L. Herna´ndez et al. / Tetrahedron: Asymmetry 17 (2006) 2813–2816
2815
(Table 1). These results might be optimized by using the
4. Experimental
reaction conditions reported by Sheldon23 for the
chemoenzymatic preparation of the cyanohydrin of 2-
chlorobenzaldehyde.
Reagents and solvents were purchased from either Baker or
Aldrich and used without any further purification. 1H
NMR spectra were recorded on a 400 MHz Varian instru-
ment in CDCl3, using tetramethylsilane (TMS) as an inter-
nal reference. Enantiomeric excesses were determined using
a Chiracel OD column (eluent: n-hexane/isopropanol mix-
tures) in a Hewlett–Packard 1050 instrument, equipped
with a diode array detector by the analysis of underivatized
cyanohydrins 2a, 2c–g; the diacetylated derivative of
cyanohydrin 2b and the naproxenate25 of cyanohydrin 2i.
Cyanohydrin 2h was derivatized as the corresponding
naproxenate and the enantiomeric excess was determined
by gas chromatography in a Hewlett–Packard 6890 instru-
ment using a HP-5 capillary column.
According to Riva,24 the use of purified HNL from Prunus
amygdalus in diisopropyl ether and a citrate buffer (pH 5.5)
gave 99% conversion and 95% ee of 2e. The use of our three
biocatalyst sources afforded similar results for enantioselec-
tivity (98%, 100% and 94% ee for capulin, peach and ma-
mey, respectively), but the extent of the reaction was
lower using mamey preparation (only 16% conversion).
On the other hand, the reactivity of the a,b-unsaturated
aldehyde 1f towards HCN addition did not show signifi-
cant differences among the three biocatalysts, with respect
to conversion (40–41%) and enantioselectivity (96–94%).
We previously reported 99% ee for 2f using defatted
mamey seed meal in diisopropyl ether and a citrate buffer,
and found that the conversion was dependent on the
concentration of the citrate buffer solution.9 It might be
possible to improve the conversion by modifying the
concentration of the citrate buffer.
4.1. Preparation of the crude enzymatic materials
Young leaves of mamey, capulin and peach were collected
from the corresponding garden tree. In order to remove
water, greasy material and some pigments that could inter-
fere with the biotransformation, the leaves were blended
sequentially three times with enough acetone to completely
cover the material. After filtering and discarding the
solvent each time, the resulting solid was air-dried in a
fumehood and stored in tight closed jars at 5 °C until use.
The addition of HCN to compound 1g, another a,b-unsat-
urated aldehyde, showed excellent enantioselectivity
(>99% ee) and moderate conversion (63% and 73%, respec-
tively) with capulin and peach crude preparations,
although for mamey the results were slightly lower (80%
ee and 50% conversion). Previously, we reported a 79%
ee and 51% conversion for cyanohydrin 2g using defatted
mamey seed meal in diisopropyl ether and citrate buffer
(pH 5.0).9
4.2. General procedure for the biotransformation of
aldehydes 1a–i into cyanohydrins 2a–i
In a typical experiment, 1.5 mL of a 1.0 M buffer solution
of KCN/citric acid (pH 4.5) was extracted three times with
diisopropyl ether (1.5 mL each time). The organic extracts
containing HCN were combined and added to a vessel con-
taining the crude enzymatic preparation (150 mg). Then
45 lL of citrate buffer (0.1 M, pH 4.5) was added, followed
by the addition of aldehyde 1a–i (0.1–0.18 mmol). The
resulting mixture was magnetically stirred at room temper-
ature for 24 h, followed by the addition of anhydrous
Na2SO4 to the reaction and filtration; the filtrate was then
evaporated under reduced pressure. The conversion per-
For the bulkier aliphatic cyclohexanecarboxyaldehyde,
1h, enantioselectivity for the reaction was low in all cases
(38%, 32% and 40% ee), although the conversion to 2h
was excellent (>99%).
In the case of cyanohydrin from 1i, the enantioselectivity
for the three biocatalysts was similar (93%, 94% and 90%
ee) with the same conversion (>99%). This result is impor-
tant, because Gotor16 reported 91% ee and 81% yield for
the preparation of cyanohydrin 2i using an almond crude
extract as the biocatalyst in diisopropyl ether and ( )2-
hydroxy-2-methylhexanonitrile as cyanide source in a
transcyanation reaction, whereas our procedure consisted
of a direct hydrocyanation process.
1
centage was measured by the H NMR of the crude prod-
uct and the enantiomeric excess of the underivatized
cyanohydrins 2a and 2c–g, the diacetylated cyanohydrin
2b and naproxenate derivative25 of cyanohydrin 2i were
measured by HPLC. The cyanohydrin obtained from alde-
hyde 1h was derivatized with (S)-naproxen chloride25 and
further analyzed by GC. Racemic cyanohydrins were
prepared from the corresponding aldehydes according to
a reported procedure, all of which showed spectral data
according to literature reports, and were used as references
during measurements.26,27
3. Conclusion
We have demonstrated that crude preparations from capu-
lin, peach and mamey leaves are excellent and efficient
sources of HNLs to prepare cyanohydrins with high levels
of enantioselectivities. This methodology represents an
inexpensive and reliable alternative, since the leaves are
available almost all year round. Remarkably, this proce-
dure can be used for preparative purposes with both ali-
phatic and aromatic substrates. In addition to the broad
substrate scope, an additional advantage is the simplicity
of manipulation and storage of the biocatalytic material.
1
Naproxenate of 2i. H NMR (400 MHz, CDCl3): d 7.71
(m, 3H), 7.35 (m, 1H), 7.13 (m, 2H), 5.33 (t, 1H), 3.91
(m, 3H), 3.41 (m, 2H), 1.75 (m, 6H).
13C NMR (100 MHz, CDCl3): d 18.0, 18.2, 31.3, 31.5, 32.3,
32.6, 44.9, 45.0, 60.6, 60.9, 119.0, 119.1, 125.5, 125.8, 126.0,
127.2, 129.0, 133.5, 134.1.