(R)-Oxynitrilase-catalyzed hydrocyanation: The first synthesis of optically active fluorinated mandelonitriles

Article abstract of DOI:10.1016/S0957-4166(01)00145-8

The first enantioselective synthesis of (R)-fluorinated mandelonitriles, by hydrocyanation under micro-aqueous conditions with almond meal as catalyst is reported.

Full text of DOI:10.1016/S0957-4166(01)00145-8

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 843–846
(
R)-Oxynitrilase-catalyzed hydrocyanation: the first synthesis of
optically active fluorinated mandelonitriles
Shiqing Han, Peiran Chen, Guoqiang Lin,* Hao Huang and Zuyi Li
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China
Received 22 January 2001; accepted 21 March 2001
Abstract—The first enantioselective synthesis of (R)-fluorinated mandelonitriles, by hydrocyanation under micro-aqueous condi-
tions with almond meal as catalyst is reported. © 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Cyanohydrins are useful intermediates in the synthesis
of a variety of natural and non-natural biologically
active compounds, owing to their potential for selective
transformations given by the presence of a single asym-
metric center bearing at least two reactive function-
alities.
In order to obtain almond meal with high catalytic
activity, it is important that the powdered almond is
defatted. From 100 g of almond kernel, the enzyme
5
activity of about 1.4×10 units for native almond meal
5
and about 3.0×10 units for the defatted almond meal.
It can be seen from Table 1 that the influence of
fluoro-substitution on e.e. values of the products varies
with the number and position of fluoro atom substi-
tuted at the phenyl ring. Compared with 100% e.e.
obtained from the reaction of unsubstituted benzalde-
Although many fluorine containing cyanohydrins and
their derivatives exhibit interesting biological activity,
the stereoselective synthesis of fluorine-containing
cyanohydrins has not been reported. The literature
1–4
7
shows that, in conventional enzymatic methods, elec-
tron deficient aromatic aldehydes are generally poor
hyde, monofluorinated benzaldehydes gave mandeloni-
triles with e.e. values ranging from 94 (entry 1) to 84%
(entry 2), the e.e. values given by difluorinated benz-
aldehydes dropped, ranging from 84 to 41% (entries 3
and 4). When tri- and pentafluorobenzaldehydes were
used, the reaction occurred without any stereoselectivity
5,6
substrates and react with low enantioselectivity, this
most probably results from the facility of the non-enzy-
matic reaction and the additional configurational labil-
ity of the cyanohydrin products.
(
entries 6 and 7).
We found that the cyanohydrination of benzaldehydes
bearing mono- or difluoro-substituents proceeded well
giving (R)-fluorinated mandelonitriles with good to
moderate enantioselectivity and yields when the sub-
strates were treated with almond meal in ‘micro-
aqueous medium’ where the reaction was carried out in
an organic solvent containing a microamount of water
with the pre-treated enzyme source. While with the
increasing number of fluoro-substitutions, none of the
products with enantioselectivity could be obtained as in
the case of trifluoro- and pentafluorobenzaldehydes
though the yields still remained high.
The position of the fluoro substituent also plays a role
on the stereoselectivity of the reaction. ortho-Substitu-
tion causes more significant reduction in the enantiose-
lectivity of the reaction than substitution at other
positions, as shown in the syntheses of 2b (84% e.e.)
versus 2a (94% e.e., entries 1 and 2). This is also shown
in the enantioselectivity of the reactions forming 2e
(41% e.e.) versus 2c (84% e.e.) and 2d (46% e.e., entries
3–5).
7,8
Fluorine, a strongly electronegative substituent, causes
the carbonyl carbon of the substrates to be more elec-
trophilic, making the non-enzymatic reaction easier.
Furthermore, fluoro-substitution enhances the acidity
of the a-hydrogen atom of the cyanohydrin product,
which facilitates racemization of the products formed in
*
Corresponding author. Fax: 86-21-64166263; e-mail: lingq@
pub.sioc.ac.cn
0
957-4166/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(01)00145-8

8
44
S. Han et al. / Tetrahedron: Asymmetry 12 (2001) 843–846
the enzymatic reaction. These factors lower the
enantioselectivity.
of 2g was 16%. In contrast, the enzymatic reaction with
almond meal under the same conditions gave a yield of
9
0% (entry 7). This indicates that almond meal does
In a comparative reaction of pentafluorobenzaldehyde
catalyze the reaction enantioselectively but the resultant
optically active product undergoes rapid racemization.
It has been reported recently that polyfluorinated
1
g, in which the substrate was subjected to cyanohydri-
11
nation conditions in the absence of a catalyst, the yield
Table 1. Conversion of fluoro-substituted benzaldehydes to fluorinated mandelonitriles catalyzed by almond meal under
micro-aqueous conditions
Entry
1
Aromatic aldehyde 1
Reaction conditions (h/°C)
24/20
Product (yield (%))
E.e. (%)a
94.2
Config.9
2a (90)
R
2
3
4
5
6
24/20
24/28
24/20
24/12
24/12
2b (96)
2c (71)
2d (92)
2e (70)
2f (37)
84
R
R
R
R
–
84.3
46.1
41.0
0
7
8
48/12
24/12
2g (90)
2h (67)
0
0
–
–
9
48/12
0
–
3
(6)
a
The mandelonitriles were acetylated and their e.e. values were determined by chiral HPLC.

S. Han et al. / Tetrahedron: Asymmetry 12 (2001) 843–846
845
arene–cyanohydrins were unstable at pH 6.5–8 (ana-
logous to our micro-aqueous medium), a trace of liber-
ated aldehyde was detected in the system. This result
indicates that racemization of 2g may occur via a
cyanohydrin–aldehyde equilibrium.
J =1.1 Hz, Ar-H); 7.43 (m, 1H, Ar-H); 7.62 (td, 1H,
2
19
J =7.5 Hz, J =1.8 Hz, Ar-H); F NMR (CDCl3/
1
2
TFA, ppm): −41.20 (m, Ar-F); IR: 3412 (w), 3080,
2928, 2255, 1618, 1592, 1494, 1460, 1234, 1043, 759;
+
MS m/z (rel. intensity (%)): 151 (M , 6). HRMS (FAB):
calcd for C H FNO: 151.0434. Found: 151.0417.
8
6
Cyanohydrination of the substrates bearing a trifl-
uoromethyl group was also carried out under the same
conditions. para-Trifluoromethyl benzaldehyde gave
the corresponding cyanohydrin 2h in 67% yield with no
optical activity, though the meta isomer was reported to
4.5. (R)-2-Hydroxy-2-(3,4-difluorophenyl)acetonitrile 2c
2
0
1
Colorless oil. [h]D +22.5 (c 2.6, CHCl ), e.e.=84%; H
3
NMR: (CDCl , ppm) 3.00 (br, s, 1H, OH); 5.54 (s, 1H,
3
1
2
19
give an e.e. value of 75%.
CH); 7.27 (m, 3H, Ar-H); F NMR (CDCl /TFA,
3
ppm): −58.3 (m, Ar-F), −58.8 (m, Ar-F); IR: 3412 (w),
3086, 2928, 2254 (CN), 1616, 1522, 1438, 1289, 1120,
1042, 875, 825, 772; MS m/z (rel. intensity (%)): 169
Racemic amine 3 was obtained in an overall yield of 6%
three steps) by LAH reduction from the corresponding
(
+
mandelonitrile, which was derived from trifluoroace-
tophenone. (R)-Oxynitrilase-catalyzed transformation
of dichloroacetophenone was also reported to give
(M , 49), 63 (35). Anal. calcd for C H F NO: C, 56.81;
8
5 2
H, 2.98; N, 8.28. Found: C, 57.02; H, 3.14; N, 8.16.
13
racemic 1-dichloromethyl-1-phenyl cyanohydrin.
4.6. (R)-2-Hydroxy-2-(2,3-difluorophenyl)acetonitrile 2d
2
0
Colorless oil. [h] +10.7 (c 3.5, CHCl ), e.e.=46.1%;
D
3
1
3
. Conclusion
H NMR: (CDCl , ppm) 3.25 (br, s, 1H, OH); 5.81 (s,
3
1
H, CH); 7.17–7.32 (m, 2H, Ar-H); 7.42 (m, 1H, Ar-H);
19
In conclusion, we have succeeded for the first time in
the preparation of some enantiomerically enriched
fluorinated mandelonitriles by (R)-oxynitrilase-cata-
lyzed reactions under micro-aqueous conditions.
F NMR (CDCl /TFA, ppm): −59.50 (m, Ar-F),
3
−65.68 (m, Ar-F); IR: 3413, 2930, 2250, 1629, 1602,
1494, 1408, 1286, 1046, 794, 759; MS: m/z (rel. intensity
(%)): 169 (M , 53). HRMS (FAB): calcd for
+
+
[C H F NO−H ]: 168.0261. Found: 168.0288.
8
5 2
4. Experimental
4.7. (R)-2-Hydroxy-2-(2,6-difluorophenyl)acetonitrile 2e
2
0
1
4.1. Preparation of almond meal
Colorless oil. [h]D +6.7 (c 1.9, CHCl ), e.e.=41%; H
3
NMR: (CDCl , ppm) 3.37 (d, 1H, OH); 5.77 (d, 1H,
3
Almond kernels were soaked in distilled water for 2 h,
peeled, air dried and then powdered in cold ethyl
acetate with a homogenizer. The powder was defatted
by a further three washes with ethyl acetate, filtered and
stored in a refrigerator.
J=6.4 Hz, CH); 7.12 (m, 2H, Ar-H), 7.35 (m, 1H,
19
Ar-H); F NMR (CDCl /TFA, ppm): −39.34 (m, Ar-
3
F), −46.73 (m, Ar-F); IR: 3437 (w), 3092, 2910, 2254,
1491, 1437, 1404, 1187, 1135, 1063, 877, 817, 799, 745;
+
MS m/z (rel. intensity (%)): 169 (M , 61). HRMS
(
FAB) calcd for C H F NO: 169.0340. Found:
8 5 2
4.2. Cyanohydrination
169.0313.
A mixture of aldehyde (1.25 mmol), almond meal (0.5
g), HCN in iso-propyl ether (IPE) (5 mL) was stirred at
the temperature and for the time period shown in Table
4.8. (±)-2-Hydroxy-2-(3,4,5-trifluorophenyl)acetonitrile
2f
1
. Flash chromatography on silica gel (eluent: ethyl
Compound 2f was obtained as its amine after reduction
with LAH. (±)-2-(3,4,5-Trifluorophenyl)ethylamine:
acetate/petroleum ether=1/4) provided the correspond-
1
8
ing cyanohydrin.
colorless crystal; mp 106–108°C; [h] 0 (c 0.2, CHCl3);
D
1
H NMR (CDCl , ppm): 2.1 (s, 3H, NH OH); 2.7–3.05
3
2
4
.3. (R)-2-Hydroxy-2-(4-fluorophenyl)acetonitrile 2a
(d, 2H, J=12.4 Hz, CH NH ); 3.05 (d, 1H, H-
2 2
19
CHNH ); 4.5 (m, 1H, CH), 7.0 (m, 2H, Ar-H);
F
2
2
0
1
Colorless oil. [h]D +36.4 (c 6.4, CHCl ), e.e.=94%; H
NMR: (CDCl , l ppm) 3.39 (br, s, 1H, OH); 5.52 (s,
NMR (CDCl /TFA, ppm): −95 (m, 1F, F-Ar); −63 (m,
3
3
2F, F-Ar); IR: 3219, 3076, 2886, 2732, 1622, 1596,
1441, 1340, 1225, 1119, 1094, 1014, 850, 794, 697; MS:
3
19
1
H, CH); 7.13 (m, 2H, Ar-H); 7.49 (m, 2H, Ar-H);
F
+
+
NMR (CDCl /TFA, ppm): −33.93 (m, Ar-F); IR: 3410
m/z (rel. intensity (%)): 192 (M +1, 49), 174 (M +1−
3
+
(
(
w), 2252, 1606, 1511, 1421, 1235, 1038, 837; MS: m/z
rel. intensity (%)): 151 (M , 11). HRMS (FAB) calcd
H O, 75). HRMS (FAB) calcd. for C H F NO (M +1−
H O): 174.0530. Found: 174.0508.
2
8
8 5
+
2
for C H FNO: 151.0433. Found: 151.0413.
8
6
4
.9. (±)-2-Hydroxy-2-(pentafluorophenyl)acetonitrile 2g
4.4. (R)-2-Hydroxy-2-(2-fluorophenyl)acetonitrile 2b
1
8
Colorless crystal; mp 55–56°C; [h] 0 (c 0.7, CHCl3);
D
2
0
1
1
Colorless oil. [h]D +21.85 (c 3.6, CHCl ), e.e.=84%; H
H NMR: (CDCl , ppm) 3.95 (s, 1H, OH); 5.83 (s, 1H,
3
3
19
NMR: (CDCl , ppm) 3.40 (br, s, 1H, OH); 5.78 (s, 1H,
CH); F NMR (CDCl /TFA, ppm): −82.5 (m, 2F,
F-Ar); −72.3 (t, 1F, F-Ar); −64.7 (m, 2F, F-Ar); IR:
3
3
CH); 7.14 (m, 1H, Ar-H); 7.24 (td, 1H, J =7.6 Hz,
1

8
46
S. Han et al. / Tetrahedron: Asymmetry 12 (2001) 843–846
494, 2953, 2266, 1660, 1515, 1137, 1052, 1002, 894, 2. Brussee, J.; Loos, W. T.; Kruse, C. G.; van der Gen, A.
+
3
7
87, 658; MS m/z (rel. intensity (%)): 223 (M , 100).
Tetrahedron 1990, 46, 979–986.
Anal. calcd for C H F NO: C, 43.05; H, 0.90; N, 6.28.
3. Griengl, H.; Klempier, N.; Pochlauer, P.; Schmidt, M.
8
2 5
Found: C, 42.97; H, 0.82; N, 6.12%.
Tetrahedron 1998, 54, 14477–14486.
4. Wajant, H.; Forster, S.; Selmar, D.; Effenberger, F.;
4
.10. (±)-2-Hydroxy-2-(4-trifluoromethylphenyl)acetoni-
Maier, K. Plant Physiol. 1995, 109, 231.
trile 2h
5. Matthews, B. R.; Jackson, W. R.; Jayatilake, G. S.;
Wilshire, C.; Jacobs, H. A. Aust. J. Chem. 1988, 41,
1697–1709.
6. Belokouv, Y.; Flego, M.; Ikonnikov, N.; Moscalenko,
M.; North, M.; Orizu, C.; Tararov, V.; Tasinazzo, M. J.
Chem. Soc., Perkin Trans. 1 1997, 1293–1295.
7. Han, S.; Lin, G.; Li, Z. Tetrahedron: Asymmetry 1998, 9,
1835–1838.
1
8
Colorless crystal; mp 67–69°C; [h] 0 (c 0.8, CHCl3);
D
1
H NMR: (CDCl , ppm) 3.1 (s, 1H, OH); 5.6 (m, 1H,
3
1
9
CH), 7.7 (m, 4H, Ar-H); F NMR (CDCl /TFA,
ppm): 14 (s, 3F, CF ); IR: 3417, 2254, 1622, 1426, 1161,
3
3
1
019, 975, 845, 763; MS: m/z (rel. intensity (%)): 201
+
(
M , 36). HRMS (FAB) calcd for C H F NO:
9 6 3
2
01.0403. Found: 201.0356.
8. Han, S.; Lin, G.; Li, Z. Tetrahedron 1999, 55, 3531–3540.
. The absolute configuration of the products has been
9
1
0
4
.11. (±)-2-Trimethylsilyoxy-2-trifluoromethyl-2-phenyl-
confirmed according to a reported method. Racemic
o-fluoromandelonitrile [(±)-2b] was kinetically resoluted
by lipase AK to give (S)-acetyl-o-fluoromandelonitrile
and (R)-o-fluoromandelonitrile (by stirring (±)-2b with
lipase AK and vinyl acetate in isopropyl ether at ambient
acetonitrile
1
8
Colorless crystal; mp 77–78°C; [h] 0 (c 0.3, CHCl3);
D
1
H NMR (CDCl , ppm) 0 (9H, Si-(CH ) ) 2.1 (s, 3H,
3
3 3
NH OH); 3.0–3.4 (d, 2H, J=13.2 Hz, CH NH ); 7.6
temperature then separating the products on a silica gel
column). (S)-Acetyl-o-fluoromandelonitrile gives [h] =
2
2
2
1
9
20
D
(
3
m, 5H, Ar-H); F NMR (CDCl /TFA, ppm): −7 (s,
3
F, CF ); IR: 3389, 1603, 1582, 1458, 1339, 1273, 1153,
−10.4 (c=1.5, CHCl ), whereas the corresponding acetyl
3
3
1
071, 1043, 941, 876, 794, 697; MS: m/z (rel. intensity
derivative of 2b obtained from the almond meal mediated
+
20
(
%)): 277 (M , 100).
reaction has an [h]D of +15.7 (c=1.0, CHCl ). By com-
3
parison of the two optical rotations, we can conclude that
the fluorine-containing mandelonitriles are of (R)-
configuration.
Acknowledgements
1
0. Sakai, T.; Miki, Y.; Tsuboi, M.; Takeuchi, H.; Ema, T.;
Uneyama, K.; Utaka, M. J. Org. Chem. 2000, 65, 2740–
2747.
The authors are grateful to The Chinese Academy of
Sciences for financial support (Grant No.
cKJ951A150605).
11. Sakai, T.; Miki, Y.; Nakatani, M.; Ema, T.; Ueyama, K.;
Utaka, M. Tetrahedron Lett. 1998, 39, 5233–5236.
1
2. Schmidt, M.; Griengl, H. In Topics in Current Chemistry;
Fessner, W.-D., Ed. Oxynitrilases: from cyanogenesis to
asymmetric synthesis. Springer-Verlag: Berlin, 1999; Vol.
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4
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