Chemoenzymatic Synthesis of a Chiral Ozanimod Key Intermediate Starting from Naphthalene as Cheap Petrochemical Feedstock

Article abstract of DOI:10.1021/acs.joc.8b03290

Ozanimod represents a recently developed, promising active pharmaceutical ingredient (API) molecule in combating multiple sclerosis. Addressing the goal of a scalable, economically attractive, and technically feasible process for the manufacture of this drug, a novel alternative synthetic approach toward (S)-4-cyano-1-aminoindane as a chiral key intermediate for ozanimod has been developed. The total synthesis of this intermediate is based on the utilization of naphthalene as a readily accessible, economically attractive, and thus favorable petrochemical starting material. At first, naphthalene is transformed into 4-carboxy-indanone within a four-step process by means of an initial Birch reduction, followed by an isomerization of the C=C double bond, oxidative C=C cleavage, and intramolecular Friedel-Crafts acylation. The transformation of the 4-carboxy-indanone into (S)-4-cyano-1-aminoindane then represents the key step for introducing the chirality and the desired absolute S configuration. When evaluating complementary biocatalytic approaches based on the use of a lipase and transaminase, respectively, the combination of a chemical reductive amination of the 4-carboxyindanone followed by a subsequent lipase-catalyzed resolution turned out to be the most efficient route, leading to the desired key intermediate (S)-4-cyano-1-aminoindane in satisfactory yield and with excellent enantiomeric excess of 99%.

Full text of DOI:10.1021/acs.joc.8b03290

Article
pubs.acs.org/joc
Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
Chemoenzymatic Synthesis of a Chiral Ozanimod Key Intermediate
Starting from Naphthalene as Cheap Petrochemical Feedstock
†,§
†
‡
,†
Florian Uthoff,†,§ Jana Lowe, Christina Harms, Kai Donsbach, and Harald Groger*
†
̈
̈
̈
Chair of Industrial Organic Chemistry and Biotechnology, Faculty of Chemistry, Bielefeld University, Universitatsstr. 25, 33615
Bielefeld, Germany
‡PharmaZell GmbH, Rosenheimer Str. 43, 83064 Raubling, Germany
S
* Supporting Information
ABSTRACT: Ozanimod represents a recently developed, promising
active pharmaceutical ingredient (API) molecule in combating multiple
sclerosis. Addressing the goal of a scalable, economically attractive, and
technically feasible process for the manufacture of this drug, a novel
alternative synthetic approach toward (S)-4-cyano-1-aminoindane as a
chiral key intermediate for ozanimod has been developed. The total
synthesis of this intermediate is based on the utilization of naphthalene
as a readily accessible, economically attractive, and thus favorable
petrochemical starting material. At first, naphthalene is transformed into
4-carboxy-indanone within a four-step process by means of an initial
Birch reduction, followed by an isomerization of the CC double bond,
oxidative CC cleavage, and intramolecular Friedel−Crafts acylation.
The transformation of the 4-carboxy-indanone into (S)-4-cyano-1-
aminoindane then represents the key step for introducing the chirality and the desired absolute S configuration. When
evaluating complementary biocatalytic approaches based on the use of a lipase and transaminase, respectively, the combination
of a chemical reductive amination of the 4-carboxyindanone followed by a subsequent lipase-catalyzed resolution turned out to
be the most efficient route, leading to the desired key intermediate (S)-4-cyano-1-aminoindane in satisfactory yield and with
excellent enantiomeric excess of 99%.
drug, its pharmaceutical relevance and importance are
underlined by the acquisition of the company Receptos by
Celgene for $7.2 billion.5
With respect to the concept for its synthesis, ozanimod can
be retrosynthetically subdivided and designed starting from
three building blocks (Scheme 1). Two of the three building
blocks are comparable in terms of their size, thus enabling an
advantageous convergent synthesis. Accordingly, the 1,2,4-
INTRODUCTION
■
The migration of lymphocytes has been associated with
sphingosine-1-phosphate receptors (S1P receptors).1 S1P
receptors belong to the G-protein-coupled proteins and are
classified into a family of five receptors, S1PR1−S1PR5.2 Drug
treatments have been developed to suppress the immune
response because S1P receptors trigger autoimmune diseases.3
The ligand for the S1P receptors is sphingosine-1-phosphate.
One approved drug for multiple sclerosis treatment is
fingolimod (FTY720-P), but its adverse effects motivate the
search for alternatives. A highly promising active pharmaceut-
ical ingredient (API) molecule is ozanimod (1), which has
been developed by Receptos and is structurally quite different
from fingolimod. The major advantages of ozanimod
(RPC1063) are the decreased tendency toward adverse effects
and the lack of a needed phosphorylation to generate the
pharmaceutically active species.2 Furthermore, it has improved
oral pharmacokinetics as well as receptor selectivity and a
shorter half life.3 A phase-2 study with ozanimod in patients
with multiple sclerosis demonstrated a dose-dependent
reduction in circulating lymphocytes. This, in turn, was
associated with a reduction in inflammatory and neuro-
degenerative brain lesions.3,4 In addition to multiple sclerosis,
this substance can also be used for curing ulcerative colitis.3
Although the active ingredient has not yet been approved as a
Scheme 1. Retrosynthetic Synthetic Access to Ozanimod 1
Special Issue: Excellence in Industrial Organic Synthesis 2019
Received: December 30, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
oxadiazole heterocycle is built up at the final step of the total
synthesis by means of an easy-to-conduct condensation
reaction.
Scheme 3. Retrosynthetic Enzymatic Opportunities for the
Synthesis of Enantiomerically Pure (S)-4-Cyano-1-
aminoindanes
One of the two key building blocks for the synthetic access
to ozanimod is the chiral 1-aminoindane species 2, which is
required in enantiomerically pure form. In initial work on the
synthesis of ozanimod,6 this 4-cyano-substituted amine 2 was
prepared from 4-bromoindanone (3) in three steps (Scheme
2). First, the bromine substituent was replaced by a cyano
Scheme 2. Established Route to the Chiral Amine Key
Intermediate (S)-2 Developed by Receptos
However, in addition to the choice of the enantioselective
biocatalytic route, a further key issue in this total synthesis of
key intermediate (S)-2 is the retrosynthetic design of an
attractive approach toward suitable substrates, such as the
prochiral 1-indanone 4. Toward this end, we envisioned a
retrosynthetic approach starting from naphthalene (8) as a
very cheap petrochemical raw material being available in bulk
quantities (Scheme 4). The initial transformation by partial
Scheme 4. Retrosynthetic Concept for Synthesizing (S)-4-
Cyano-1-aminoindane ((S)-2) Starting from Naphthalene
(8)
group via a palladium-catalyzed coupling reaction; subse-
quently, an imine was formed through a condensation with the
chiral Ellman’s auxiliary as an amine component, followed by a
diastereoselective reduction.6 Although the desired 1-amino-
indane (S)-4-cyano-1-aminoindane 2 ((S)-2) is formed with
high enantiomeric excess, the need for a high catalytic loading
of the palladium catalyst with 5 mol % in combination with
cyanide as a reagent and a stoichiometric amount of a chiral
auxiliary represent limitations (marked in red in Scheme 2).
Furthermore, in part, yields do not exceed a moderate range. It
also has to be taken into account that a brominated starting
material has to be prepared and that the insertion of bromine
via bromination represents a somewhat more costly type of
reaction (compared with, e.g., chlorination). Thus despite the
straightforward and elegant formation of the 1,2,4-oxadiazole
heterocycle when starting from the 1-aminoindane and the
nitrile,6 with respect to a technical process, the current
synthesis of the 4-cyano-1-aminoindane (S)-2 as an
intermediate consists of the utilization of, in part, relatively
expensive components (brominated substrate, chiral auxiliary,
palladium catalyst) as well as toxic reagents such as a metal
cyanide.
As potential alternative approaches toward the 1-amino-
indane key building block (S)-2, one can conceive various
synthetic options based on biocatalysis (Scheme 3).7 Because
of the high enantioselectivity of enzymes, we were interested to
study such reactions as key steps. Instead of a diastereose-
lective reduction of the preformed chiral imine (S)-5, the
prochiral ketone 4 can be directly reductively aminated in an
asymmetric fashion by means of, for example, a transaminase.
A further biocatalytic option is the “classic” chemical
reductive amination leading to racemic amine rac-2, followed
by an enzymatic kinetic resolution by means of a lipase-
catalyzed acylation. Although such a kinetic resolution route is
limited, by definition, to a maximum yield of 50%, the high
efficiency of lipases in such types of resolutions together with
the option to subsequently recycle the undesired enantiomer
make this route attractive as well. Both routes were evaluated
and, in part, optimized in our work.
reduction furnishing 1,2-dihydronaphthalene (which could be
done, for example, through Birch reduction,8 followed by an
isomerization) and the subsequent oxidatively cleavage via an
ozonolysis-type reaction8 lead to the diacid (7). A Friedel−
Crafts acylation9 of this diacid 5 then gives the 1-indanone
derivative 6, now bearing a carboxylate (as a nitrile precursor)
in the desired four-position. It should be added that the
acylation can take place only in the meta-position, thus
avoiding side products with an undesirable substituent pattern.
The carboxylic acid moiety of the indanone skeleton in 6 can
then be interconverted into the desired nitrile group either
prior to or after the formation of the chiral amine moiety
through the biocatalytic reactions described above (e.g., by the
initial formation of 4 and the subsequent transamination or
chemical synthesis of rac-2, followed by enzymatic resolution),
thus forming the desired chiral key building block (S)-2.
In the following, we report our results on such a
chemoenzymatic approach for the enantioselective synthesis
of the ozanimod key intermediate (S)-2 based on an evaluation
of both biocatalysis concepts, namely, lipase-catalyzed
resolution and transaminase-catalyzed reductive amination, as
key steps to introduce the chiral amine moiety in an
asymmetric fashion. In addition, the optimization of the
favored route for (S)-2 is described as well as a “back
B
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
integration” of the synthesis of suitable 1-indanone inter-
mediates, for example, 4 and 6, starting from naphthalene (8).
acid chloride as the acyl donor led to an increased byproduct
formation, alternatives were studied. Sulfuric acid acts as an
efficient catalyst and increases the electrophilicity of the free
carboxylic acid 7, leading to a successful aromatic substitution
when following a literature protocol.9 However, such
conditions described in the literature led to a high byproduct
formation and low yield of 10% for the desired indanone-type
product 6. We were pleased to find that by substrate
concentration dilution to 56 mM and product isolation by
continuous liquid−liquid extraction the yield of 6 is increased
to (nonoptimized) 42%. Note that the resulting high purity
does not require further purification. Subsequently, the acid 6
is converted to the corresponding nitrile 4 via a primary amide
11. Ethyl chloroformate proved to be a favorable activating
reagent, whereas attempts using the more reactive thionyl
chloride were not successful. The mixed anhydride reacts
quantitatively to the primary amide 11 in the presence of
aqueous ammonia. Without isolating the mixed anhydride, a
yield of 77% for 11 over these two steps is achieved (Scheme
5).
RESULTS AND DISCUSSION
■
To start with the synthesis of suitable substrates for the
biotransformations, in particular, 4-aminoindanone (4), the
initial step consists of a partial reduction of naphthalene (8) to
1,2-dihydronaphthalene (Scheme 5). Although being aware
Scheme 5. Multi-Step Transformation of Naphthalene (8)
into 4-Cyano-1-indanone (4) as a Substrate for the
Biotransformations
The next step consists of converting the amide moiety in 11
into the cyano group, thus leading to the desired prochiral
intermediate 4. For this dehydration, various reagents known
for this type of transformation were investigated, and the
results are shown in Figure 1. The desired product 4 is formed
that a direct partial hydrogenation might be an even more
straightforward approach for technical purposes, for practical
reasons, in our lab synthesis this step was carried out as a two-
step interconversion via a Birch reduction, followed by an
isomerization. For the Birch reduction, sodium in the presence
of tert-butanol was used as a reducing agent. The product
spectrum can vary in dependency on the chosen solvent
conditions.8 Naphthalene (8) has been converted to 1,4-
dihydronaphthalene 9 in 86% yield. Isomerization then leads
to 1,2-dihydronaphthalene 10, and after workup, this product
is isolated in 98% yield (with a ratio of 10:9 of 97:3, thus
corresponding to 95% yield referring to pure 10). For an
isomerization, both basic and Brønsted-acid-catalyzed con-
ditions are conceivable. Because in the literature,10,11 only a
limited number of data on such reaction conditions can be
found, we determine the preferred reaction conditions for this
reaction within a reaction parameter screening. No rearrange-
ment is observed by acid catalysis, whereas the rearrangement
under basic conditions is strongly dependent on the type of
base and solvent. Whereas no rearrangement is found when
using toluene and tetrahydrofuran (THF), the treatment of 9
with potassium tert-butylate in tert-butanol results in the
formation of the desired isomer 10. Under reflux and thus
thermodynamic control, the isomer ratio reaches 97:3 in
combination with a yield of 90% (Scheme 5).
Figure 1. Dehydration of primary amides with different reagents.
when using all of the applied reagents, but the yields vary
broadly. The best result is obtained when utilizing trifluoro-
acetic anhydride (17) under mild reaction conditions and with
a short reaction time, leading to product 11 in excellent yield.
Subsequently, we focus on the formation of the amine
moiety with the desired absolute configuration at the
stereogenic center according to the biocatalytic concepts
described in the Introduction (see also Scheme 3). First, the
results of our study on the asymmetric synthesis of amine (S)-
2 and related derivatives with a modified substitution pattern
in the four-position by means of a transaminase as a biocatalyst
are presented. Transaminases catalyze the conversion of a
carbonyl compound to a primary amine, while an amine donor
is transformed into the corresponding ketone, and, in
The isolated double bond in 10 is then oxidatively cleaved
by means of potassium permanganate following a literature
known protocol,9 leading to the desired diacid 7 in 75% yield
after filtering off the manganese dioxide (Scheme 5). The
product 7 requires no further purification and can be used
directly for the subsequent Friedel−Crafts-type acylation.
Because the “classic” way with aluminum trichloride and an
C
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
particular, in recent years, this methodology turned out to be
highly suitable for the asymmetric synthesis of amines from
ketone substrates.7,12 The ketone substrates used in this study
are the four-substituted indanones 4 and 19 synthesized in the
total synthesis as well as 4-bromoindanone (3) and
acetophenone (18) for comparison.
addition, according to this literature,15 the racemic amines rac-
24−26 were formed, leading to yields between 28 and 37%
(Scheme 6). The amines are easily isolated via extraction and
need no further purification prior to their use as substrates for
the lipase-catalyzed resolution.
For our studies, two transaminases are investigated that
differ with respect to the needed cosubstrate (Figure 2). The
Scheme 6. Synthesis of the Racemic Amines via a Two-Step
Synthesis Consisting of Ketoxime Formation and Reduction
With the various racemic amines in hand, we next focus on
the enzymatic resolution utilizing lipases to obtain the required
(S)-enantiomer of the amine. For the identification of a
suitable lipase racemic 1-aminoindane, rac-24, is chosen as a
commercially readily available model substrate being structur-
ally related to the ozanimod precursor amines rac-2, rac-25,
and rac-26. In addition, diethylmalonate is used as an acylation
̈
reagent in the initial screening experiments, as Groger et al.
recently showed that this acyl donor enables efficient
enzymatic resolutions of 1-arylethylamines.16,17 The results,
which are described in the Supporting Information (Table S3),
reveal lipase B from Candida antarctica (CAL-B) as the most
suitable enzyme for the kinetic resolution of 1-aminoindane
rac-24.
Figure 2. Enzymatic transamination with two different cosubstrates.
The next step is the optimization of the kinetic resolution
catalyzed by CAL-B, and toward this end, various acyl donors
and solvents are examined. Racemic 1-aminoindane, rac-24, is
again chosen as a model substrate. Toluene, methyl tert-butyl
ether (MTBE), 2-methyltetrahydrofuran (2-MTHF), methyl-
cyclohexane (MCH), and n-heptane are investigated as
solvents. Toluene is chosen because it is a readily accessible
and low-price solvent.18 Lipases are successfully used in apolar
solvents; therefore, MCH and n-heptane were selected,19 and
also MTBE was reported to be a suitable solvent for enzymatic
kinetic resolution.20 2-MTHF represents a recently introduced
green solvent, which can be obtained from biorenewable
resources.21 Diethyl malonate 28 and ethyl methoxyacetate
2714b are used as acyl donors. Ethyl methoxyacetate 27 is an
acyl donor used by BASF in the synthesis of chiral
amines.7,13,14 The related isopropyl methoxyacetate 29 was
tested in a later experiment under optimal conditions. In the
literature, kinetic resolution using isopropyl methoxyacetate 29
is reported to proceed with high E values.14d,22 The goal of this
study was to achieve E values of at least 20 because E values in
this range appear to be suitable for industrial applications.23 In
Figure 3, the results of the kinetic resolution with the different
acyl donors (27−29) and solvents at 60 °C are shown. The
reaction is performed for 26 h, and the E values are calculated
from the ee values of the remaining amine and amide formed in
the resolution.
transaminase from Arthrobacter sp.12 (ArS-TA), which accepts
isopropylamine (20) as an amine donor, shows a low activity
against the model substrate acetophenone in aqueous buffer
solution. In the organic medium, it loses almost all of the
activity. In addition, 4-cyano-1-indanone (7) is not converted.
In contrast, when using the transaminase from Vibrio
fluvialis12a,c,d (VF-TA) for all studied indanones, at least a
reasonable activity is observed (Figure 2). For this enzyme, L-
alanine ((S)-22) serves as an amine donor, and the coupling to
an enzyme cascade using a lactate dehydrogenase (LDH) and a
glucose dehydrogenase (GDH) together with glucose as a
reducing agent shifts the equilibrium to the product side.
Surprisingly, the ee values for the bromo- and cyano-
substituted indanones 3 and 4 are <60%, which is remarkable
because most examples of transaminases known in the
literature give very high enantioselectivities.7,12 However, we
were pleased to find that indanone 19 bearing a methyl
carboxylate substituent in the four-position is converted with
good conversion and excellent enantioselectivity (Figure 2).
As an alternative biocatalytic route, next we focus on the
evaluation of a lipase-catalyzed amine resolution7,13 for the
synthesis of the ozanimod key intermediate (S)-2. A well-
known example for the kinetic resolution of racemic amines is
the synthesis of enantiomerically pure 1-phenylethylamine
established by BASF.7,13,14 However, with respect to the
synthesis of the racemic amine substrate rac-2, it turned out
that little is known about the chemical reductive amination of
ketones that tolerates a nitrile group. Following a method
reported by Gillen et al.,15 the ketoxime has been formed with
hydroxylamine and subsequently reduced with Zn to the amine
rac-2, which was obtained in nonoptimized 43% yield. In
When conducting the kinetic resolution in n-heptane with
diethyl malonate 28, an E value between 9 and 12 is obtained
(Figure 3), whereas the kinetic resolution of 1-aminoindane,
rac-24, with CAL-B in n-heptane with ethyl methoxyacetate 27
gives an increased E value of 56−60. The kinetic resolution in
MTBE shows similar E values for both acyl donors: For diethyl
D
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Figure 4. CAL-B-catalyzed resolutions of racemic four-substituted 1-
aminoindanes as ozanimod intermediates.
Therefore, −CN, −Br, and COOCH3 are chosen as easily
transformable substituents in the four-position of 1-amino-
indane to form the heterocycle in a later step of the total
synthesis of ozanimod. When studying these substrates rac-2,
rac-25, and rac-26, the best E value is found for the resolution
of 4-cyano-1-aminoindane, rac-2, with an E value of 31. For 4-
bromo-1-aminoindane, rac-25, a slightly lower E value of 26 is
measured, whereas the methyl ester rac-26 leads to a decreased
E value of 11. This low E value might be caused by steric
hindrance effects in the active site of the enzyme.
With the efficient enzymatic resolution of, in particular, 4-
cyano-substituted amine in hand, next we conduct the
synthesis of rac-2 and its biocatalytic resolution in a row to
gain a better insight into the overall process efficiency of this
chemoenzymatic synthesis of the desired ozanimod inter-
mediate, (S)-2 (Scheme 7). First, the oxime is formed by
means of hydroxylamine hydrochloride and reduced with zinc
to the racemic amine rac-2, which is then acetylated with
Figure 3. Solvent and acyl donor screening for CAL-B-catalyzed
reactions.
malonate 28, an E value between 21 and 24 is reached, and for
ethyl methoxyacetate 27, an E value of 19−20 is found. For the
kinetic resolution in toluene with diethyl malonate 28, an E
value between 35 and 36 is reached, and with methoxyacetate
27, the E value is between 28 and 31. For the reaction of 2-
methyl-THF with CAL-B and diethyl malonate 28, an E value
of 28−32 is found, and when using ethyl methoxyacetate 27, E
values of 32−49 are obtained. Thus 2-methyl-THF seems to be
a green alternative compared with n-heptane. The kinetic
resolution in MCH shows similar E values for both acyl
donors, with 25−40 for diethyl malonate 28 and 25−39 for
ethyl methoxyacetate 27. Furthermore, the best temperature
for the kinetic resolution of 1-aminoindane, rac-24, in the
presence of lipase B from C. antarctica as a biocatalyst is
determined, with no significant changes in E value between
reaction temperatures of 60 and 80 °C.
Scheme 7. Chemoenzymatic Synthesis of Enantiomerically
Pure 4-Cyano-1-aminoindane, (S)-2
Finally, under the optimal reaction conditions (2-methyl-
THF, 60 °C) a further biotransformation was carried out using
isopropyl methoxyacetate, 29, as an acyl donor. We were
pleased to find a further increase of the E value up to 113 in
this experiment (Figure 3). Furthermore, an increase of the
substrate concentration up to 1 M also turned out to be
successful. These results and the high E values obtained for the
resolution of the model substrate rac-24 made this racemic
resolution an attractive starting point to transfer the best
reaction conditions (60 °C, 2-MTHF) to the four-substituted
1-aminoindanes being of relevance for the ozanimod synthesis.
The results of these biotransformations using such substrates
in the presence of lipase B from C. antarctica are summarized
in Figure 4.
E
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
isopropyl methoxyacetate 29 in the presence of the lipase
CAL-B and in 2-methyl-THF as a solvent for 20 h at a reaction
temperature of 60 °C. Subsequently, the crude extract is
extracted at various pH values, and the solvent is removed in
vacuo, thus furnishing the amine (S)-2 with excellent 99% ee in
an overall yield of 19% (based on ketone 4). The enantiomeric
purity is proven via the comparison of HPLC chromatograms
of the commercial available (S)-2 with isolated amine 2 from
the reaction. The chromatograms are shown in the Supporting
Information (Figures S43, S54, and S55). The modest yield is
not due to the kinetic racemate resolution because this step
works very well with >99% and 58% conversion. A challenge of
this synthetic sequence for future work is the “classic chemical”
reductive amination process with (nonoptimized) yields being
so far in the range between 28 and 35%.
As for the technical purpose, the biocatalyst loading is a
crucial reaction parameter in terms of the reduction of the
costs of the biocatalyst; next, the overall amount of the used
lipase CAL-B is decreased (Figure 5), and, in addition, the
same enzyme charge is used for several cycles to demonstrate a
proof-of-concept for recycling of the enzyme in this process
(Figure 6).
Figure 6. Recycling experiment with CAL-B.
is carried out, and the enzyme is used six times. As indicated by
the comparable conversions, ee values for the amine (S)-24 and
ee values for the amide (R)-30 obtained through all of the
reaction cycles, there is no loss of activity of CAL-B after reuse
over six reaction cycles. Thus besides showing a high
robustness and reproducibility, this biocatalytic resolution
process is also efficient in terms of biocatalyst loading and
recyclability.
CONCLUSIONS
■
In summary, a chemoenzymatic synthetic route toward (S)-2
as a chiral key intermediate for API ozanimod is presented.
The total synthesis of this intermediate is based on the
utilization of naphthalene as a readily accessible starting
material, which is transformed into 4-carboxy-indanone within
a four-step process by means of an initial Birch reduction,
followed by an isomerization of the CC double bond,
oxidative CC cleavage, and intramolecular Friedel−Crafts
acylation. For the transformation of the 4-carboxy-indanone
into (S)-2 as a key step for introducing the chirality and the
desired absolute S configuration, complementary biocatalytic
approaches based on the use of a lipase and a transaminase,
respectively, were evaluated. As a key enantioselective step,
lipase-catalyzed resolution turned out to be the most efficient
route, leading to the desired key intermediate (S)-2 in
satisfactory yield and with excellent enantiomeric excess of
99%. For industrial application, the replacement of the initial
Birch reduction of naphthalene by an efficient electrochemical
reaction step would be desirable, and the development of such
technical transformation represents a task for future process
research work.
Figure 5. Reduction of catalyst amount in CAL-B-catalyzed resolution
of the model substrate1-aminoindane, rac-24.
As for the reduction of the biocatalyst, the enzyme per
substrate ratio is stepwise reduced from originally 40 mg
enzyme per mmol of amine substrate (rac-24) to 20, 10, and 5
mg/mmol (Figure 5). It is noteworthy that the reaction also
works with decreased CAL-B amount, and after 3 h, nearly the
same ee values are found when using 10, 20, and 40 mg of
lipase CAL-B per mmol of substrate. However, when further
reducing the CAL-B amount to only 5 mg of CAL-B per mmol
of substrate, the ee value after 3 h is at only 60%. In this case,
however, when increasing the reaction time, the reaction also
reaches completion. With this reduced CAL-B amount, the
reaction is also conducted with 4-cyano-1-aminoindane, rac-2,
and after 5 h, a conversion of 57% and an ee value of 99% for
the desired amine (S)-2 are reached.
EXPERIMENTAL SECTION
■
Chemistry: General Remarks. NMR spectra are recorded on
Bruker Avance 500 and Bruker DRX 500 spectrometers. Chemical
shifts (ppm) are given relative to a tetramethylsilane (TMS) standard.
The deuterated solvents present the reference for all spectra
(chloroform-d: 7.26 ppm; dimethylsulfoxide: 2.50 ppm). Multiplets
are assigned as s (singlet), d (doublet), t (triplet), dd (doublet of
doublet), and m (multiplet). Gas chromatography analysis is
Furthermore, recycling of the biocatalyst CAL-B has been
studied (Figure 6). Therefore, the acylation of 1-aminoindane,
rac-24, with isopropyl methoxyacetate, 29, in toluene at 60 °C
F
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
3
3
performed on a GC-2010 Plus instrument from a Shimadzu
Phenomenex ZB-SMS capillary column (polydimethylsiloxane with
5% polyphenylmethylsiloxane, 30 m, 0.25 mm i.d., 0.25 μm film
thickness) using argon as a carrier gas. All HPLC chromatograms are
recorded on a Jasco LC Net II/ADC machine with PU-2080 pumps.
Chiral columns (Chiralpak AD-H, OB-H, and OJ-H) for the
separation of the enantiomers for analytical purposes are commer-
cially available from Daicel Chemical Industries. All of the reagents
were purchased from Sigma-Aldrich, Alfa Aesar, ApplChem, Roth,
Oxchem, Merck, TCI, VWR, Fluorochem, Deutero, Acros Organics,
Amano Enzyme, Oriental Yeast, and Fisher Scientific and were used
without further purification.
8.03 (d, J = 7.6 Hz, 1H), 8.38 (d, J = 8.0 Hz, 1H). 13C{1H}-NMR
(126 MHz, CDCl3) δ 206.4, 156.8, 138.4, 136.6, 129.8, 128.6, 128.1,
127.6, 36.2, 27.4. MS m/z: 174.8 [M−H]−. The analytical data are in
agreement with the literature.27
Derivatization of the 4-Carboxy Substituent of Indanone. 4-
Carboxy-1-indanone (6, 200 mg, 1.14 mmol, 1.00 equiv) and
triethylamine (551 μL, 5.79 mmol, 1.5 equiv) are dissolved in THF
(5 mL) and cooled to 0 °C. Ethyl chloroformate (540 μL, 5.67 mmol,
5.00 equiv) is added. The reaction mixture is stirred for 2 h at rt. After
quenching with water (3 mL), the mixture is extracted with DCM (3
× 7 mL). The combined organic layers are washed with water (15
mL) and dried with MgSO4. The solvent is removed to give 262.6 mg
of a brown oil (85% yield). (Ethyl carbonic) 1-Oxo-2,3-dihydro-1H-
indene-4-carboxylic anhydride: 1H NMR (500 MHz, CDCl3) δ: 1.45 (t,
Reduction of Naphthalene and Rearrangement toward 1,2-
Dihydronaphthalene. Naphthalene (8, 10.00 g, 78 mmol, 1.00
equiv) is dissolved in Et2O (150 mL). Small pieces of sodium (5.00 g,
217 mmol, 2.78 equiv) are given to the solution. The atmosphere over
the sodium is nitrogen flushed. tBuOH (14.5 g, 196 mmol, 2.50
equiv) is dissolved in Et2O (50 mL) and dropped into the sodium
suspension. After stirring for 16 h at rt, the reaction mixture is
quenched with water and extracted with ethyl acetate (three times).
The combined organic layers are dried with Na2SO4 and separated
from the solvent in vacuo. Conversion is calculated by the NMR
spectrum, determining an amount of 8.73 g of 1,4-dihydronaph-
thalene (86%). 1,4-Dihydronaphthalene (9): 1H NMR (500 MHz,
3J = 7.1 Hz, 3H), 2.77 (m, 2H), 3.55 (m, 2H), 4.45 (q, J = 7.2 Hz,
3
3
3
2H), 7.56 (m, 1H), 8.05 (d, J = 7.6 Hz, 1H), 8.32 (d, J = 8.0 Hz,
1H). 13C{1H}-NMR (126 MHz, CDCl3) δ: 205.8, 158.0, 148.9,
138.8, 137.0, 129.6, 127.9, 125.8, 66.0, 36.0, 27.4, 14.0. MS m/z:
271.1 [M+Na]+, HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for
C13H12O5Na 271.0582. Found 271.0577.
The mixed anhydride (100 mg, 0.40 mmol, 1.00 equiv) is given to
an ammonia solution (2.5 mL, 25%, 14.7 equiv) and heated to 60 °C.
The reaction mixture is stirred for 24 h at rt. The mixture is extracted
with DCM (3 × 5 mL). The combined organic layers are washed with
water (15 mL) and dried with MgSO4. The solvent is removed to give
63.77 mg of a brown oil (91% yield, corresponding to a total yield of
77% over two steps from compound 6). 1-Oxo-2,3-dihydro-1H-indene-
3
3
CDCl3) δ: 3.41 (d, J = 1.3 Hz, 2H), 5.94 (t, J = 1.5 Hz, 2H), 7.09
(m, 4H). 13C{1H}-NMR (126 MHz, CDCl3) δ: 134.2, 128.4, 125.9,
124.8, 29.8. The analytical data are in agreement with the literature.24
For the rearrangement, 1,4-dihydronaphthalene (9, 7.92 g, 60.1
mmol, 1 equiv) is dissolved in tert-butanol (6 mL). Potassium tert-
butylate (2.5 g, 22.3 mmol, 0.4 equiv) is given to the solution, and the
mixture is stirred for 12 h at 60 °C under an argon atmosphere. The
reaction mixture is admixed with dichloromethane (DCM) (10 mL)
and washed three times with water (15 mL). The solvent is removed
in vacuo, and the product is recovered as a slightly yellowish liquid.
The reaction control is carried out by GC, and the purity of the
product is determined by 1H NMR, leading to a yield of 7.43 g of 1,2-
dihydronaphthalene (97%; ratio of 10:9 of 97:3, thus corresponding
1
4-carboxamide (11): H NMR (500 MHz, CDCl3) δ: 2.76 (m, 2H),
3.49 (m, 2H), 7.49 (m, 1H), 7.89 (dd, 3J = 7.6 Hz, 3J = 1.1 Hz, 1H),
7.93 (dd, J = 7.6 Hz, J = 1.1 Hz, 1H). 13C{1H}-NMR (126 MHz,
CDCl3) δ: 206.4, 168.7, 154.7, 138.5, 132.8, 132.0, 127.7, 126.8, 36.2,
26.3. MS m/z: 173.8 [M−H]−.
3
3
The primary amide (11, 150 mg, 0.85 mmol, 1.00 equiv) and
triethylamine (356 μL, 2.55 mmol, 3.00 equiv) are suspended in
toluene (5 mL). Phosphorus pentoxide (48 mg, 0.34 mmol, 0.40
equiv) is added, and the mixture is heated to 80 °C. The reaction
mixture is stirred for 5 h. After quenching with water (3 mL), the
mixture is extracted with DCM (3 × 7 mL). The combined organic
layers are washed with water (15 mL) and dried with MgSO4. The
solvent is removed to give 39.0 mg of a brown oil (29% yield). 1-Oxo-
2,3-dihydro-1H-indene-4-carbonitrile (4): 1H NMR (500 MHz, CDCl3)
δ: 2.82 (m, 2H), 3.36 (m, 2H), 7.54 (tt, 3J = 7.6 Hz, 3J = 0.9 Hz, 1H),
7.91 (dd, 3J = 7.5 Hz, 3J = 1.2 Hz, 1H), 7.99 (m, 1H). 13C{1H}-NMR
(126 MHz, CDCl3) δ: 204.5, 157.7, 138.2, 137.7, 128.2, 128.1, 116.1,
111.5, 35.8, 25.3. MS m/z: 179.0 [M−H]+.
1
to 95% of pure 10). 1,2-Dihydronaphthalene (10): H NMR (500
3
MHz, CDCl3) δ: 2.34 (m, 1H), 2.82 (t, J = 8.2 Hz, 1H), 6.05 (m,
1H), 6.48 (m, 1H), 6.85−7.29 (m, 4H). 13C{1H}-NMR (126 MHz,
CDCl3) δ: 135.4, 134.2, 128.5, 127.8, 127.5, 126.8, 126.4, 125.9, 27.5,
23.2. The analytical data are in agreement with the literature.25
Oxidative Cleavage of the CC Double Bond in 1,2-
Dihydronaphthalene. 1,2-Dihydronapthalene (10, 15.00 g, 115
mmol, 1.00 equiv) is dissolved in methylene chloride (50 mL) and
then slowly added dropwise to a potassium permanganate solution
(2000 mL, 230 mmol, 230 mM, 4.00 equiv). The reaction mixture is
stirred for 3 h at 0 °C. After adding sodium hydroxide (32 g, 800
mmol, 6.96 equiv) the solution is filtrated to separate manganese
dioxide. The filtrate is extracted by methylene chloride (three times
1000 mL). The aqueous layer is acidified with concentrated
hydrochloric acid (70 mL, 840 mmol, 12 M, 7.30 equiv) to pH 12
to give a precipitate. After filtration, the diacid 7 is isolated with a
yield of 75% (15.53 g, 80 mmol). 3-(2-Carboxyphenyl)propionic acid
General Procedures. Reductive Amination. 1-Indanone and
four-substituted 1-indanone derivatives (1 equiv) and hydroxylamine
hydrochloride (1.5 equiv) are dissolved in ethanol and water (1:1 v/
v). Meanwhile, sodium hydroxide (1.75 equiv) dissolved in water is
added to the suspension. The mixture is heated to reflux for 90 min.
The crude product is filtered over Celite and washed with water. The
oxime is dissolved in acetic acid. Under an argon atmosphere, zinc
dust (5 equiv) is added, and the suspension is stirred at room
temperature for 48 h. The reaction mixture is filtered over Celite and
washed with ethyl acetate, and the solvent is removed in vacuo. The
oil is dissolved in ethyl acetate and hydrogen chloride (1:1 v/v) and
extracted twice with 2 M hydrogen chloride. The pH value of the
aqueous phase is adjusted to 10, and afterward extracted three times
with ethyl acetate. The organic phase is washed with brine and dried
over MgSO4. The solvent is removed in vacuo. The remaining solvent
is removed in a Schlenk flask under an argon atmosphere.
1
3
(7): H NMR (500 MHz, CDCl3) δ: 2.73 (t, J = 7.8 Hz, 2H), 3.42
(t, 3J = 7.9 Hz, 2H), 7.35 (m, 2H), 7.52 (m, 1H), 8.06 (d, 3J = 7.7 Hz,
1H). 13C{1H}-NMR (126 MHz, CDCl3) δ: 174.7, 167.6, 142.8,
132.7, 131.7, 131.4, 131.2, 127.2, 36.3, 30.0. MS: m/z = 192.8 [M−
H]−, MS m/z: 217.0 [M+Na]+. The analytical data are in agreement
with the literature.26
Intramolecular Friedel−Crafts Acylation under the For-
mation of 4-Carboxylic-Acid-Substituted Indan-1-one. 3-(2-
Carboxyphenyl)propionic acid (7, 500 mg, 2.57 mmol, 1.00 equiv) is
dissolved in sulfuric acid (30 mL). The reaction mixture is stirred for
2 h at 130 °C. The complete reaction mixture is given on ice (20 g).
The solution is stored at 8 °C for 65 h to give a precipitate. After
filtration, 209.6 mg (42% yield) of the analytically pure but brown-
colored product 6 (analytically determined by NMR) is isolated. 1-
Oxo-2,3-dihydro-1H-indene-4-carboxylic acid (6): 1H NMR (500 MHz,
CDCl3) δ: 3.16−3.22 (m, 2H), 3.54−3.58 (m, 2H), 7.62 (m, 1H),
2,3-Dihydro-1H-inden-1-amine (rac-24). The product is isolated
in a yield of 0.18 g (1.38 mmol, 37%). 1H NMR (500 MHz, CDCl3)
δ: 1.70−1.76 (m, 1H), 2.51−2.56 (m, 1H), 2.83−2.87 (m, 1H),
3
2.95−3.02 (m, 1H), 4.37 (t, J = 7.5 Hz, 1H), 7.19−7.25 (m, 2H),
7.33−7.35 (m, 1H). MS m/z 134.5 [M+H]+.
4-Bromo-2,3-dihydro-1H-inden-1-amine (rac-25). The product is
1
isolated in a yield of 0.36 g (1.68 mmol, 28%). H NMR (500 MHz,
CDCl3) δ: 1.61−1.64 (m, 1H), 2.42−2.50 (m, 1H), 2.69−2.75 (m,
1H), 2.91−2.97 (m, 1H), 4.33 (t, J = 7.5 Hz, 1H), 7.00 (t, J = 7.6
3
3
G
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
3
3
Hz, 1H), 7.17 (d, J = 7.4 Hz, 1H), 7.27 (d, J = 7.9 Hz, 1H).
13C{1H}-NMR (126 MHz, CDCl3) δ: 144.2, 143.1,131.9, 129.0,
123.5, 120.4, 56.8, 31.8, 31.6. MS (ESI) m/z: 211.9 [M+H]+, 213.9
[M+H]+. The analytical data are in agreement with the literature.28
1-Amino-2,3-dihydro-1H-indene-4-carbonitrile (rac-2). The
product is isolated in a yield of 28 mg (0.18 mmol, 35%). 1H
NMR (500 MHz, CDCl3) δ: 2.11−2.17 (m, 1H), 2.60−2.69 (m, 1H),
water and extracted with methylene chloride. The crude product is
purified by one acidic extraction at pH 1 and one basic (pH 13)
extraction (three times for each of them).
General Procedures. Derivatization for Calculation of
Enantiomeric Excess (Lipase). The amine sample was acetylated
with acetyl chloride (1.1 equiv) and triethylamine (1.5 equiv) in
methylene chloride for 1 h. The suspension is washed with hydrogen
chloride (1:1 v/v). The solvent is removed in vacuo. Enantiomeric
excess of the amide is determined via HPLC.
3
3.05−3.10 (m, 1H), 3.28−3.33 (m, 1H), 4.65 (t, J = 7.5 Hz, 1H),
7.37 (t, 3J = 7.6 Hz, 1H), 7.56 (d, 3J = 7.4 Hz, 1H), 7.70 (d, J = 7.9
3
Hz, 1H). 13C{1H}-NMR (126 MHz, CDCl3) δ: 148.2, 141.7, 132.6,
129.6, 126.1, 117.3, 109.5, 55.8, 31.3, 30.0. MS (ESI) m/z: 159.0 [M
+H]+. The analytical data are in agreement with the literature.6
Methyl 1-Amino-2,3-dihydro-1H-indene-4-carboxylate (rac-26).
N-(2,3-Dihydro-1H-inden-1-yl)acetamide (36). The product is
1
isolated in a yield of 65 mg (0.38 mmol, 51%). H NMR (500 MHz,
CDCl3) δ: 1.80−1.86 (m, 1H), 2.03 (s, 3H), 2.64−2.69 (m, 1H),
3
2.86−2.95 (m, 1H), 2.89−2.94 (m, 1H), 5.48 (q, J = 7.7 Hz, 1H),
1
7.20−7.25 (m, 3H), 7.28−7.30 (m, 1H). 13C{1H}-NMR (126 MHz,
CDCl3) δ: 177.9, 143.6, 142.7, 128.3, 126.9, 125.0, 124.1, 55.3, 33.8,
30.4, 22.8. MS (EI) m/z: 175.1 [M+H]+.
The product is isolated in a yield of 0.30 g (1.57 mmol, 30%). H
NMR (500 MHz, CDCl3) δ: 1.69−1.76 (m, 1H), 2.55−2.61 (m, 1H),
3.10−3.17 (m, 1H), 3.37 (s, 3H), 3.45−3.53 (m, 1H), 4.39 (t, 3J = 7.5
Hz, 1H), 7.32 (t, 3J = 7.6 Hz, 1H), 7.54 (d, 3J = 7.4 Hz, 1H), 7.91 (d,
3J = 7.9 Hz, 1H).
Biology. General Remarks. All transaminase plasmids are
purchased from Thermo Scientific as codon-optimized genes and
overexpressed in E. coli BL21(DE3). Transaminases are used as a
lyophilized crude extract. The CAL-B (Candida antarctica lipase B) is
purchased from Sigma-Aldrich.
Ethyl-3-((2,3-dihydro-1H-inden-1-yl)amino)-3-oxopropanoate
(31). The product is isolated in a yield of 76 mg (0.32 mmol, 86%).
1H NMR (500 MHz, CDCl3) δ: 1.28 (t, 3J = 7.1 Hz, 3H), 1.85−1.90
(m, 1H), 2.62−2.66 (m, 1H), 2.88−2.94 (m, 1H), 3.00−3.05 (m,
1H), 3.37 (d, 4J = 2.2 Hz, 2H), 4.19 (q, 3J = 7.1 Hz, 2H), 5.51 (q, 3J =
7.7 Hz, 1H), 7.19−7.26 (m, 3H), 7.28−7.32 (m, 1H).
N-(2,3-Dihydro-1H-inden-1-yl)-2-methoxyacetamide (30). The
1
Activity Assay for Transaminases. The activity of the trans-
aminase is measured by a photometer assay. The U mg−1 of the ω-
transaminase is assayed employing enantiomerically pure 1-phenyl-
ethylamine (S)-3 as the substrate (2.5 mM) for VF-TA, sodium
pyruvate (2.5 mM) as an acceptor, and pyridoxal phosphate (PLP, 0.1
mM) in phosphate buffer (0.1 M, pH 8.0). A typical sample is
prepared using 1 mg of lyophilized crude enzyme in phosphate buffer
(1 mL, pH 8, 100 mM). The substrate/buffer solution (980 μL) is
heated to 30 °C. The cuvette is filled to 1 mL with enzyme solution.
The activity is measured by means of the photometer by UV
detection (245 nm) in time-course mode.
General Procedures. Transamination with Vibrio fluvialis
Transaminase (VF-TA). L-Alanine (275 mM) and glucose (160
mM) are dissolved in a phosphate buffer (0.1 M, pH 8.0). To the
solution are given the lyophilizates of transaminase (575 U/mmol
substrate) and GDH (1230 U/mmol substrate). LDH (575 U/mmol
substrate), organic solvent (MeOH 25% v/v of buffer), solution of
PLP (12.5% v/v of buffer, 10 mM, pH 8.0), and acetophenone (50
mM) are added to the suspension. The reaction mixture is stirred for
24 h at 30 °C. After the complete reaction time, the mixture is
incubated by hydrochloric acid (50% v/v, 1 M) for 19 h at 30 °C. The
pH is changed to 14 with sodium hydroxide solution (10 M) before
the suspension is extracted by methylene chloride (three times). The
combined organic layers are dried with Na2SO4 and separated from
the solvent in vacuum. Conversion is calculated by the NMR
spectrum, and enantiomeric excess is determined after derivatization
to the corresponding amide by chiral HPLC. The results are shown in
Table S1.
Transamination with Arthrobacter sp. Transaminase in DMSO.
First, isopropylamine (75 μL, 0.90 mmol, 2.1 eq) is dissolved in
DMSO (3.0 mL). The solution is added to phosphate buffer (540 μL,
0.1 M, pH 8.0). To the solution is given the lyophilizate of the
transaminase of Arthrobacter sp. (76 mg), and PLP solution (4.3 mL,
10 mM, pH 8.0) and acetophenone (50 μL, 0.43 mmol, 1.00 eq) are
added to the suspension. The reaction mixture is stirred for 24 h at 45
°C. After the corresponding reaction time, the mixture is extracted by
methylene chloride (3 × 10 mL). The combined organic layers are
dried with Na2SO4 and separated from the solvent in vacuum.
Conversion is calculated according to NMR spectrum, and
enantiomeric excess is determined after derivatization to the
corresponding amide by chiral HPLC. The results are shown in
Table S2.
product is isolated in a yield 102 mg (0.50 mmol, 67%). H NMR
(500 MHz, CDCl3) δ: 1.85−1.90 (m, 1H), 2.63−2.70 (m, 1H),
4
2.90−2.96 (m, 1H), 3.02−3.07 (m, 1H), 3.41 (s, 3H), 3.97 (d, J =
1.1 Hz, 2H), 5.55 (q, 3J = 7.9 Hz, 1H), 7.21−7.25 (m, 4H). 13C{1H}-
NMR (126 MHz, CDCl3): 169.5, 143.5, 143.6, 128.1, 126.9, 124.9,
124.2, 72.1, 59.3, 54.2, 34.2, 30.4. MS (EI) m/z: 205.1 [M+H]+
N-(4-Bromo-2,3-dihydro-1H-inden-1-yl)acetamide (37). The
product is isolated in a yield of 65 mg (0.38 mmol, 51%). 1H
NMR (500 MHz, CDCl3) δ: 1.80−1.86 (m, 1H), 2.09 (s, 3H) 2.60−
3
2.66 (m, 1H), 2.85−2.91 (m, 1H), 3.04−3.10 (m, 1H), 5.55 (q, J =
7.7 Hz, 1H), 7.09 (t, 3J = 7.6 Hz, 1H), 7.22 (d, 3J = 7.4 Hz, 1H), 7.22
3
(d, J = 7.39 Hz, 1H). 13C{1H}-NMR (126 MHz, CDCl3): 170.0,
145.3, 143.8, 131.3, 128.8, 123.1, 120.4, 55.7, 33.1, 31.7, 23.5. MS
(EI) m/z: 253.0 [M+H]+.
Ethyl 3-((4-Bromo-2,3-dihydro-1H-inden-1-yl)amino)-3-oxopro-
panoate (33). The product is isolated in a yield of 76 mg (0.32
1
3
mmol, 86%). H NMR (500 MHz, CDCl3) δ: 1.27 (t, J = 7.2 Hz,
3H),. 1.85−1.93 (m, 1H), 2.61−2.66 (m, 1H), 2.86−2.92 (m, 1H),
4
3
3.06−3.12 (m, 1H), 3.36 (d, J = 1.3 Hz, 2H), 4.18 (q, J = 7.1 Hz,
2H), 5.56 (q, 3J = 7.7 Hz, 1H), 7.08 (t, 3J = 7.6 Hz, 1H), 7.22 (d, 3J =
7.4 Hz, 1H), 7.39 (d, J = 7.39 Hz, 1H). 13C-{1H}-NMR (126 MHz,
3
CDCl3) δ: 169.7, 169.7, 145.0, 143.8, 131.3, 128.8, 123.1, 120.4, 61.8,
55.7, 41.2, 33.1, 31.7, 14.2. MS (EI) m/z: 325.0 [M+H]+.
N-(4-Bromo-2,3-dihydro-1H-inden-1-yl)-2-methoxyacetamide
(32). The product is isolated in a yield of 11 mg (0.04 mmol, 50%).
1H NMR (500 MHz, CDCl3) δ: 1.79−1.85 (m, 1H), 2.56−2.62 (m,
1H), 2.82−2.89 (m, 1H), 3.98−3.05 (m, 1H), 3.34 (s, 3H), 3.90 (d,
4J = 1.1 Hz, 2H), 5.54 (q, 3J = 7.9 Hz, 1H), 7.03 (t, 3J = 7.6 Hz, 1H),
3
3
7.15 (d, J = 7.4 Hz, 1H), 7.34 (d, J = 7.9 Hz, 1H). 13C{1H}-NMR
(126 MHz, CDCl3) δ: 169.5, 145.0, 143.8, 131.3, 128.8, 123.1, 120.4,
72.1, 59.3, 59.3, 33.0, 31.8. MS (EI) m/z: 283.0 [M+H]+.
N-(4-Cyano-2,3-dihydro-1H-inden-1-yl)acetamide (37). The
product is isolated in a yield of 10 mg (0.05 mmol, 60%). 1H
NMR (500 MHz, CDCl3) δ: 1.87−1.92 (m, 1H), 2.14 (s, 3H) 2.63−
3
2.69 (m, 1H), 2.87−2.93 (m, 1H), 3.08−3.15 (m, 1H), 5.55 (q, J =
3
3
7.7 Hz, 1H), 7.30 (dd, J = 7.8 Hz, 1H), 7.52 (t, J = 7.8 Hz, 2H).
13C{1H}-NMR (126 MHz, CDCl3) δ: 169.6, 147.4, 144.9, 131.7,
128.8, 127.9, 117.6, 109.3, 53.8, 33.4, 30.0, 23.5. MS (EI) m/z: 253.0
[M+H]+, 255.0 [M+H]+.
Ethyl 3-((4-Cyano-2,3-dihydro-1H-inden-1-yl)amino)-3-oxopro-
panoate (34). The product is isolated in a yield of 15 mg (0.06
1
3
mmol, 69%). H NMR (500 MHz, CDCl3) δ: 1.28 (t, J = 7.2 Hz,
3H) 1.93−2.00 (m, 1H), 2.68−2.73 (m, 1H), 3.03−3.07 (m, 1H),
General Procedures. Derivatization for Calculation of
Enantiomeric Excess (Transaminase). DMAP (0.8 equiv) is
dissolved in acetic anhydride (20.0 equiv). The amine (1.0 equiv)
is dissolved in EtOAc and given to the DMAP solution. After the
mixture is stirred at room temperature, the reaction is quenched with
4
3
3.21−3.26 (m, 1H), 3.41 (d, J = 2.2 Hz, 2H), 4.19 (q, J = 7.1 Hz,
2H), 5.56 (q, 3J = 7.7 Hz, 1H), 7.30 (dd, 3J = 7.7 Hz, 1H) 7.52 (dt, 3J
= 7.7 Hz, 2H). 13C{1H}-NMR (126 MHz, CDCl3) δ: 172.9, 169.8,
H
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
147.4, 144.8, 131.7, 128.9, 127.9, 117.6, 59.3, 53.9, 42.7, 33.3, 30.3,
14.2. MS (EI) m/z: 325.0 [M+H]+, 327.0 325.0 [M+H]+.
are dissolved in the appropriate solvent with acyl donor (1.1 equiv).
The reactions are carried out in screw cap glasses with a volume of 5
mL. CAL-B (40 mg/mmol amine) is added to the reaction solution.
After 26 h, a sample is taken, which then is acetylated with acetyl
chloride (1.1 equiv) and triethylamine (1.5 equiv) in methylene
chloride for 1 h. The suspension is washed with hydrogen chloride
(1:1 v/v). The solvent is removed in vacuo. Enantiomeric excess and
conversion are determined via HPLC. The results are shown in Table
S6.
N-(4-Cyano-2,3-dihydro-1H-inden-1-yl)-2-methoxyacetamide
(9). The product is isolated in a yield of 12 mg (0.05 mmol, 64%). 1H
NMR (500 MHz, CDCl3) δ: 1.92−2.00 (m, 1H), 2.67−2.71 (m, 1H),
4
3.07−3.12 (m, 1H), 3.21−3.25 (m, 1H), 3.40 (s, 3H), 3.96 (d, J =
3
3
2.5 Hz, 2H), 5.58 (q, J = 8.1 Hz, 1H), 7.30 (dd, J = 7.67 Hz, 1H),
7.51 (dd, J = 7.4 Hz, 2H). 13C{1H}-NMR (126 MHz, CDCl3) δ:
3
169.6, 147.3, 144.7, 131.6, 128.8, 127.7, 117.4, 109.2, 71.8, 59.2, 53.7,
33.2, 29.9. MS (ESI) m/z: 253.1 [M+Na]+, 483.1 [2M+Na]+. IR
(neat)/cm−1: 3216, 2920, 2850, 2222, 1654, 1541, 1447, 795. Anal.
Calcd for C13H14N2O2: C, 67.81; H, 6.13; N, 12.17. Found: C, 67.83;
H, 6.21; N, 11.73.
CAL-B-Catalyzed Reaction with 4-Cyano-1-aminoindane. 4-
Cyano-1-indanone (4, 401 mg, 2.551 mmol) and hydroxylamine
hydrochloride (0.27 g, 3.84 mmol) are dissolved in 10 mL of ethanol
and water (1:1 v/v). Meanwhile, sodium hydroxide (0.17 g, 4.51
mmol) dissolved in 1 mL water is added to the suspension. The
mixture is heated to reflux for 90 min. The crude product is filtered
over Celite and washed with water. The oxime is dissolved in 10 mL
of acetic acid under an argon atmosphere, zinc dust (0.83, 12.77
mmol) is added, and the suspension is stirred at room temperature for
86 h. The reaction mixture is filtered over Celite and washed with
ethyl acetate, and the solvent is removed in vacuo. The oil is dissolved
in 10 mL of ethyl acetate and hydrogen chloride (1:1 v/v) and
extracted with 2 M hydrogen chloride (2 × 10 mL). The pH value of
the aqueous phase is adjusted to 10 and afterward extracted with ethyl
acetate (3 × 10 mL). The organic phase is washed with brine and
dried over MgSO4. The solvent is removed in vacuo. The remaining
solvent is removed in a Schlenk flask under an argon atmosphere,
furnishing racemic 4-cyano-1-aminoindane (rac-2, 150 mg, 0.948
mmol) in 37% yield. Next, racemic 4-cyano-1-aminoindane (rac-2,
150 mg, 0.948 mmol), ethyl methoxyacetate (29, 0.11 g, 0.99 mmol),
and CAL-B (60 mg) are dissolved in 2-MTHF (5 mL) and stirred at
60 °C for 20 h. The sample is acetylated with acetyl chloride (1.1
equiv) and triethylamine (1.5 equiv) in methylene chloride for 1 h.
The suspension is washed with hydrogen chloride (1:1 v/v) and
sodium hydrogen carbonate. The solvent is removed in vacuo.
Enantiomeric excess and conversion are determined via HPLC. The
desired product (S)-4-cyano-1-aminoindane ((S)-2, 75 mg, 0.474
mmol) is isolated in a yield of 50% (corresponding to an overall yield
of 19% for the two steps when starting from 4) and with an ee of 99%,
corresponding to an E value of 24.
Methyl-1-acetamido-2,3-dihydro-1H-indene-4-carboxylate (39).
1
The product is isolated in a yield of 7 mg (0.03 mmol, 40%). H
NMR (500 MHz, CDCl3) δ: 1.80−1.85 (m, 1H), 2.04 (s, 3H) 2.61−
2.66 (m, 1H), 3.12−3.18 (m, 1H), 3.33 (s, 3H), 3.35−3.40 (m, 1H),
3
5.50 (q, 3J = 7.9 Hz, 1H), 7.29 (t, 3J = 7.6 Hz, 1H), 7.48 (d, J = 7.5
Hz, 2H), 7.48 (d, 3J = 7.8 Hz, 1H). 13C{1H}-NMR (126 MHz,
CDCl3) δ: 170.0, 167.3, 145.8, 145.1, 130.1, 128.6, 127.1, 126.9, 54.4,
52.0, 33.7, 23.6. MS (EI) m/z: 233.1 [M+H]+.
Methyl 1-(2-Methoxyacetamido)-2,3-dihydro-1H-indene-4-car-
boxylate (35). The product is isolated in a yield of 5 mg (0.03
1
mmol, 52%). H NMR (500 MHz, CDCl3) δ: 1.85−1.90 (m, 1H),
2.62−2.68 (m, 1H), 3.11−3.16 (m, 1H), 3.40 (s, 3H), 3.43−3.49 (m,
1H), 3.47 (d, 3J = 5.3 Hz, 2H), 5.55 (q,3J = 8.1 Hz, 1H), 7.30 (t, 3J =
3
3
7.7 Hz, 1H), 7.47 (d, J = 7.5 Hz, 1H), 7.92 (d, J = 7.7 Hz, 1H).
13C{1H}-NMR (126 MHz, CDCl3) δ: 171.8, 169.6, 145.8, 144.8,
130.6, 130.1, 128.7, 127.2, 72.0, 53.6, 52.0, 33.6, 31.4. MS (EI) m/z:
263.1 [M+H]+.
General Procedure. Resolution of 1-Aminoindane Catalyzed
by Lipases. 1-Aminoindane (rac-24, 1.0 equiv) and the acyl donor
(1.1 equiv) are dissolved in organic solvent. Lipase is added and
heated to 60 °C. At fixed times, samples are taken. The samples are
acetylated with acetyl chloride (1.1 equiv) and triethylamine (1.5
equiv) in methylene chloride for 1 h. The suspension is washed with
hydrogen chloride (1:1 v/v). The solvent is removed in vacuo.
Enantiomeric excess and conversion are determined via HPLC.
Solvent, Acyl Donor, Temperature Screening of CAL-B-Catalyzed
Reactions. For the determination of optimal solvent, 1-aminoindane
(rac-24, 30 mg, 0.22 mmol) and diethyl malonate (28, 40 mg, 0.25
mmol) or ethyl methoxyacetate, (27, 29 mg, 0.29 mmol) are dissolved
in the solvent (384 μL), and CAL-B (9 mg) is added. The reactions
are carried out in screw cap glasses of 5 mL. The following solvents
are tested: toluene, MTBE, 2-MTHF, MCH, and n-heptane. The
reaction with isopropyl methoxyacetate (29, 25 mg, 0.19 mmol) is
carried in 2-MTHF (384 μL). Samples are taken after 6, 26, and 48 h.
The enzyme is filtered and washed with DCM. These samples are
acetylated with acetyl chloride (1.1 equiv) and triethylamine (1.5
equiv) in methylene chloride for 1 h. The suspension is washed with
hydrogen chloride (1:1 v/v). The solvent is removed in vacuo.
Enantiomeric excess and conversion are determined via HPLC. The
Recycling Experiments with CAL-B. 1-Aminoindane (rac-24, 67
mg, 0.50 mmol), isopropyl methoxyacetate (29, 73 mg, 0.55 mmol),
and CAL-B (20 mg) are dissolved in 5 mL of toluene and stirred at 60
°C. After 7 h, a 200 μL sample is taken. The sample is acylated with
acetyl chloride (9 mg, 0.11 mmol) and triethylamine (15 mg, 0.15
mmol). The reaction is repeated for several days. The sample is
acetylated with acetyl chloride (1.1 equiv) and triethylamine (1.5
equiv) in methylene chloride for 1 h. The suspension is washed with
hydrogen chloride (1:1 v/v) and sodium hydrogen carbonate. The
solvent is removed in vacuo. Enantiomeric excess and conversion are
determined via HPLC. The results are shown in Table S7.
ASSOCIATED CONTENT
* Supporting Information
■
1
conversion is carried out using H NMR spectroscopy and HPLC.
S
The enantiomeric excess is determined via chiral HPLC. The results
are shown in Table S4.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.8b03290.
CAL-B-Catalyzed Reaction with 1 M Substrate Loading. For the
determination of CAL-B-catalyzed reaction with 1 M substrate
loading, 1-aminoindane (rac-24, 665 mg, 5.03 mmol), ethyl
methoxyacetate (28, 770 mg, 7.09 mmol), and CAL-B (0.20 g) are
dissolved in 2-MTHF (5 mL). The reaction is stirred at 60 °C.
Samples are taken after 6 and 26 h. The enzyme is filtered off and
washed with DCM. This sample is acetylated with acetyl chloride (1.1
equiv) and triethylamine (1.5 equiv) in methylene chloride for 1 h.
The suspension is washed with hydrogen chloride (1:1 v/v). The
solvent is removed in vacuo. Enantiomeric excess and conversion are
Experimental procedures related to the conducted
biotransformations as well as NMR, GC, and HPLC
data (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: harald.groeger@uni-bielefeld.de.
■
1
determined via HPLC. The conversion is determined via H NMR
ORCID
spectroscopy and HPLC. The enantiomeric excess is determined via
chiral HPLC. The results are shown in Table S5.
CAL-B-Catalyzed Reaction with 1-Aminoindane Derivatives. For
the CAL-B-catalyzed reactions, 1-aminoindane derivatives (1.0 equiv)
̈
Harald Groger: 0000-0001-8582-2107
Author Contributions
§F.U. and J.L. contributed equally.
I
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Squire, C.; Straatman, H.; Wells, A. S. Efficient Synthesis of (S)-1-(5-
Fluoropyrimidin-2-yl)ethylamine Using an ω-Transaminase Biocata-
lyst in a Two-Phase System. Org. Process Res. Dev. 2013, 17, 1117−
1122. (d) Nobili, A.; Steffen-Munsberg, F.; Kohls, H.; Trentin, I.;
Funding
This research was financially supported by the company
PharmaZell GmbH, Raubling, Germany.
Notes
̈
Schulzke, C.; Hohne, M.; Bornscheuer, U. T. Engineering the Active
The authors declare no competing financial interest.
Site of the Amine Transaminase from Vibrio fluvialis for the
Asymmetric Synthesis of Aryl−Alkyl Amines and Amino Alcohols.
ChemCatChem 2015, 7, 757−760. (e) Pavlidis, I. V.; Weiß, M. S.;
Genz, M.; Spurr, P.; Hanlon, S. P.; Wirz, B.; Iding, H.; Bornscheuer,
U. T. Identification of (S)-selective transaminases for the asymmetric
synthesis of bulky chiral amines. Nat. Chem. 2016, 8, 1076−1082.
(f) Cuetos, A.; Garcia-Ramos, M.; Fischereder, E.; Diaz-Rodriguez,
A.; Grogan, G.; Gotor, V.; Kroutil, W.; Lavandera, I. Catalytic
Promiscuity of Transaminases: Preparation of Enantioenriched Beta-
Fluoroamines by Formal Tandem Hydrodefluorination/Deamination.
Angew. Chem., Int. Ed. 2016, 55, 3144−3147.
REFERENCES
■
(1) Brinkmann, V. Sphingosine 1-phosphate receptors in health and
disease: mechanistic insights from gene deletion studies and reverse
pharmacology. Pharmacol. Ther. 2007, 115, 84−105.
(2) Cohen, J. A.; Arnold, D. L.; Comi, G.; Bar-Or, A.; Gujrathi, S.;
Hartung, J. P.; Cravets, M.; Olson, A.; Frohna, P. A.; Selmaj, K. W.
Safety and efficacy of the selective sphingosine 1-phosphate receptor
modulator ozanimod in relapsing multiple sclerosis (RADIANCE): a
randomised, placebo-controlled, phase 2 trial. Lancet Neurol. 2016, 15,
373−381.
(3) Scott, F. L.; Clemons, B.; Brooks, J.; Brahmachary, E.; Powell, R.;
Dedman, H.; Desale, H. G.; Timony, G. A.; Martinborough, E.;
Rosen, H.; et al. Ozanimod (RPC1063) is a potent sphingosine-1-
phosphate receptor-1 (S1P1) and receptor-5 (S1P5) agonist with
autoimmune disease-modifying activity. Br. J. Pharmacol. 2016, 173,
1778−1792.
(4) Sandborn, W. J.; Feagan, B. G.; Wolf, D. C.; D’Haens, G.;
Vermeire, S.; Hanauer, S. B.; Ghosh, S.; Smith, H.; Cravets, M.;
Frohna, P. A.; et al. Ozanimod Induction and Maintenance Treatment
for Ulcerative Colitis. N. Engl. J. Med. 2016, 374, 1754−1762.
(5) Jarvis, L. ACQUISITIONS Celgene buys Receptos to capture
potential immunology drug. Chem. Eng. News 2015, 93 (29), 9.
(6) Martinborough, E.; Boehm, M.; Yeager, A. R.; Tamiya, J.;
Huang, L.; Brahmachary, E.; Moorjani, M.; Timony, G. A.; Brooks, J.;
Peach, R.; et al. Selective Sphingosine 1-Phosphate Receptor
Modulators and Combination Therapy Therewith. Patent
WO2015066515A1, 2013.
(13) Breuer, M.; Ditrich, K.; Habicher, T.; Hauer, B.; Keßeler, M.;
Sturmer, R.; Zelinski, T. Industrial Methods for the Production of
̈
Optically Active Intermediates. Angew. Chem., Int. Ed. 2004, 43, 788−
824.
(14) (a) Balkenhohl, F.; Hauer, B.; Landner, W.; Pressler, U.
̈
̈
Racematspaltung primarer und sekundarer Amine durch Enzym-
katalysierte Acylierung. Patent DE19934332738, 1994. (b) Ditrich, K.
Verfahren zur Racemisierung optisch aktiver Amine. Patent
DE19606124 A1, 1996. (c) Balkenhohl, F.; Ditrich, K.; Hauer, B.;
Ladner, W. Optisch aktive Amine durch Lipase-katalysierte
Methoxyacetylierung. J. Prakt. Chem./Chem.-Ztg. 1997, 339, 381−
384. (d) Ditrich, K. Optically Active Amines by Enzyme-Catalyzed
Kinetic Resolution. Synthesis 2008, 2008, 2283−2287.
(15) Gillen, K. J.; Gillespie, J.; Jamieson, C.; Maclean, J. K. F.; Moir,
E. M.; Rankovic, Z. Indane Derivatives. PCT Patent Applic.-
WO2010115952, 2010.
̈
(16) Uthoff, F.; Reimer, A.; Liese, A.; Groger, H. Enantioselective
synthesis of chiral amines through enzymatic resolution under
solvent-free conditions with malonate as reagent for acylation. Sustain.
Chem. Pharm. 2017, 5, 42−45.
(7) Reviews on the enzymatic synthesis of amines: (a) Guo, F.;
Berglund, P. Transaminase biocatalysis: optimization and application.
Green Chem. 2017, 19, 333−360. (b) Hohne, M.; Bornscheuer, U. T.
Biocatalytic Routes to Optically Active Amines. ChemCatChem 2009,
̈
̈
(17) Simon, S.; Oßwald, S.; Roos, J.; Groger, H. Efficient Enzymatic
Amine Resolution at High Substrate Input Using Diethyl Malonate as
an Acyl Donor of Low Hazard Potential. Z. Naturforsch., B: J. Chem.
Sci. 2012, 67b, 1123−1126.
̈
1, 42−51. (c) Buchholz, S.; Groger, H. Biocatalytic Concepts for the
Synthesis of Optically Active Amines. In Biocatalysis in the
Pharmaceutical and Biotechnology Industries; Patel, R. N., Ed.; CRC
Press: New York, 2006; Chapter 34, pp 829−847.
(18) Passos, H.; Freire, M. G.; Coutinho, J. A. P. Ionic liquid
solutions as extractive solvents for value-added compounds from
biomass. Green Chem. 2014, 16, 4786−4815.
(8) Menzek, A.; Altundas, A.; Gultekin, D. A new, safe and
̈
convenient procedure for reduction of naphthalene and anthracene:
synthesis of tetralin in a one-pot reaction. J. Chem. Res. 2003, 2003,
752−753.
(19) Anderson, E. M.; Larsson, K. M.; Kirk, O. One Biocatalyst−
Many Applications: The Use of Candida Antarctica B-Lipase in
Organic Synthesis. Biocatal. Biotransform. 1998, 16, 181−204.
(9) Tan, J.; Zhou, Z.; Yan, J.; Zhang, M.; Wang, J. Novel Chemical
Total Synthesis Preparation Method for Coronalon. PatentCN
201110361988, 2011.
̀
(20) Poulhes, F.; Vanthuyne, N.; Bertrand, M. P.; Gastaldi, S.; Gil,
G. Chemoenzymatic Dynamic Kinetic Resolution of Primary Amines
Catalyzed by CAL-B at 38−40 °C. J. Org. Chem. 2011, 76, 7281−
7286.
(10) Wu, W.; Verkade, J. G. EtNP(NMe2)NP(NMe2)3: An
efficient non-ionic base catalyst for the isomerization of allylic
compounds and methylene-interrupted dienes. ARKIVOC 2004, 88−
95.
(21) Aycock, D. F. Solvent Applications of 2-Methyltetrahydrofuran
in Organometallic and Biphasic Reactions. Org. Process Res. Dev. 2007,
11, 156−159.
(11) Guney, M.; Cosķ un, A.; Topal, F.; Dasţ an, A.; Gulci̧ n, I.;
̈ ̈
́
̈
(22) Thalen, L. K.; Backvall, J. E. Development of dynamic kinetic
resolution on large scale for ( )-1-phenylethylamine. Beilstein J. Org.
Chem. 2010, 6, 823−829.
Supuran, C. T. Oxidation of cyanobenzocycloheptatrienes: Synthesis,
photooxygenation reaction and carbonic anhydrase isoenzymes
inhibition properties of some new benzotropone derivatives. Bioorg.
Med. Chem. 2014, 22, 3537−3543.
(23) Bornscheuer, U. T.; Kazlauskas, R. J. Hydrolases in Organic
Synthesis: Regio- and Stereoselective Biotransformations, 2nd ed.; Wiley
and Sons: Weinheim, Germany, 2006.
(12) For selected examples of reductive ketone aminations using
transaminases, see: (a) Shin, J. S.; Yun, H.; Jang, J. W.; Park, I.; Kim,
B. G. Purification, characterization, and molecular cloning of a novel
amine:pyruvate transaminase from Vibrio fluvialis JS17. Appl.
Microbiol. Biotechnol. 2003, 61, 463−471. (b) Savile, C. K.; Janey, J.
M.; Mundorff, E. C.; Moore, J. C.; Tam, S.; Jarvis, W. R.; Colbeck, J.
C.; Krebber, A.; Fleitz, F. J.; Brands, J.; Devine, P. N.; Huisman, G.
W.; Hughes, G. J. Biocatalytic asymmetric synthesis of chiral amines
from ketones applied to sitagliptin manufacture. Science 2010, 329,
(24) Yoo, B. I.; Kim, Y. J.; You, Y.; Yang, J. W.; Kim, S. W. Birch
Reduction of Aromatic Compounds by Inorganic Electride [Ca2N]+·
in an Alcoholic Solvent: An Analogue of Solvated Electrons. J. Org.
Chem. 2018, 83, 13847−13853.
(25) Cheng, W.-M.; Shang, R.; Fu, Y. Irradiation-induced palladium-
catalyzed decarboxylative desaturation enabled by a dual ligand
system. Nat. Commun. 2018, 9, 5215−5224.
(26) Genrich, F.; Harms, G.; Schaumann, E.; Gjikaj, M.; Adiwidjaja,
G. Functionalized esters as bis-electrophiles in a silicon-induced
305−309. (c) Meadows, R. E.; Mulholland, K. R.; Schu
Golden, M.; Kierkels, H.; Meulenbroeks, E.; Mink, D.; May, O.;
̈
rmann, M.;
J
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
domino synthesis of annulated carbocycles. Tetrahedron 2009, 65,
5577−5587.
(27) Krumm, T.; Bandemer, K.; Boland, W. Induction of volatile
biosynthesis in the Lima bean (Phaseolus lunatus) by leucine- and
isoleucine conjugates of 1-oxo- and 1-hydroxyindan-4-carboxylic acid:
evidence for amino acid conjugates of jasmonic acid as intermediates
in the octadecanoid signalling pathway. FEBS Lett. 1995, 377, 523−
529.
̇
̈ ̈
̈
(28) Akbaba, Y.; Akıncıoglu, A.; Gocȩ r, H.; Goksu, S.; Gulci̧ n, I.;
̌
Supuran, C. T. Carbonic anhydrase inhibitory properties of novel
sulfonamide derivatives of aminoindanes and aminotetralins. J.
Enzyme Inhib. Med. Chem. 2014, 29, 35−42.
K
DOI: 10.1021/acs.joc.8b03290
J. Org. Chem. XXXX, XXX, XXX−XXX