Lipase catalyzed resolution of the quaternary stereogenic center in ketone-derived benzo-fused cyclic cyanohydrins

Article abstract of DOI:10.1016/j.tetasy.2011.07.004

The enantioselective preparation of both enantiomers of the benzo-fused cyanohydrin derived from atetralone via lipase catalyzed processes is described. The (S)-enantiomer has been obtained as the remaining substrate in the CAL-A catalyzed aminolysis of the methoxyacetylated derivative, using a structurally related amine as the nucleophile. The (R)-enantiomer has been obtained as the product of CAL-A or PSL-C catalyzed hydrolysis of the methoxyacetylated derivative in organic solvents. The resolutions of several structural analogs of this cyanohydrin have been also tested under similar conditions.

Full text of DOI:10.1016/j.tetasy.2011.07.004

Tetrahedron: Asymmetry 22 (2011) 1218–1224
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Lipase catalyzed resolution of the quaternary stereogenic center
in ketone-derived benzo-fused cyclic cyanohydrins
⇑
⇑
Jesús A. Rodríguez-Rodríguez, Vicente Gotor , Rosario Brieva
Departamento de Química Orgánica e Inorgánica and Instituto Universitario de Biotecnología de Asturias, Universidad de Oviedo, 33006 Oviedo (Asturias), Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The enantioselective preparation of both enantiomers of the benzo-fused cyanohydrin derived from
a-
Received 30 May 2011
Accepted 1 July 2011
Available online 4 August 2011
tetralone via lipase catalyzed processes is described. The (S)-enantiomer has been obtained as the remain-
ing substrate in the CAL-A catalyzed aminolysis of the methoxyacetylated derivative, using a structurally
related amine as the nucleophile. The (R)-enantiomer has been obtained as the product of CAL-A or PSL-C
catalyzed hydrolysis of the methoxyacetylated derivative in organic solvents. The resolutions of several
structural analogs of this cyanohydrin have been also tested under similar conditions.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
and performing an enzyme-catalyzed acylation of an hydroxyl
group remote from the stereogenic center.7
Optically active cyanohydrins have considerable synthetic po-
tential in organic synthesis because they can be readily trans-
formed into a wide variety of chiral molecules, such as hydroxyl
carbonyl compounds or b-aminoalcohols.1 In particular, ketone-
derived cyanohydrins are valuable intermediates in the prepara-
tion of compounds with a quaternary stereocenter.
With the aim of extending the biocatalytic methods to the reso-
lution of new ketone-derived cyanohydrin structures, we focused
our attention in the benzo-fused cyanohydrin derivative of a-tetra-
lone. This cyanohydrin is an intermediate for the preparation of the
spirooxazolidinediones, described as therapeutic agents for the
treatmentof chronic diabetic complications,8 or amidino derivatives
that can be competitive inhibitors of trypsin-like proteases, such as
thrombin.9 Herein, we report a study of lipase catalyzed acetylation
of 1-hydroxy-1,2,3,4-tetrahydronaphthalene carbonitrile and the
hydrolysis of several acylated derivatives. The best found conditions
were also tested on several structural analogs of this cyanohydrin.
The stereoselective synthesis of aldehyde-derived cyanohydrins
has been extensively investigated, but only a few approaches to
the synthesis of ketone-derived cyanohydrins have been de-
scribed. Together with the Lewis acid catalyzed asymmetric syn-
thesis,1c biocatalytic methods represent a promising but also
challenging alternative for the preparation of enantiomerically
pure ketone-derived cyanohydrins.2 The kinetic resolution of steri-
cally demanding cyanohydrins, as a particular type of tertiary alco-
hols,3 has been scarcely reported and, in general, attempts
resolution with commercially available lipases or proteases4 and
whole cells5 are limited to a small number of structures and, usu-
ally, with only moderate success. Recently, the resolution of sev-
eral acetates of tertiary alcohols including ketone-derived
cyanohydrins by recombinant carboxyl estearases has been stud-
ied. Most of the alcohols were resolved with high enantiopurity
but, in the case of cyanohydrins, only the resolution of 1-cyano-
2,2,2-trifluoro-1-phenyl acetate has been successfully carried
out.6 In a previous report, we have also observed that the lipase
catalyzed acylation of 1-cycloalkyl-1-hydroxy-1-phenylcyanohyd-
rins by several lipases did not give any conversion. Nevertheless
the adverse steric interactions caused by the quaternary stereo-
genic center could be circumvented by extending the substrate
2. Results and discussion
Our initial experiments were designed to test the lipase
catalyzed acetylation of 1-hydroxy-1,2,3,4-tetrahydronaphthalene
carbonitrile ( )-2a which has been synthesized in good yield from
a-tetralone, 1a (Scheme 1).
From the three lipases tested, only lipase A from Candida ant-
arctica (CAL-A) catalyzed the reaction. No conversion was ob-
served in the processes catalyzed by lipase B from Candida
antarctica (CAL-B) or the immobilized form of the Pseudomonas
cepacia lipase, PSL-C.
The reaction catalyzed by CAL-A showed very high enantiose-
lectivity and the enantiomerically pure acylated product (R)-3a
was obtained. The assignment of the absolute configurations of
the products and the remaining substrate is described in the Sec-
tion 4. However, the conversion in this acylation process was
very low, only 13% yield after 6 days of reaction. Similar results
were obtained in different solvents, such as 1,4-dioxane, tert-
butyl methyl ether or vinyl acetate. In all of the conditions tested,
⇑
Corresponding authors. Tel.: +34 985 102 994; fax: +34 985 103 448.
E-mail addresses: vgs@uniovi.es (V. Gotor), rbrieva@uniovi.es (R. Brieva).
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.07.004

J. A. Rodríguez-Rodríguez et al. / Tetrahedron: Asymmetry 22 (2011) 1218–1224
1219
NC OH
O
of 1-cycloalkyl-1-hydroxy-1-phenylcyanohydrins.7 CAL-A and PSL-
C catalyzed the hydrolysis of substrate ( )-4a and showed the
same stereochemical preference. However, the expected hydrox-
yacetate derivative 5a was not the main product of the reaction,
but the cyanohydrin (R)-2a and the ketone 1a, that comes from
decomposition of the cyanohydrin. Depending on the biocatalyst
and the reaction conditions, a mixture of the three products was
achieved (Table 1).
1) (CH3)3SiCN/Cu(OTf)2 / H2CCl2
2) HCl 1M / CH3CN
87% yield
2a
( )-
1a
NC OH
NC
O
O
Vinyl acetate
+
( )-2a
1a
+
CAL-A / toluene
6 days, c =13%
recovered
yield > 80%
O
(R)-3a
(S)-2a
ee = 15 %
ee > 99 (%)
O
O
NC
O
NC OH
O
Cl
Scheme 1. Enzymatic acetylation of ( )-2.
O
O
Py / CH2Cl2
most of the remaining cyanohydrin decomposed into the
a-tetra-
lone 1a. Increasing the temperature or reaction time did not im-
prove the conversion because of the competing dehydrocyanation
( )-2a
4a
( )-
Lipase
reaction; more than 80% of
the cases.
a-tetralone 1a was recovered in all
H2O or buffer/
organic solvent
O
Another common approach for the resolution of racemic alco-
hols is the enzymatic hydrolysis of a suitable derivative. As a great
variety of derivatives can be tested, we carried out a preliminary
study in order to find an appropriate substrate for these reactions.
We focused especially substrates, which we had used in previous
enzymatic resolutions.
We started testing the enzymatic hydrolysis of the acetylate
derivative ( )-3a. Under the conditions shown in Scheme 2, only
CAL-A catalyzed the reaction. The enantioselectivity and the con-
version of the process were moderated, after 7 days of reaction a
37% yield of cyanohydrin was detected by HPLC with 76% enantio-
O
OH
O
NC
O
NC O
NC OH
+
O
O
+
+
(R)-2a
Scheme 3. Enzymatic hydrolysis of ( )-4.
1a
(S)-4a
(R)-5a
For both enzymes, when the hydrolysis was carried out using a
mixture of buffer: organic solvent 1:2 as the reaction media, the
cyanohydrin was completely converted into the ketone, even when
citrate buffer pH 5 was used (entries 1 and 3). When the process
was catalyzed by CAL-A (entry 3), it was fast and enantioselective,
after 5 h of reaction, the remaining substrate (S)-4a showed 93%
enantiomeric excess.
meric excess. A 9% yield of a-tetralone 1a was also detected. As in
the case of the acylation, attempts to improve the conversion of the
reaction were unsuccessful because of the increment of the cyano-
hydrin decomposition into the ketone.
When the reactions were carried out in an organic solvent using
5 equiv of H2O, the formation of the a-tetralone 1a can be partially
O
NC
O
NC
O
NC OH
O
O
CAL-A
+
+
avoided (entries 2, 4–6). Lipase PSL-C showed low enantiosectivity
under these conditions (entry 2). On the other hand, it is notewor-
thy that the high enantioselectivity achieved when the hydrolysis
catalyzed by CAL-A was carried out in tert-butyl methyl ether:
the remaining substrate (S)-4a showed 99% enantiomeric excess
and 12% of the enantiopure product (R)-5a was also achieved.
Even though the mixture of products obtained in these pro-
cesses has scarce practical utility, several interesting conclusions
can be established from the results obtained: both enzymes prefer-
ably catalyzed the hydrolysis of the ester closest to the quaternary
center and the remaining substrate (S)-4a can be obtained in high
enantiomeric purity in processes catalyzed by CAL-A.
5 equiv. H2O
tBuOEt
7 days
1a
3a
( )-3a
(S)-
(R)-2a
Yield: 9 %
ee:
54 %
55 %
37%
76%
Scheme 2. Enzymatic hydrolysis of ( )-3a.
Another attempt to improve the conversion by carrying out the
reaction in a mixture of 1,4-dioxane/buffer (2:1) was also unsuc-
cessful: after 6 days of reaction a 16% yield of
a-tetralone 1a was
detected and 84% yield of the racemic remaining substrate was
recovered.
Taking into account the results obtained with substrates ( )-3a
and ( )-4a, we selected the methoxyacetyl derivative ( )-6a as a
suitable substrate for our study (Scheme 4). The high reactivity
In order to avoid the possible steric hindrance, we proceeded to
modify the substrate by preparation of the derivative ( )-4a,
(Scheme 3). This strategy was applied in the enzymatic resolution
Table 1
Enzymatic hydrolysis of ( )-4, at 40 °C
Entry
Lipase
Organic solvent
H2O (equiv)
t (h)
Yielda (%) 1a
Yielda (%)
eea (%)
Yielda (%)
eea (%)
Yielda (%)
eea (%)
(R)-2a
(R)-2a
(S)-4a
(S)-4a
(R)-5a
(R)-5a
c
1
2
3
4
5
6
PSL-Cb
PSL-Cb
CAL-A
CAL-A
CAL-A
CAL-A
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
THF
40
34
58
8
14
5
—
66
—
48
35
51
—
8
—
54
76
37
60
—
42
44
44
32
11
—
93
76
86
99
—
—
—
—
7
—
—
—
—
>99
>99
5
156
5
144
168
48
c
5
5
5
tBuOMe
12
a
b
c
Determined by chiral HPLC.
Carried out at 30 °C.
1,4-Dioxane/citrate buffer pH 5 (2:1).

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J. A. Rodríguez-Rodríguez et al. / Tetrahedron: Asymmetry 22 (2011) 1218–1224
of this ester should improve the reaction rate in comparison with
the acetyl derivative. On the other hand, while the methoxyacetyl
derivative ( )-6a is structurally related to ( )-4a, only one hydro-
lytic product is possible.
of reaction were required in order to achieve 48% conversion (Ta-
ble 2, entry 4).
Taking into account the high reaction rate observed in diethyl
ether, we tested the process at a lower temperature (entry 6). Un-
der these conditions, the reaction was slower and no improvement
in the enantioselectivity was observed.
O
O
As for the previously tested substrates, the hydrolysis catalyzed
by PSL-C was slower than with CAL-A and conversions between
25% and 30% were obtained after longer reaction times, although
the enantioselectivities were higher (Table 2, entries 8–12). The
highest enantioselectivity (E = 93) was obtained in the reaction
carried out in 1,4-dioxane (Table 2, entry 8): the reaction product
can be obtained in 97% enantiomeric excess but the conversion
was only 27% after ten days of reaction. A longer reaction time of
16 days improved the conversion to 31%, but the enantiomeric
excess of the product slightly decreased (Table 2, entry 9). In gen-
eral, a small decrease in the enantioselectivity was observed in all
the processes catalyzed by PSL-C at longer times than those shown
in Table 2.
Taking into account that in organic solvents, different nucleo-
philes can be used instead of water, to optimize the biocatalytic sys-
tem, we examined the influence of the nucleophile in the alcoholysis
or aminolysis catalyzed by CAL-A (Scheme 5). We carried out the
reactions under the best found conditions for the hydrolysis.
The selected nucleophiles, n-propanol 7 and benzyl amine 9 are
commonly esed in enzymatic alcoholysis and aminolysis processes
respectively; alcohol ( )-8 and amine ( )-10 were selected because
they are structurally related to the cyanohydrin. The enhancement
of the lipase enantioselectivity in the enzymatic aminolysis by the
use of a leaving group structurally matched with a racemic amine
has already been demonstrated.11
O
NC
O
NC OH
X
NC
O
O
O
lipase
+
n
+
H2O or buffer/
organic solvent
n
n
n
X
X
X
2a-c
6a-c
(R)-
6a-c
(S)-
( )-
1a-c
a, X = CH2, n = 1
b, X = O, n = 1
(the absolute configuration was not confirmed)
c, X = CH2, n = 0
Scheme 4. Enzymatic hydrolysis of ( )-6a–c.
The preliminary processes carried out using a mixture of citrate
buffer (pH 5)/1,4-dioxane, 1:2 (Table 2, entry 1) confirmed us that
( )-6a was a suitable substrate for enzymatic hydrolysis catalyzed
by CAL-A. High enantioselectivity was observed in this process; the
remaining substrate was obtained in 97% enantiomeric excess with
a yield of 44% (isolated compound) after 4 days of reaction. The
cyanohydrin obtained as the product was unstable under these
conditions and 51% of the a-tetralone 1a was isolated (similar re-
sults were obtained using a phosphate buffer pH 7).
Under the same conditions, in the hydrolysis catalyzed by PSL-C
lower enantiosectivity was observed and the reaction was very
slow (Table 2, entry 7).
It is noteworthy that when the hydrolysis is catalyzed by CAL-A,
it yields the remaining substrate in very high enantiomeric purity
Table 3 summarizes the results obtained in these alcoholysis
and aminolysis processes. When alcohols were used as nucleo-
philes (Table 3, entries 1 and 2), the reaction rate was similar to
the hydrolytic process in the organic solvent, but the enantioselec-
tivity decreased from E = 40 (Table 2, entry 5) to E = 27 or E = 11
(Table 3, entries 1 and 2).
and allows the recycling of a-tetralone 1a.
Despite these good results, we considered it of interest to find
reaction conditions under which the stability of the cyanohydrin
was higher, in order to obtain the (R)-enantiomer. We studied
the enzymatic hydrolysis in organic solvents using 5 equivalents
of H2O (Table 2, entries 2–6 and 8–12).
The results obtained with the CAL-A catalyzed aminolysis
(Table 3, entries 3 and 4) depends very much on the nucleophile
used. As expected, the presence of an amine in the reaction media
increased the dehydrocyanation of the product, which was com-
Under these conditions, the formation of a-tetralone 1a was not
detected in any of the experiments. In the processes catalyzed by
CAL-A, we observed a strong influence of the organic solvent
(Table 2, entries 2–6); the best results were obtained when diethyl
ether was used as solvent: 55% conversion was achieved after only
two hours of reaction and the remaining substrate (S)-6a was ob-
tained in 98% enantiomeric excess (Table 2, entry 5).
pletely converted into the
a-tetralone 1a. The reaction with ben-
zyl amine was fast and 91% conversion was achieved in only one
hour of reaction, although the enantiomeric excess of the remain-
ing substrate was very low (Table 3, entry 3). Note the result ob-
tained using amine ( )-10 as a nucleophile (Table 3, entry 4). The
Similar enantioselectivity was observed when the process was
carried out in THF, although the reaction was slower and four days
Table 2
Enzymatic hydrolysis of ( )-6a, in organic solvents
Entry Lipase Organic solvent H2O (equiv) T (°C) t (days) Yield (%) 1a Yielda (%) (S)-6a eesa (%) (S)-6a Yielda (%) (R)-2a eepa (%) (R)-2a
C
b (%) Eb
c
1
2
3
4
5
6
7
8
CAL-A 1,4-Dioxane
CAL-A 1,4-Dioxane
CAL-A tBuOMe
CAL-A THF
CAL-A Et2O
CAL-A Et2O
30
30
30
30
30
15
30
30
30
30
30
30
4
4
2 (h)
4
2 (h)
4
51
—
—
—
—
—
40
—
—
—
—
—
44d
—
—
—
—
—
60
—
—
—
97
97
79
80
98
79
11
35
43
41
28
27
5
—
—
—
5
5
5
5
—
—
—
—
—
—
—
—
—
—
—
68
61
88
80
84
—
97
96
95
94
94
59
56
48
55
49
—
27
31
30
23
22
22
10
38
40
30
—
93
75
58
42
42
5
c
PSL-C
PSL-C
PSL-C
PSL-C
PSL-C
PSL-C
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
tBuOMe
THF
Et2O
13
5
5
5
5
5
10
16
10
11
9
9
10
11
12
—
—
a
Determined by chiral HPLC.
b
Conversion, c = ees/(ees + eep), enantiomeric ratio, E = ln[(1 ꢀ c)(1 ꢀ ees)]/ln [(1 ꢀ c)((1 + ees)].10 These equations are used when only one product was obtained in the
kinetic resolution.
c
1,4-Dioxane/citrate buffer pH 5 (2:1).
Isolated yield.
d

J. A. Rodríguez-Rodríguez et al. / Tetrahedron: Asymmetry 22 (2011) 1218–1224
1221
O
O
NC OH
X
NC
O
NC
O
O
O
lipase
+
n
n
n
X
X
H-Nu
O
2a-c
(R)-
6a-c
( )-
5 equiv.
(S)-6a-c
O
Nu
O
a, X = CH2, n = 1
b, X = O, n = 1
(the configuration was not confirmed)
c, X = CH2, n = 0
n
X
1a-c
OH
NH2
NH2
Nu-H =
OH
n
9
7
8
( )-
10a,
n = 1
( )-
( )-10b, n = 0
Scheme 5. Enzymatic alcoholysis or aminolysis of ( )-6a–c.
Table 3
Enzymatic alcoholysis or aminolysis of ( )-6a in diethyl ether at 30 °C using 5 equiv of nucleophile
Yielda (%) (S)-6a
ees (%) (S)-6a
Yielda (%) (R)-2a
eep (%) (R)-2a
cb (%)
Eb
a
a
Entry
Lipase
Nucleophile
t (h)
Yield (%) 1a
1
2
3
4
CAL-A
CAL-A
CAL-A
CAL-A
7
( )-8
9
3
2
1
—
—
25
65
—
—
nd
nd
91
69
—
22
49
—
27
11
91
61
9
39
8
—
( )-10
23
>99c
—
61c
>100d
a
b
c
Determined by chiral HPLC.
Conversion, c = ees/(ees + eep), enantiomeric ratio, E = ln[(1 ꢀ c)(1 ꢀ ees)]/ln [(1 ꢀ c)(1 + ees)].10
Determined by chiral HPLC after extraction with 1 M HCl in order to remove the excess amine ( )-10.
Estimated from the conversion and the ees, both determinated by HPLC.
d
enantiomeric excess of the remaining substrate (S)-6a was very
high under these conditions; taking into account this value and
the conversion determined by HPLC, the estimated enantioselec-
tivity was higher than 100. This result confirms that the lipase
enantioselectivity in the aminolysis processes can be enhanced
by the use of a nucleophile that is structurally related to the leav-
ing group of the ester.
Finally, in order to establish the scope of this methodology, the
best conditions used in the biocatalytic resolution of the methoxy-
acetylated cyanohydrin ( )-6a, were applied to the resolution of
analog cyanohydrin esters ( )-6b and ( )-6c. Substrates ( )-6b
and ( )-6c were synthesized in high yields from the corresponding
ketones. Table 4 summarizes the results obtained in the enzymatic
processes.
For both substrates, reactions catalyzed by CAL-A were gener-
ally fast but showed low enantioselectivity (Table 4, entries 1–3
and 5–7). On the contrary, processes catalyzed by PSL-C (Table 4,
entries 4 and 8), showed better enantioselectivity, especially in
the case of substrate ( )-6c, but slow reaction rates; using this en-
zyme, cyanohydrin (R)-2c was obtained in very high enantiomeric
purity, and 27% conversion was achieved after 14 days of reaction
(Table 4, entry 8).
The use of structurally related amines as nucleophiles (Table 4,
entries 3 and 7) did not appreciably improve the enantioselectivity
of these processes. We also observed that the cyanohydrin derived
from
than (R)-2a or (R)-2c and
of the conditions tested (Table 4, entries 1–4).
a-chromanone, 2b was more unstable in the reaction media
a
-chromanone, 1b was achieved in all
Table 4
Enzymatic resolution of ( )-6b and ( )-6c, in organic solvent at 30 °C
a
a
Entry Substrate Enzyme Solvent
Nucleophile 5 equiv t (h) Yielda (%) 1b,c Yielda (%) 6b,cd ees (%) 6b,cd Yielda (%) 2b,cd eep (%) 2b,cd cb (%) Eb
1
2
3
4
5
6
7
8
( )-6b
( )-6b
( )-6b
( )-6b
( )-6c
( )-6c
( )-6c
( )-6c
CAL-A
CAL-A
CAL-A
PSL-C
CAL-A
CAL-A
CAL-A
PSL-C
1,4-Dioxane H2Oc
2
2
2
48
1
1
38
78
63
5
32
—
62
9
7
83
68
—
39
—
7
64
<1
9
<1
22
7
—
—
9
—
—
—
—
—
42
—
—
—
—
—
—
2
Et2O
Et2O
H2O
( )-10a
13
30
12
—
—
—
27
60
—
16
—
1,4-Dioxane H2O
1,4-Dioxane H2Oc
Et2O
Et2O
H2O
( )-10b
2
336
59
—
—
33
1,4-Dioxane H2O
34
—
92
27
Configurations of the remaining substrate 6b and product 2b have not been assigned.
a
Determined by chiral HPLC.
b
Conversion, c = ees/(ees + eep), enantiomeric ratio, E = ln[(1 ꢀ c)(1 ꢀ ees)]/ln [(1 ꢀ c)((1 + ees)].10 These equations were used when only one product was obtained in the
kinetic resolution.
c
1,4-Dioxane/citrate buffer pH 5 (2:1).
Configurations: remaining substrate: (S)-6c, product (R)-2c for both enzymes.
d

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J. A. Rodríguez-Rodríguez et al. / Tetrahedron: Asymmetry 22 (2011) 1218–1224
A more exhaustive study of the lipase catalyzed resolution of
4.1.2. ( )-3,4-Dihydro-4-hydroxy-2H-1-benzopyran-4-
these substrates needs to be carried out in the future.
carbonitrile, ( )-2b
Brown oil (crude). 1H NMR (CDCl3, 300.13 MHz): d 7.60 (d, 1H,
3JHH 7.7), 7.35–7.29 (m, 1H), 7.05–6.99 (m, 1H), 6.90 (d, 1H, JHH
3
8.2), 4.40–4.25 (m, 2H), 3.30 (br s, 1H), 2.52–2.37 (m, 2H); 13C
NMR (CDCl3, 75.5 MHz): d 153.6 (C), 131.7 (CH), 128.1 (CH),
121.4 (CH), 128.8 (CN), 120.1 (C), 117.8 (CH), 64.24 (C), 61.0
3. Conclusions
Herein, we have described an easy methodology for the enan-
tioselective preparation of both enantiomers of the benzo-fused
(CH2), 34.6 (CH2); IR (NaCl) 3424, 2238 cmꢀ1
.
cyanohydrin derived from
a-tetralone, via a lipase catalyzed pro-
cess. Notwithstanding a ketone-derived cyanohydrin is a challeng-
ing substrate for an enzymatic resolution, we have demonstrated
that successful results can be achieved in these processes, using
commercially available lipases as a catalyst, by an adequate opti-
mization of the reaction parameters. In this case, the selection of
an adequate derivative as a substrate and the nucleophile has been
crucial in the enzymatic process.
4.1.3. ( )-2,3-Dihydro-1-hydroxy-1H-indene-1-carbonitrile ( )-
2c
Brown oil (crude). 1H NMR (CDCl3, 300.13 MHz): d 7.59 (d, 1H,
3JHH 1.3), 7.57–7.30 (m, 3H), 2.96–2.23 (m, 3H), 3.30 (br s, 1H),
2.77–2.70 (m, 1H), 2.67–2.45 (m, 1H); 13C NMR (CDCl3,
75.5 MHz): d 143.1 (C), 141.2 (C), 130.4 (CH), 127.5 (CH), 125.31
(CH), 123.4 (CH), 120.7 (CN), 75.38 (C), 41.0 (CH2), 29.3 (CH2); IR
(NaCl) 3509, 2234 cmꢀ1
.
4. Experimental
4.2. Preparation of ( )-1-cyano-1,2,3,4-tetrahydro naphthalene-
1-yl acetate ( )-3a
General. Enzymatic reactions were carried out in a Gallenkamp
incubatory orbital shaker. Immobilized Candida antarctica lipase
B, CAL-B (Novozym 435, 7300 PLU/g), was a gift from Novo Nor-
disk co., immobilized CAL-A (lipase NZL-101, 6.2 U/g) is commer-
cially available from Codexis, immobilized Pseudomonas cepacia
lipase (PSL-C, 783 U/g) is commercially available from Amano
Pharmaceuticals. Chemical reagents were purchased from Aldrich,
Fluka, Lancaster or Prolabo. Solvents were distilled over an appro-
priate desiccant under nitrogen. Flash chromatography was per-
formed using Merck Silica Gel 60 (230–400 mesh). Optical
rotations were measured using a Perkin-Elmer 343 polarimeter
and are quoted in units of 10ꢀ1 deg cm2 gꢀ1.1H NMR, 13C NMR
and DEPT spectra were recorded in a Bruker AC-300, Bruker AC-
300 DPX or Bruker NAV-400 spectrometer using CDCl3 as solvent.
The chemical shift values (d) are given in ppm. APCI+ and ESI+
using a Hewlett–Packard 1100 chromatograph mass detector or
EI+ with a Hewlett–Packard 5973 mass spectrometer were used
to record mass spectra (MS). IR spectra were recorded in a UNI-
CAM Mattson 3000 FT. The enantiomeric excesses were
determined by chiral HPLC analysis on a Hewlett-Packard 1100,
To a solution of ( )-2a (100 mg, 0.6 mmol) and DMAP (147 mg,
1.2 mmol) in ethyl acetate (3 mL), acetic anhydride (286 lL,
3 mmol) was added under nitrogen atmosphere. The mixture was
stirred at room temperature for 2 h. Next, the mixture was washed
with water (3 ꢁ 1 mL) and the organic phase was dried over
Na2SO4. The solvent was removed under reduced pressure and
the crude residue was purified by flash chromatography on silica
gel (dichloromethane) to afford the product ( )-3a as a white solid
(87 mg, 70%). Mp = 95–97 °C. 1H NMR (CDCl3, 300.13 MHz): d 7.72
(d, 1H, 3JHH 7.3 Hz), 7.37–7.30 (m, 2H), 7.17 (d, 1H, 3JHH 6.6), 2.91–
2.86 (m, 2H), 2.65–2.55 (d, 2H), 2.13 (s, 3H), 2.04–1.95 (m, 2H); 13
C
NMR (CDCl3, 75.5 MHz): d 168.8 (CO), 137.0 (C), 131.4 (C), 129.8
(CH), 129.4 (CH), 128.9 (CH), 126.8 (CH), 118.7 (CN), 71.8 (C),
33.0 (CH2), 28.3 (CH2), 21.3 (CH3), 18.3 (CH2); IR (neat, NaCl):
m
2232, 1745 cmꢀ1; HRMS-ESI+ calcd for [C13H13NO2Na]+ (M+Na)+
m/z 238.0838, found 238.0846.
4.3. Enzymatic acylation of ( )-2a
LC liquid chromatograph, using
a CHIRALPCK OJ-H column
(4.5 ꢁ 250 mm).
A reaction mixture containing ( )-2a (20 mg, 0.12 mmol), the li-
pase (20 mg) and vinyl acetate (53 lL, 0.6 mmol) in toluene (1 mL)
4.1. Synthesis of cyanohydrins ( )-2a–c
or using vinyl acetate as the solvent (1 mL), was shaken (generally
at 30 °C and 250 rpm) in an orbital shaker. The progress of the
reaction was monitored by HPLC analysis of samples that were pre-
pared by taking aliquots (0.1–0.2 mL) until achievement of the re-
quired conversion. The enzyme was then removed by filtration and
washed with the corresponding organic solvent. The solvent was
evaporated under reduced pressure and the crude residue was
purified by flash chromatography on silica gel (hexane/EtOAc 2:3).
To a stirred solution of ketone 1a–c (3.4 mmol) in dry dichlo-
romethane (5 mL), TMSCN (1.2 mL, 8.9 mmol) and Cu(OTf)2
(124 mg, 0.3 mmol) were added under an N2 atmosphere. The
resulting mixture was stirred for 48 h at rt. After this time, 1 M
HCl (2 mL) and acetonitrile (2 mL) were added and the resulting
mixture was stirred for 1 h at rt. Then, the reaction mixture
was quenched with water (6 mL) and extracted with dichloro-
methane (3 ꢁ 10 mL). The organic phase was washed with brine
separated and dried over Na2SO4; the solvent was removed under
reduced pressure to give a brown oil as a crude residue in 88–
95% yield, which was used for the acylation without further
purification.
4.4. General procedure for the preparation of derivatives ( )-4a
and ( )-6a–c
The corresponding acyl chloride (11.5 mmol) was added drop-
wise to a stirred solution of the corresponding cyanohydrin ( )-
2a–c (2.3 mmol) in pyridine (11.5 mmol) and dry CH2Cl2 (20 mL)
under an N2 atmosphere. The resulting mixture was stirred for
4 h in the case of derivative ( )-4a and 12 h in the case of deriva-
tives ( )-6a–c, at rt. After this time, the reaction mixture was
washed with 1 M HCl (3 ꢁ 10 mL) and extracted with CH2Cl2
(2 ꢁ 50 mL). The organic phases were combined and dried over
Na2SO4. The solvent was removed under reduced pressure and
the crude residue was purified by flash chromatography on silica
gel [dichloromethane for derivatives ( )-4a and ( )-6b and hexane/
4.1.1. ( )-1-Hydroxy-1,2,3,4-tetrahydronaphthalene carbonitrile
( )-2a
Brown oil (crude). 1H NMR (CDCl3, 300.13 MHz): d 7.67–7.64
3
(m, 1H), 7.29–7.26 (m, 2H), 7.13–7.10 (m, 2H), 2.84 (t, 2H, JHH
7.3), 2.40–2.31 (m, 1H), 2.25–2.21 (m, 1H), 2.12–1.91 (m, 3H);
13C NMR (CDCl3, 75.5 MHz): d 136.0 (C), 135.1 (C), 129.21 (CH),
129.00 (CH), 128.0 (CH), 126.6 (CH), 122.0 (CN), 69.8 (C), 37.7
(CH2), 28.3 (CH2), 18.6 (CH2); IR (NaCl) 3401, 2235 cmꢀ1
.

J. A. Rodríguez-Rodríguez et al. / Tetrahedron: Asymmetry 22 (2011) 1218–1224
1223
EtOAc 4:1 for derivatives ( )-6a and ( )-6c to afford the corre-
4.6. Procedure for the enzymatic aminolysis of ( )-6a–c
sponding product].
The reaction mixture, containing the corresponding acylated
cyanohydrin (100 mg), lipase (100 mg) and amine ( )-10a
(5 equiv) in diethyl ether (4 mL), was shaken at 30 °C and
250 rpm in an orbital shaker. The progress of the reaction was
monitored by HPLC analysis of samples that were prepared by tak-
ing aliquots (0.1–0.2 mL) until achievement higher than 50%. The
enzyme was then removed by filtration and washed with diethyl
ether. The combined organic filtrate was washed with 1 M HCl
(3 ꢁ 10 mL) and dried over Na2SO4; the solvent was evaporated
under reduced pressure and the crude residue was purified by flash
chromatography on silica gel (hexane/EtOAc 4:1).
4.4.1. ( )-1-Cyano-1,2,3,4-tetrahydronaphthalen-1-yl
acetoxyacetate ( )-4a
Yield 83%. White solid. Mp: 56–58 °C. 1H NMR (CDCl3,
3
300.13 MHz): d 7.68 (d, 1H, JHH 7.6 Hz), 7.38–7.26 (m, 2H), 7.18
3
(d, 1H, JHH 6.9), 4.63 (s, 2H), 2.90–2.85 (m, 2H), 2.62–2.58 (m,
2H), 2.14 (s, 3H), 2.05–1.96 (m, 2H); 13C NMR (CDCl3, 75.5 MHz):
d 170.09 (CO), 165.7 (CO), 137.1 (C), 130.7 (C), 130.1 (CH), 129.53
(CH), 128,8 (CH), 126.8 (CH), 118.1 (CN), 73.1 (C), 60.42 (CH2),
33.0 (CH2), 28.2 (CH2), 20.1 (CH3), 18.2 (CH2); IR (KBr) 2242,
1770,1754 cmꢀ1. HRMS-ESI+ calcd for [C15H15NO2Na]+ (M+Na)+
m/z 296.0893, found 296.0908.
4.7. Assignation of the absolute configuration of cyanohydrin
(R)-2a and the remaining substrate (S)-6a
4.4.2. ( )-1-Cyano-1,2,3,4-tetrahydronaphtalen-1-yl
metoxyacetate, ( )-6a
Yield 83%. White solid. Mp: 75–77 °C. 1H NMR (CDCl3,
The hydrolysis catalyzed by CAL-A was carried out as follows:
the reaction mixture, containing ( )-6a (75 mg), CAL-A (75 mg),
H2O (27 lL, 5 equiv) and diethyl ether (3 mL), was shaken for
1.5 h at 30 °C and 250 rpm in an orbital shaker. After this time,
the enzyme was removed by filtration and washed with diethyl
ether. The solvent was evaporated under reduced pressure to
3
300.13 MHz): d 7.71 (d, 1H, JHH 9.3 Hz), 7.38–7.30 (m, 2H), 7.18
3
(d, 1H, JHH 9.0), 4.07 (s, 2H), 3.47 (s, 3H), 2.91–2.86 (m, 2H),
2.64–2.59 (m, 2H), 2.04–1.96 (m, 2H); 13C NMR (CDCl3,
75.5 MHz): d 168.2 (CO), 137.1 (C), 130.9 (C), 130.0 (CH), 129.51
(CH), 128,9 (CH), 126.8 (CH), 118.3 (CN), 72.4 (C), 69.6 (CH2),
59.49 (CH3), 33.2 (CH2), 28.2 (CH2), 18.3 (CH2); IR (KBr) 2242,
1767 cmꢀ1. HRMS-ESI+ calcd for [C14H15NO2Na]+ (M+Na)+ m/z
268.0944, found 238.0945.
obtain a crude residue. Next, trimethylsilyl chloride (136
added to a stirred solution of the crude residue in dichloromethane
(3 mL) and pyridine (71 L). The reaction mixture was stirred for
lL) was
l
6 h at rt. After which the solvent was evaporated under reduced
pressure. The resulting mixture was purified by flash chromatogra-
phy on silica gel (dichloromethane) to obtain the pure products
(ꢀ)-6a and (+)-1-trimethylsilyloxy-1,2,3,4-tetrahydronaphtalene-
1-carbonitrile, (+)-7a, whose specific rotation sign was compared
with that reported for the (R)-enantiomer.12
The absolute configuration of product 5a was assigned as fol-
lows: the reaction mixture obtained in the enzymatic hydrolysis
of ( )-4a was analyzed by chiral HPLC and only one enantiomer
of 5a is detected. Then, the reaction mixture was acetylated and
the comparison of the HPLC chromatograms taken before and after
the acetylation showed the conversion of the peak corresponding
to 5a into (R)-4a.
4.4.3. ( )-1-Cyanochroman-1-yl methoxylacetate, ( )-6b
Yield 46%. White solid. Mp: 100–102 °C. 1H NMR (CDCl3,
3
300.13 MHz): d 7.73 (d, 1H, JHH 7.9 Hz), 7.38–7.32 (m, 1H), 7.05–
3
7.00 (m, 1H), 6.90 (d, 1H, JHH 8.4), 4.43–4.36 (m, 1H), 4.33–4.24
(m, 1H), 4.08 (s, 3H), 3.47 (s, 3H), 2.77–2.73 (m, 2H); 13C NMR
(CDCl3, 75.5 MHz): d 168.2 (CO), 154.1 (C), 132.3 (C), 130.1 (CH),
117.71 (CH), 117.5 (CN), 116.3 (C), 69.5 (CH2), 67.81 (C), 60.9
(CH2), 59.5 (CH3), 32.4 (CH2); IR (NaCl) 2361, 1643 cmꢀ1. HRMS-
ESI+ calcd for [C13H13NO4Na]+ (M+Na)+ m/z 270.0737, found
270.0725.
4.4.4. ( )-1-Cyanoindan-1-yl methoxylacetate, ( )-6c
Yield 80%. White solid. Mp: 58–60 °C. 1H NMR (CDCl3,
3
4.8. Determination of the conversion and ee by HPLC analysis
300.13 MHz): d 7.75 (d, 1H, JHH 7.2 Hz), 7.45–7.31 (m, 3H), 4.04
(s, 2H), 3.45 (s, 3H, CH3), 3.23–3.01 (m, 2H), 2.98–2.88 (m, 1H),
2.77–2.69 (m, 1H); 13C NMR (CDCl3, 75.5 MHz): d 168.6 (CO),
137.5 (C), 130.9 (CH), 127.5 (C), 126.1 (C), 125.1 (CH), 117.4 (CN),
78.3 (C), 69.3 (CH2), 59.4 (CH3), 39.3 (CH2), 29.56 (CH2); IR (NaCl)
2245, 1763 cmꢀ1. HRMS-ESI+ calcd for [C13H13NO3Na]+ (M+Na)+
m/z 254.0788, found 254.0793.
Chiralpack OJ-H, 40 °C, hexane/2-propanol (90:10), UV 210 nm,
0.8 mL minꢀ1, tR 6.75 min 1a; tR 13.32 min [(R)-2a]; tR 11.29 min
[(S)-2a]; tR 23.71 min [(R)-3a]; tR 17.46 min [(S)-3a]; tR 35.64 min
[(S)-4a]; tR 40.46 min [(R)-4a]; tR 28.30 min [(R)-5a]; tR 24.67 min
[(S)-5a]; tR 24.35 min [(R)-6a], tR 22.25 min [(S)-6a].
Chiralpack OJ-H, 30 °C, hexane/2-propanol (90:10), UV 210 nm,
0.8 mL minꢀ1, tR 10.45 min 1b; tR 11.96 min and tR 13.8 min [(R)-
2b] and [(S)-2b]; tR 16.91 min and tR 19.12 min [(R)-6b] and [(S)-
6b] (configuration has not been assigned).
4.5. General procedure for the enzymatic hydrolysis of ( )-3a,
( )-4a and ( )-6a–c
Chiralpack OJ-H, 40 °C, hexane/2-propanol (90:10), UV 210 nm,
0.8 mL minꢀ1, tR 8.02 min 1c; tR 14.00 min [(R)-2c]; tR 12.69 min
[(S)-2c]; tR 22.32 min [(R)-6c], tR 23.11 min [(S)-6c].
The reaction mixture, containing the corresponding acylated
cyanohydrin (20 mg), lipase (20 mg) and H2O (5 equiv) in the cor-
responding organic solvent (1 mL), [or 1,4-dioxane (0.6 mL) and
citrate buffer pH 5 (0.3 mL)] was shaken (generally at 30 °C and
250 rpm) in an orbital shaker. The progress of the reaction was
monitored by HPLC analysis of samples that were prepared by tak-
ing aliquots (0.1–0.2 mL) until the required conversion. The en-
zyme was then removed by filtration and washed with the
corresponding organic solvent. The solvent was evaporated under
reduced pressure and the crude residue was analyzed by HPLC.
The isolation and purification of the products were carried out after
the derivatization of the cyanohydrin to the trimethylsilyl-deriva-
tive as is described below in order to assign their absolute
configuration.
4.8.1. (S)-1-Cyano-1,2,3,4-tetrahydronaphtalen-1-yl metoxyac
etate (S)-6a
½
a 2D5
ꢂ
¼ þ22:0 (c 1, CH2Cl2), ee >99%.
4.8.2. (S)-1-Cyanoindan-1-yl methoxylacetate (S)-6c
½
a 2D5
ꢂ
¼ ꢀ17:5 (c 1, CH2Cl2), ee 34%.
4.8.3. (R)-1-Trimethylsilyloxy-1,2,3,4-tetrahydronaphtalene-1-
carbonitrile (R)-7a
½
a 2D5
ꢂ
¼ þ11:5 (c 1, CH2Cl2), ee = 85%.

1224
J. A. Rodríguez-Rodríguez et al. / Tetrahedron: Asymmetry 22 (2011) 1218–1224
4. (a) Wiggers, M.; Holt, J.; Kourist, R.; Bartsch, S.; Arends, I. W. C. E.; Minnaard, A.
J.; Bornscheuer, U. T.; Hanefeld, U. J. Mol. Catal. B: Enzym. 2009, 60, 82–85; (b)
Holt, J.; Arends, I. W. C. E.; Minnaard, A. J.; Hanefeld, U. Adv. Synth. Catal. 2007,
4.8.4. (R)-1-Trimethylsilyloxyindane-1-carbonitrile (R)-7c
¼ þ10:4 (c 0.5, CH2Cl2), ee = 92%.
½ ꢂ
a 2D5
ˇ
349, 1341–1344; (c) Konigsberger, K.; Prasad, K.; Repic, O. Tetrahedron:
Asymmetry 1999, 10, 679–687.
Acknowledgment
5. (a) Paravidino, M.; Holt, J.; Romano, D.; Singh, N.; Arends, I. W. C. E.; Minnaard,
A. J.; Orru, R. V. A.; Molinari, F.; Hanefeld, U. J. Mol. Catal. B: Enzym. 2010, 63, 87–
92; (b) Ohta, H.; Kimura, Y.; Sugano, Y.; Sugai, T. Tetrahedron 1989, 45, 5469–
5476; (c) Ohta, H.; Kimura, Y.; Sugano, Y. Tetrahedron 1988, 29, 6960–6967.
6. Nguyen, G.-S.; Kourist, R.; Paravidino, M.; Hummel, A.; Rehdorf, J.; Orru, R. V.
A.; Holt, J.; Hanefeld, U.; Bornscheuer, U. T. Eur. J. Org. Chem. 2010, 2752–2758.
7. Recuero, V.; Ferrero, M.; Gotor-Fernández, V.; Brieva, R.; Gotor, V. Tetrahedron:
Asymmetry 2007, 18, 994–1002.
8. Inghardt, T.; Karlsson, O.; Linschoten, M.; Nystrom, J.-E. WO 2000035869, 2000;
CAN, 133:58704.
9. Schnur R. C. U.S. Patent 4,226,875, 1980; Chem. Abstr. CAN 94:103345.
10. Chen, C. S.; Fujimoto, Y.; Girdaukas, G.; Sih, C. J. Am. Chem. Soc. 1982, 104, 7294.
11. González-Sabín, J.; Gotor, V.; Rebolledo, F. Tetrahedron: Asymmetry 2004, 15,
418–488.
This work was supported by the Ministerio de Ciencia e Innova-
ción of Spain (Project CTQ 2007-61126).
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