natural sources. Optically pure (1S)-camphor, however, is much more difficult to obtain. The synthesis of racemic camphor from α-pinene proceeds via an intermediary racemic isobornyl ester, which is then hydrolyzed and oxidized to give camphor. We reasoned that enantioselective hydrolysis of isobornyl esters would give facile access to optically pure isoborneol and camphor isomers, respectively. While screening of a set of commercial lipases and esterases in the kinetic resolution of racemic monoterpenols did not lead to the identification of any enantioselective enzymes, the cephalosporin Esterase B from Burkholderia gladioli (EstB) and Esterase C (EstC) from Rhodococcus rhodochrous showed outstanding enantioselectivity (E>100) towards the butyryl esters of isoborneol, borneol and fenchol. The enantioselectivity was higher with increasing chain length of the acyl moiety of the substrate. The kinetic resolution of isobornyl butyrate can be easily integrated into the production of camphor from α-pinene and thus allows the facile synthesis of optically pure monoterpenols from a renewable side-product.

Full text of DOI:10.1002/cbic.202100187

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Simple Plug-In Synthetic Step for the Synthesis of
(À )-Camphor from Renewable Starting Materials
Elia Calderini,[a] Ivana Drienovská,[a] Kamela Myrtollari,[a, b] Michaela Pressnig,[a]
Volker Sieber,[c, d] Helmut Schwab,[a] Michael Hofer,*[d] and Robert Kourist*[a]
Racemic camphor and isoborneol are readily available as
industrial side products, whereas (1R)-camphor is available from
natural sources. Optically pure (1S)-camphor, however, is much
more difficult to obtain. The synthesis of racemic camphor from
α-pinene proceeds via an intermediary racemic isobornyl ester,
which is then hydrolyzed and oxidized to give camphor. We
reasoned that enantioselective hydrolysis of isobornyl esters
would give facile access to optically pure isoborneol and
camphor isomers, respectively. While screening of a set of
commercial lipases and esterases in the kinetic resolution of
racemic monoterpenols did not lead to the identification of any
enantioselective enzymes, the cephalosporin Esterase B from
Burkholderia gladioli (EstB) and Esterase C (EstC) from Rhodo-
coccus rhodochrous showed outstanding enantioselectivity (E>
100) towards the butyryl esters of isoborneol, borneol and
fenchol. The enantioselectivity was higher with increasing chain
length of the acyl moiety of the substrate. The kinetic resolution
of isobornyl butyrate can be easily integrated into the
production of camphor from α-pinene and thus allows the
facile synthesis of optically pure monoterpenols from a renew-
able side-product.
Introduction
from conifers and thus are considered a promising feedstock for
the synthesis of chemical products from renewable resources as
a substitute for petrol-based chemicals.[4] In particular, the
monoterpenoids isoborneol, borneol, and camphor find appli-
cation in many products such as food flavoring, cosmetics, and
cleaning products.[5] Diverse biological activities such as anti-
inflammatory, vasorelaxant and neuroprotective, among others,
make them valuable ingredients for health-related
formulations.[6] The monoterpene camphor and its correspond-
ing alcohols of camphor, borneol, and isoborneol, find
application as fragrances and in traditional Chinese medicine.
The pure enantiomers of isoborneol and borneol are frequently
found in essential oils from many plants, and they have shown
a wide array of biological and antimicrobial activities.[7,8]
Derivatives of these monoterpenoids also find application as
chiral ligands in asymmetric synthesis.[9,10]
Enzyme catalysis is an efficient and environmentally friendly
method for the production and modification of bio-based
chemicals.[1] Furthermore, the demand for enantiopure com-
pounds and synthetic pathways to access single enantiomers is
constantly increasing in the chemical and pharmaceutical
industry due to stricter requirements by the national regulatory
agencies.[2,3] In this context, the intrinsic selectivity of enzymes
at mild conditions is generally an advantage over organo-
metallic catalysts and other chemo-catalytic methods for
asymmetric synthesis.[2] Several monoterpenes accumulate as
side-products during processes such as the cellulose production
[a] Dr. E. Calderini, Dr. I. Drienovská, K. Myrtollari, Dr. M. Pressnig,
Prof. Dr. H. Schwab, Prof. Dr. R. Kourist
Institute of Molecular Biotechnology
(+)-camphor (or (1R)-camphor) can be isolated from various
natural sources and is widely applied in cosmetics and natural
medicine, whereas racemic camphor can be obtained from α-
pinene, a side-product from turpentine production. Isomer-
ization of α-pinene yields camphene and afterward, the
addition of organic acids to camphene leads to isomerization
and the formation of rac-isobornyl esters. Finally, hydrolysis and
oxidation yield rac-camphor (Scheme 1).[11] (À )-Camphor is a
constituent of essential oils of plants from the genus Matricaria.
The synthesis of pure ingredients of essential oils is a strategy
to alter the composition and thus their olfactory properties. The
current synthesis of (À )-camphor proceeds from optically pure
alpha-pinene, which itself must be isolated from plants.
Graz University of Technology, Petersgasse 14, 8010 Graz (Austria)
E-mail: kourist@tugraz.at
[b] K. Myrtollari
Henkel AG & Co. KGaA, Adhesive Research/Bioconjugates
Henkelstr. 67, 40191 Düsseldorf (Germany)
[c] Prof. Dr. V. Sieber
Chemistry of Biogenic Resources
Technical University of Munich
Schulgasse 16, 94315 Straubing (Germany)
[d] Prof. Dr. V. Sieber, Dr. M. Hofer
Bio, Electro and Chemocatalysis BioCat
Fraunhofer Institute for Interfacial Engineering and Biotechnology
Schulgasse 11a, 94315 Straubing (Germany)
E-mail: michael.hofer@igb.fraunhofer.de
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cbic.202100187
Chemical reduction of rac-camphor produces a mixture of
racemic borneol and isoborneol which is called ‘synthetic
borneol.[12] Reduction of (+)-camphor leads to a mixture of
(+)-borneol and (À )-isoborneol. As the separation of the
isomers is costly, they are usually marketed as a mixture under
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Scheme 1. Synthesis route for the production of racemic camphor from α-pinene.[11] Esterase-catalyzed kinetic resolution of bornyl esters would substitute
non-selective ester hydrolysis and allow the isolation of both isomers in pure form.
the assignation ‘semisynthetic borneol’.[13] Pure (1R)-borneol can
be extracted from the resin and essential oil of woody plants of
the families Dipterocarpaceae, Lamiaceae, Valerianaceae, and
Asteraceae. Borneol and isoborneol can be easily oxidized to
camphor chemically or enzymatically.[11,14] Pure (À )-borneol can
be isolated from the herbaceous plant Blumea balsamifera.[15]
While (+)-camphor and rac-camphor are easily available,
(À )-camphor is more difficult to obtain as it is produced by
reduction of (À )-borneol which can only be extracted from
plants.[15,16] Its scarce availability and high price make its
synthesis an interesting goal.
Previously, we isolated the first enantioselective borneol-
dehydrogenases from Salvia officinalis L.[17] While these enzymes
showed high enantioselectivity in the kinetic resolution of
borneol and isoborneol, the necessity to recycle the NAD+
-cofactor represents a disadvantage. We envisioned that a
hydrolase could catalyze the kinetic resolution either with a
selective transesterification of isoborneol or with selective
hydrolysis of an isobornyl ester. Resolution of norborneol and
similar compounds by microbial wild type strains has been
reported,[18] but to our knowledge, no isolated enzymes with
satisfactory selectivity have been reported for this reaction.
Here we report screening and identification of a highly selective
esterase for the kinetic resolution of isobornyl butyrate. The
esterase-catalyzed resolution can substitute the non-selective
hydrolytic step in the existing synthetic route from α-pinene to
camphor, thus providing a facile procedure for the manufacture
of pure (1S,2S,4S)-(+)-isoborneol and (1R,2R,4R)-(À )-isobornyl
butyrate.
active sites can convert tertiary alcohols and their esters.[20] Yet,
the selectivity towards sterically hindered substrates is often
very low and in most cases hard to predict. At first, 10
commercially available lipases were screened for the enantiose-
lective conversion of borneol or isoborneol by transesterifica-
tion using vinyl butyrate as acyl donor.[21] This set includes
several widely applied lipases and is thus representative of
commercially used lipases. For instance, lipases A and B from
the yeast Candida antarctica are generally cheap and robust
catalysts with high activity and selectivity in a large number of
classic and dynamic kinetic resolutions.[22] None of the tested
lipases showed any significant selectivity toward rac-borneol,
resulting in eeP-values from below 2%. Generally, a conversion
lower than 15% was observed even at high enzyme loadings
(SI, Figure S2, A and B). Similarly, no satisfactory selectivity was
achieved in the kinetic resolution of isoborneol, however,
higher conversions were achieved compared to the kinetic
resolution of borneol (SI, Figure S2C). The screening of
commercial catalysts underlined the difficulty to identify
suitable hydrolases for the bulky monoterpenols.
Kinetic resolution using esterases
In a prescreen of a collection of 15 bacterial esterases in the
conversion of sterically demanding esters, five had shown
activity towards monoterpenes such as 5-endo-norbonen-2-yl
acetate or tetrahydrofuran-3-acetate (data not shown). As 5-
endo-norbonen-2-yl acetate shares the same bicyclic structure
as esters from isoborneol and borneol, the five esterases that
converted this substrate were tested in the conversion of the α-
acetate or butyrate esters of isoborneol and borneol (Table 2).
In a pre-screen by thin-layer chromatography, two out of five of
the tested esterases, EstB from Burkholderia gladioli and EstC
from Rhodococcus rhodochrous, showed activity in the hydrol-
ysis of bornyl and isobornyl acetate, respectively (data not
shown). For EstB, we investigated the wild-type enzyme and
mutant EstB NK70 (originally mutant 214_27). This variant with
seven amino acid substitutions was generated by directed
evolution and contains the mutation A311V, which is believed
to stabilize a relatively unstable loop. EstB NK70 has increased
Results and Discussion
Kinetic resolution using lipases
Several α/β-hydrolase-fold class enzymes are known to accept
esters from very bulky alcohols as substrates. This includes
hydrolases with a particularly large active site such as lipase A
from Pseudozyma antarctica (previously Candida antarctica), the
cephalosporine-esterase EstB from Burkholderia gladioli,[19] and
several esterases having a so-called GGG(A)X-motif in their
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temperature stability (40-fold higher activity and a 6-fold longer
97% ee. From rac-bornyl acetate, (À )-borneol was produced
with 90% ee.
°
half-life time at 45 C) and higher tolerance against water-
soluble co-solvents, which we considered advantageous in view
of a later implementation of the kinetic resolution.[23] After the
identification of active enzymes, we then proceed to determine
the preference of the enzymes towards p-nitrophenol esters of
different lengths (C2, C4, C5, C6). This revealed a clear
preference of EstB and EstB NK70 towards shorter esters
whereas EstC seems to prefer longer ones (SI, Figure S1). The
low activity of EstB towards p-nitrophenyl hexanoate was a bit
surprising given that the enzyme shows excellent activity
towards Cephalosporin C, a carboxyl amide with a somewhat
larger acyl moiety than hexanoic acid.[23]
Investigation of a representative set of 10 α/β-hydrolase-
fold lipases and four α/β-hydrolase-fold esterases showed that
none of these enzymes had an enantioselectivity level that
would suffice for kinetic resolution of isoborneol esters. For
such an application, enantioselectivity of E�20 is the minimum,
albeit at this enantioselectivity level the final yield of the
optically pure product of interest would be well below 50%.[25]
Esterase B from B. gladioli is known to convert esters of
bulky alcohols (such as linalyl acetate),[19] EstB differs structurally
from the classic esterases and lipases and exhibits homology
with esterases of class VIII and with class C β-lactamases.[26] The
natural substrate is presumably Cephalosporin C, a carboxyl
amide. Moreover, its catalytic machinery differs considerably
from α/β-fold esterases. This regards the position of the
catalytic serine 105 and the nature of the catalytic triad, which
does not consist of the typical motif Ser-His-Asp/Glu, but of
Ser105, Lys78, and/or Trp348.[26] While the enzyme has been
shown to accept sterically very demanding tertiary alcohols, the
selectivity towards all racemic substrates investigated has been
low so far.[19] Surprisingly, this unique hydrolase was the only
one from a representative set of lipases and esterases that
showed high enantioselectivity (E>100) in the kinetic resolu-
tion of isoborneol esters, respectively. It is well-known that
many esterases and lipases show higher enantioselectivity and
activity towards esters with longer chains of the same
alcohol.[27] This empiric rule is confirmed by EstB and EstC that
show both much higher enantioselectivity towards the butyrate
ester than towards the acetate esters of isoborneol and borneol,
respectively. Interestingly, compared to EstC, EstB shows inverse
enantiopreference towards esters from borneol and isoborneol
as it preferentially produces (+)-borneol from racemic bornyl
butyrate, whereas it produces (+)-isoborneol from racemic
isobornyl butyrate. The different endo- and exo-position of the
ester group in α-position of the bicyclic molecules leads to an
inversion of the preference of the enzyme. A similar inverse
In the kinetic resolution of racemic esters of isoborneol and
borneol, all esterases except the commercial esterase from
porcine liver showed better selectivity with butyryl moiety
compared to the shorter chain acetate moiety, resulting in up
to 7-fold higher E values towards the butyryl esters. Further-
more, both EstB variants showed high enantioselectivity in the
kinetic resolution of isobornyl butyrate, whereas EstC showed
high selectivity for bornyl butyrate (Table 1). Interestingly, EstB
converted all substrates but only showed selectivity for two,
EstC, in contrast, showed no conversion with isobornyl and
fenchyl esters as substrate. Conversion of isobornyl hexanoate
by EstB, its variant NK70 and EstC yielded no product (data not
shown), which was surprising since the corresponding p-nitro-
phenyl ester was converted, albeit with much lower activity
than esters of shorter acids.
Surprisingly, EstB from B. gladioli showed high activity and
excellent enantioselectivity in the kinetic resolution of rac-
isobornyl butyrate giving rise to (+)-isoborneol (or (1S,2S,4S)-
1,7,7-trimethyl-bicyclo[2.2.1]heptan-2-ol in high optical purity
(>98% ee, Scheme 2). Hydrolysis of rac-isobornyl acetate
produced (+)-isoborneol in 89% ee (Scheme 2). The oxidation
of (+)-isoborneol is simple and gives rise to (À )-camphor (or
(1S)-camphor). In contrast, EstC showed high enantioselectivity
towards rac-bornyl butyrate and produced (À )-borneol in
°
Table 1. Kinetic resolution of bornyl butyrate and isobornyl butyrate using 1 mM substrate at 30 C and 600 rpm. Data reported correspond to values
obtained at 24-h intervals. Only esterases with detectable activity are listed.
[b]
[a]
Substrate
Enzyme
Origin
Conversion[a]
(%)
eeS
eeP
E-value[c]
Main product
enantiomer
(%)
(%)
rac-2a
rac-2b
rac-1a
rac-1b
rac-2a
rac-2b
rac-1a
rac-1b
rac-3b
rac-2a
rac-2b
rac-1a
rac-1b
rac-2a
rac-2b
Esterase
Esterase
Esterase
Esterase
EstB
EstB
EstB
EstB
EstB
EstB NK70
EstB NK70
EstB NK70
EstB NK70
EstC
Porcine liver
Porcine liver
Porcine liver
Porcine liver
B. gladioli
B. gladioli
B. gladioli
B. gladioli
B. gladioli
B. gladioli
B. gladioli
49.4%�4.3%
59.4%�1.2%
43.7%�0.1%
42.6%�1.7%
34.2%�7.2%
90.4%�1.9%
16.9%
0.3%
0.2%
62%
16%
16%
94%
19%
44%
20%
10%
70%
27%
44%
11%
45%
0.3%
0.1%
80%
21%
31%
10%
91%
98%
90%
31%
37%
91%
98%
89%
97%
1
1
17
2
2
3
26
>100
23
2
4
30
>100
19
n.d.
n.d.
(À )-1
(À )-1
(+)-2
(+)-2
(+)-1
(+)-1
(+)-3
(+)-2
(+)-2
(+)-1
(+)-1
(À )-2
(À )-2
31.2%�0.1%
18%�0.03%
24.8%�2.0%
65.5%�1.2%
23.2%�0.1%
31.2%�0%
B. gladioli
B. gladioli
R. rhodochrous
R. rhodochrous
11.2%�1.5%
31.8%�0.1%
EstC
>100
[a] Determined by chiral gas chromatography. [b] Calculated from the formula: C=ees/(ees +eep), where C corresponds to conversion. [c] Calculated from
conversion and eeP according to Chen et al.[24]
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Scheme 2. Enantioselective kinetic resolution of racemic acetyl and butyryl esters of isoborneol (1), borneol (2), and fenchol (3) employing esterases.
preference towards these bicyclic monoterpenoids was ob-
served in the stereoselective oxidation of borneol and iso-
borneol by plant dehydrogenases.[17]
available building block that can be obtained both from non-
renewable and renewable resources, and giving its high
chemical similarity with acetate we envision that the current
process conditions to produce isobornyl acetate will apply to
the product of isobornyl butyrate with minor adjustments. The
inexpensive isobornyl esters can be then used as substrates for
EstB from B. gladioli followed by the further oxidation of
(+)-isoborneol to the highly valuable (À )-camphor. In contrast,
EstC from R. rhodochrous can catalyze the kinetic resolution of
the bornyl esters affording (À )-borneol which can be oxidized
to the valuable (À )-camphor as well.
Implementation of enzyme catalysis can either replace one
or more existing steps of a given synthetic route, or a new
synthetic route is assembled “de novo”. The latter is typically
hard to implement as it requires a complete redesign of a
certain chemical process, whereas replacing only one step is
considerably easier. While the yield limitation of kinetic
resolutions is considered a disadvantage, it is still very popular
in industrial processes due to the extremely high simplicity.[28]
The EstB-catalyzed kinetic resolution process can be easily
integrated into an existing chemical route. In this particular
case, the resulting isobornyl and bornyl esters are both used as
fragrances, in food preparations and as chiral ligands, or they
can be hydrolyzed and oxidized to produce the natural
(+)-camphor and (À )-camphor isomers. The biocatalytic step
simply substitutes the unselective chemical hydrolysis. Unlike
oxidoreductases, the esterase does not require the supply of
external cofactors or additional steps for their regeneration.
This work underlines the simplicity of this approach where the
production of the highly valuable (1S)-camphor using an
Turpentine oils, a waste by-product from industrial wood
processing were identified as possible substituents for fossil-
derived chemicals.[31,32] The identification of stereoselective
enzymes for the synthesis of high-value compounds from bio-
based side-stream can substitute petrol chemicals and the
isolation of natural product from plants (which is problematic
due to considerable accumulation of waste), respectively, and
makes thus a contribution towards the development of clean
and economically successful processes.
enzyme can outweigh the cost of chromatographic separation Conclusion
of enantiomers. Racemic borneol and bornyl acetate as starting
materials for EstC are not readily available but are common
compounds in turpentine from different trees and can be
isolated during the refinement.[29,30]
From an industrial application point of view, the corre-
sponding acetate substrates are more attractive giving that
isobornyl acetate is already an intermediate in the chemical
synthesis of racemic camphor.[11] We expect that the selectivity
of the enzymes towards the acetyl esters can be increased by
protein engineering.[20] Nevertheless, butyric acid is an easily
We identified two highly selective esterases for the kinetic
resolution of isobornyl and bornyl esters. EstB from B. gladioli is
particularly interesting as it is highly specific for isobornyl
butyrate, an intermediate of the chemical synthesis of camphor
from α-pinene. We showed that achieving conversion and high
selectivity with these bulky esters is a complex task and none of
a representative set of commercially used α/β-hydrolase-fold
lipases showed mentionable selectivity. EstB is an esterase that
exhibits homology with esterases of class VIII and with class C
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β-lactamases[26] and possibly its particular and unique structure
is required to achieve high selectivity with the substrates used
in this study.
We anticipate that the integration of the kinetic resolution
of isobornyl esters in the current industrial process can be
extremely simple and lead to a more valuable product:
(À )-camphor. This work highlights the high potential of
biocatalysis for side-product valorization while not being
invasive in the process design.
pressure. The product was purified with column chromatography
(cyclohexane:ethyl acetate, 97:3) and an isolated yield of 60% was
reached. Fenchyl butyrate was identified based on the 1H-NMR-
spectra from literature.[38]
Enzymatic kinetic resolution with esterases
All kinetic resolutions were carried out using 1 mM of either bornyl,
isobornyl or fenchyl esters in 50 mM potassium phosphate buffer
°
pH 7.5 at 30 C for 24 h with stirring at 600 rpm. All esterases
described in Table 2 were applied as cell-free extract (9 g/L total
protein content). All reactions were carried out in duplicate,
samples of 200 μl were periodically taken, extracted with 200 μl of
ethyl acetate, dried over sodium sulfate and submitted to GC-FID
analysis.
Experimental Section
Materials
All chemicals were purchased from either Sigma Aldrich Chemie
(Steinheim, Germany), Alfa-Aesar (Thermo Fisher (Kandel) GmbH,
Germany), c-LEcta (Leipzig, Germany) unless otherwise stated.
p-Nitrophenyl ester assay
In a microtiter plate, cleared cell lysate (10 μL), either directly after
cell disruption or after 10-fold dilution, were added to Tris-HCl
buffer (180 μL 100 mM Tris-HCl, 150 mM NaCl, pH 7.5). Absorbance
over time (λ=410 nm) was measured in the spectrophotometer
(BioTek, USA) immediately after the addition of 10 μL of 20 mM
DMSO stock of one of the p-nitrophenyl esters: acetate, butyrate,
valerate, or hexanoate. The esterase activity in CFE was calculated
based on the calibration as the initial reaction velocity (μmol/min).
The extinction coefficient calculated for p-nitrophenolate with the
calibration curve at the stated pH and buffer was 17700 M/cm.
Enzymes
Enzyme production
°
Esterases were produced in terrific broth (TB) media at 20 C
overnight according to the standard protocol after adding the
appropriate antibiotic and inducer listed in Table 2.[26,33]
Synthesis of monoterpenol esters
Acknowledgments
Bornyl, isobornyl, and fenchyl esters: To 1 g of either borneol or
isoborneol (50 mM) and dimethylaminopyridine (DMAP, 6 eq.) in
dry dichloromethane (130 mL, respectively), acetyl, butyryl, or
hexanoyl chloride (4 eq.) was added dropwise and the solution was
stirred overnight. The mixture was washed with 1 M HCl (2×60 mL)
and distilled water (2×60 mL). The organic layer was dried over
anhydrous Na2SO4 before the solvent was removed under reduced
pressure. Products were purified with column chromatography
(cyclohexane:ethyl acetate, 95:5 for the acetyl esters and 90:10 for
the butyryl esters) and isolated yields of 70–85% were reached.[34]
Bornyl acetate,[35] bornyl butyrate,[36] isobornyl butyrate,[37] and
isobornyl hexanoate[37] were identified based on the 1H-NMR-
spectra from literature.
We thank c-LEcta (Leipzig, Germany) and Sigma-Aldrich, for
providing a set of lipases. The authors acknowledge financial
support by the German Federal Ministry of Education and
Research (BMBF) for the project CbP-camphor based polymers
within the bio-economy international program (grant No.
031B0501). This project has received funding from the European
Union’s Horizon 2020 research and innovation programme under
the Marie Skłodowska-Curie grant agreement No 860414.
Conflict of Interest
Fenchyl butyrate: To 100 mg of fenchol and dimeth-
ylaminopyridine (DMAP, 6 eq.) in dry dichloromethane (13 mL),
butyryl chloride (4 eq.) was added dropwise and the solution was
stirred overnight. The mixture was washed with 1 M HCl (2×6 mL)
and distilled water (2×6 mL). The organic layer was dried over
anhydrous Na2SO4 before the solvent was removed under reduced
The authors declare no conflict of interest.
Keywords: asymmetric catalysis
esterases · kinetic resolution
· borneol · camphor ·
Table 2. List of enzymes used in this work and organism of origin.
[1] W. Zhang, J.-H. Lee, S. H. H. Younes, F. Tonin, P.-L. Hagedoorn, H. Pichler,
Y. Baeg, J.-B. Park, R. Kourist, F. Hollmann, Nat. Commun. 2020, 11, 1–8.
[2] J. Albarrán-Velo, D. González-Martínez, V. Gotor-Fernández, Biocatal.
Biotransform. 2018, 36, 102–130.
[3] V. Nosek, J. Míšek, Chem. Commun. 2019, 55, 10480–10483.
[4] M. Hofer, H. Strittmatter, V. Sieber, ChemCatChem 2013, 5, 3351–3357.
[5] S. A. Holstein, R. J. Hohl, Lipids 2004, 39, 293–309.
Enzyme
Organism
Plasmid/Antibiotic/Inducer
Esterase
Esterase EstB
Porcine liver
Burkholderia gladioli
Immobilized (Commercial)
pK214_estB (wt)/kan /IPTG
pJexpress401_estA/kan/ARA
Esterase EstA Burkholderia gladioli
Esterase EstA Rhodococcus rhodochrous pMS470_estA/amp/IPTG
Esterase EstC Rhodococcus rhodochrous pMS470d8-estC/amp/IPTG
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Esterase EstA Arthrobacter nicotianae
pMS470_estA/amp/IPTG
pK214_estB (NK70)/kan /IPTG
Esterase EstB
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mutant[a]
[a] Mutations confirmed by sequencing: S17L G132S E251G A311V E316K.
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Manuscript received: April 19, 2021
104, 7294–7299.
Revised manuscript received: May 24, 2021
Accepted manuscript online: May 25, 2021
Version of record online: ■■■, ■■■■
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FULL PAPERS
Enzymes are a valuable tool for
upcycling side-stream from
Dr. E. Calderini, Dr. I. Drienovská, K.
Myrtollari, Dr. M. Pressnig, Prof. Dr. V.
Sieber, Prof. Dr. H. Schwab, Dr. M.
Hofer*, Prof. Dr. R. Kourist*
renewable sources. We have found
that EstB from Burkholderia gladioli
could achieve high enantiopurity in
upcycling α-pinene, a side-product
from turpentine production, whereas
common lipases and esterases could
not. This reaction can be easily inte-
grated with the existing chemical
synthetic route for the production of
the racemic camphor with minimal
adjustments as the route goes
through an intermediate isobornyl
acetate.
1 – 7
Simple Plug-In Synthetic Step for
the Synthesis of (À )-Camphor from
Renewable Starting Materials