Enzyme-promoted desymmetrisation of prochiral bis(cyanomethyl) sulfoxide

Article abstract of DOI:10.1002/adsc.200700033

Prochiral bis(cyanomethyl) sulfoxide has been successfully transformed into the corresponding optically active mono amide and mono acid with enantiomeric excesses ranging from low (10%) to very high (up to 99%) using a broad spectrum of nitrilehydrolysing

Full text of DOI:10.1002/adsc.200700033

FULL PAPERS
DOI: 10.1002/adsc.200700033
Enzyme-Promoted Desymmetrisation of Prochiral
Bis(cyanomethyl) Sulfoxide
a,
a
a
´
Piotr Kiełbasinski, * Michał Rachwalski, Marian Mikołajczyk,
Małgorzata Szyrej,b Michał W. Wieczorek,b Roel Wijtmans,c
and Floris P. J. T. Rutjesc,*
Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Department of Heteroorganic Chemistry,
a
´
Sienkiewicza 112, 90-363 Łódz, Poland
Fax: (+48)-42-684-7126; e-mail: piokiel@bilbo.cbmm.lodz.pl
Jan Długosz University, Institute of Chemistry and Environmental Protection, al. Armii Krajowej 13/15, 42-201
Cze˛stochowa, Poland
Radboud University Nijmegen, Institute for Molecules and Materials, Toernooiveld 1, 6525 ED Nijmegen, The
Netherlands
b
c
Fax: (+31)-24-365-3393; e-mail: f.rutjes@science.ru.nl
Received: January 19, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Prochiral bis(cyanomethyl) sulfoxide has high (up to 99%) using a broad spectrum of nitrile-
been successfully transformed into the corresponding hydrolysing enzymes.
optically active mono amide and mono acid with en-
antiomeric excesses ranging from low (10%) to very Keywords: asymmetric synthesis; cyanides; enzyme
catalysis; nitrilases; sulfoxides
Introduction
Three different enzyme classes can be involved in
enzymatic nitrile hydrolysis. Nitrilases are able to con-
Nitriles constitute a versatile group of compounds, vert nitriles directly into the corresponding acids with-
due to the ease of cyanide introduction, and subse- out the amide as an intermediate. Nitrile hydratases
quent conversion into various other functional groups, (NHases), on the other hand, transform nitriles into
for example, amines, amides or acids.[1] Possibilities to the corresponding amides, which under the influence
introduce a cyanide function include substitution of of amidases can be converted into the appropriate
alkyl halides by cyanide anion,[1,2] the reaction of aryl acids (Figure 1).[8,9]
halides under the influence of copper cyanide,[3] dehy-
Although many enzymatic transformations of ni-
dration of amides,[4] reaction of ketones with tosyl- triles, which have been reported thus far, involve
methyl isocyanide,[5] and the Sandmayer reaction.[6] straightforward achiral substrates, stereoselective
The hydrolysis, however, of nitriles to the correspond- transformations involving racemic or prochiral sub-
ing amides or carboxylic acids usually requires rather strates are particularly interesting.[10] Mechanisms in-
harsh conditions, for example, very concentrated acids fluencing the stereoselectivity of the nitrile hydrolysis
or bases, heavy metal salts or elevated temperatures.
An alternative for such reactions can be found in
the use of nitrile-converting enzymes. In the past two
decades the enzymatic hydrolysis of nitriles has been
recognised as a method to afford a broad spectrum of
useful carboxylic acids and amides.[7] This method is
very useful from the synthetic point of view owing to
the mildness under which the reactions can be carried
out such as in aqueous buffers at neutral pH and at
ambient temperature.
are complicated and may vary depending on the kind
Figure 1. Nitrile hydrolysing enzymes.
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of substrates. Thus, in one of the first publications de- Results and Discussion
scribing the hydrolysis of racemic nitriles, the resolu-
tion was assumed to occur in the amide hydrolysis Synthesis of Prochiral Bis(cyanomethyl) Sulfoxide (2)
step under influence of the amidase.[11] The lack of en-
antiomeric enrichment of the recovered substrate and The starting material, prochiral bis(cyanomethyl) sulf-
opposite absolute configurations of the resulting oxide (2), was synthesised via a two-step method
amide and acid were taken as proof for such a mecha- from chloroacetonitrile which, upon treatment with
nistic proposal. However, changing the substrate anhydrous sodium sulfide in DME, provided bis(cya-
caused an opposite effect leading to the conclusion nomethyl) sulfide (1) in 75% yield. The latter was
that the hydratases were responsible for the observed oxidised in crude form with sodium periodate in a 1:1
enantioselectivity.[11] A similar assumption was made mixture of water and ethanol to give the desired sulf-
in the case of the desymmetrisation of prochiral dini- oxide 2 in 60% yield after purification (Figure 2). An
triles, although no amide intermediate was detect- alternative method for the synthesis of 2, previously
ed.[12] Clearly, both types of enzymes have been described by Singh and Gandhi[18] using thioacetamide
shown to exhibit stereoselectivity. In view of the un- as a sulfur source, in our hands appeared to be unsuc-
predictable nature of enantioselective enzymatic ni- cessful.
trile hydrolysis,[13] a strong interest remains in investi-
gating a variety of new enzymatic transformations of
nitriles.
Previous work of our group on the enzymatic de-
symmetrisation of prochiral compounds and enzymat-
ic resolution of racemic substrates led to particularly
efficient syntheses of several classes of optically
active heteroatom-containing compounds.[14–17] In all
cases simple hydrolytic enzymes were applied and ap-
peared capable of recognising stereogenic or proster-
eogenic centres located on heteroatoms or hetero-
atom-bound carbon atoms. In the course of our inves-
Figure 2. Synthesis of bis(cyanomethyl) sulfoxide (2).
tigations we turned our attention to heteroorganic ni-
triles which, in the light of the facts presented above, Enzymatic Hydrolysis of Prochiral Bis(cyanomethyl)
seemed interesting both from mechanistic and practi- Sulfoxide (2)
cal points of view. Therefore, we decided to investi-
gate whether nitrile-hydrolysing enzymes are capable The prochiral substrate 2 was subjected to a general
of accepting substrates with a prochiral heteroatom. It enzymatic resolution protocol (vide infra), which can
should be emphasised that such dinitriles have never give rise to the five possible products 3–7 (Figure 3).
been used as substrates for these types of enzymes. Three of them (3, 5 and 6) may, in principle, be pro-
This contribution focuses on the investigation of duced in each enantiomeric form. The hydrolyses
enzyme-mediated desymmetrisation of bis(cyano- were performed in different buffer solutions using a
methyl) sulfoxide.
broad spectrum of nitrile-converting enzymes, namely
a whole cell preparation from Rhodococcus erythrop-
olis NCIMB 11540 (freeze dried cells)[13] and a series
of commercially available nitrilases. In all cases the
Figure 3. Possible products of hydrolysis of (2).
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Desymmetrisation of Prochiral Bis(cyanomethyl) Sulfoxide
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Table 1. Enzymatic hydrolysis of prochiral 2
Entry Enzyme
Buffer, pH
After column chromatography After purification with DOWEXꢁ 50W
[a]
Product, yield [%]
Product, yield [a]D
ee
Abs.
conf.
[%]
[%]
1
Nitrilase 101
KH2PO4/K2HPO4, 7.2 2, 45.0
3, 1.75
+10.0
41[c]
(S)
3, 26.3
KH2PO4/K2HPO4, 7.2 7, 55.8
KH2PO4/K2HPO4, 7.2 3, 26.3
7, 46.5
KH2PO4/K2HPO4, 7.2 3, 8.8
5, 43.5
2
3
Nitrilase 102
Nitrilase 103
7, 38.8
3, 8.8
7, 42.6
0
-8.0
0
+20.0
+12.0
0
+18
-15.0
+12.0
+8.0
0
-
33[c]
0
(R)
-
(S)
(S)
-
(S)
(R)
(S)
(S)
4
Nitrilase 105
3, 4.4
82[c]
52[c]
0
5, 26.1
7, 19.4
3, 3.5
5, 20.8
3, 4.4
7, 23.2
5
6
7
8
Nitrilase 107
KH2PO4/K2HPO4, 7.2 3, 17.5
5, 34.8
74[c]
66[c]
49[c]
33[c]
Whole cells[d] KH2PO4/K2HPO4, 7.2 3, 13.2
5, 79.1
5, 61.7
Nitrilase 103
K3PO4, DTT
EDTA, pH 7.5
K3PO4, DTT
EDTA, pH 7.5
2, 60.0
3, 39.5
2, 30.0
3, 27.2
5, 43.5
2, 50.0
3, 21.9
5, 4.4
3, 27.2
-8.0
33[b]
(R)
Nitrilase 107
3, 8.8
5, 28.7
+12.0
+15.0
49[c]
66[c]
(S)
(S)
9
Nitrilase 108
K3PO4, DTT
EDTA, pH 7.5
3, 14.9
5, 2.6
3, 57.0
5, 19.9
5, 35.6
7, 16.5
+10.0
+15.0
+24.14
-17.5
-22.6
0
41[c]
66[c]
(S)
(S)
10
11
Nitrilase 104
Nitrilase 107
KH2PO4/K2HPO4, 7.2 3, 61.4
>99[c] (S)
5, 27.2
KH2PO4/K2HPO4, 7.2 5, 50.0
7, 23.2
77[c]
(R)
>99[b] (R)
0
-
[a]
[b]
[c]
[d]
In D2O.
Determined by chiral HPLC.
Optical purity, determined by comparison of [a]D values for the cases, in which ees were determined by chiral HPLC
Rhodococcus erythropolis NCIMB 11540 (freeze dried cells).
reaction time was 48 h. The results are summarised in
Table 1.
Nevertheless, both enantiomerically enriched forms
of 3 and 5 could be obtained in a ratio depending on
Inspection of Table 1 shows that, of the five expect- the enzyme and conditions used. In the first set of ex-
ed products, only three that is, cyanomethylsulfinyl- periments, we used a potassium phosphates buffer so-
acetamide (3), cyanomethylsulfinylacetic acid (5) and lution of pH 7.2, following the suggestion of Turner.[1]
sulfinyldiacetic acid (7), were formed in different The use of buffers containing potassium phosphates,
ratios and various enantioselectivities. All these prod- DTT (dithiotreitol) and EDTA with pH 7.5 gave gen-
ucts were isolated by column chromatography and erally poorer results. In these cases, we isolated about
spectroscopically analysed. Their structures were es- 50% of unreacted bis(cyanomethyl) sulfoxide (en-
1
tablished by H NMR, 13C NMR, MS (CI) and ele- tries 7–9). Our investigations showed, that the enzy-
mental analysis and the absolute configurations were matic hydrolysis of prochiral bis(cyanomethyl) sulfox-
determined by X-ray analysis (see Supporting Infor- ide (2) was not always chemoselective since in several
mation). We encountered severe difficulties in sepa- cases more than one product was formed. In cases of
rating the desired products via chromatography. After chiral products, the stereoselectivity of these enzymes
standard silica gel column chromatography, products towards the heteroatom prostereogenic centre (the
3, 5 and 7 were still found to contain an inorganic sulfinyl group) varied from low to excellent depend-
substance (combustion analysis revealed the presence ing on the nitrilase, being generally lower in compari-
of an inorganic ash). Therefore, the products were son to those reported for C-prochiral hydroxyglutaro-
subjected to additional purification involving elution nitriles.[19] An interesting observation is that some ni-
through a strongly acidic ion exchange column. Al- trilases behaved as nitrile hydratases leading to the
though this procedure enabled us to remove the inor- preferential formation of the amide. Similar findings
ganic contamination, it also in most cases led to a have been previously reported by Effenberg and Oss-
lowered overall yield of the reaction products.
wald[20] and recently by Sheldon et al.,[21] who noticed
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that such an activity was profoundly enhanced by sub-
strates bearing electron-withdrawing substituents,
lower temperature and higher pH. The ee values of
amides 3 were determined by chiral HPLC (Varian
Pro Star 210, Chiralpak AS). In most instances the
enzyme led to the (S)-amide as the predominant
product, although in some cases (R)-selectivity was
observed (entries 3 and 7). In one instance, the mono-
amide 3 was obtained in excellent enantioselectivity
of over 99%, albeit in a yield of 57% (entry 10). In
order to determine the ee of acid 5, the free acid had
to be transformed into the corresponding phenacyl
derivative 8, following the general methodology de-
scribed by Nagao et al. (Figure 4).[22] This involved
generation of the carboxylate, followed by alkylation
with bromoacetophenone in DMF.
Figure 4. Preparation of benzoylmethyl cyanomethylsulfinyl-
acetate (8).
This transformation also allowed us to determine
the absolute configuration of the acid 5, which ap-
peared to depend on the nitrilase involved. While the
enantioselectivities varied, in a single instance an ex-
cellent ee of >99% of the (R)-isomer was obtained
Figure 5. Hydrolysis of hydroxyglutaronitriles.
(entry 11), besides formation of
amount of the diacid.
a considerable
It is also interesting to compare the outcome of the Conclusions
experiment with the whole cells (entry 6) with the
result of enzymatic hydrolysis of the corresponding Enzymatic hydrolysis of prochiral bis(cyanomethyl)
hydroxy-substituted glutarodinitriles 9 and 10 that sulfoxide was achieved for the first time using a broad
was published previously (Figure 5).[23] In the latter spectrum of nitrile-hydrolysing enzymes under mild
cases, the corresponding (S)-enantiomers of 11 and 12 conditions (buffer solution of pH 7.2, 308C). Three
were obtained in excess, with a significantly better se- products formed were cyanomethylsulfinylacetamide,
lectivity for the benzyloxy than for the small hydroxy cyanomethylsulfinylacetic acid and sulfinyldiacetic
substituent. This low ee value is well in line with rela- acid, in different ratios and varying enantioselectivity
tively low selectivity for the desymmetrisation of the ranging from 10 to 99% ee. Further investigations
sulfoxide.
concerning enzymatic transformations of the sub-
As far as the mechanism of the above desymmetri- strates bearing other heteroatom prostereogenic cen-
sation is concerned, two courses of the reaction must tres will be continued.
be considered: In the first one the mono amide 3 and
the mono acid 5 are formed concurrently, in the
second one the amide is formed first and then subse- Experimental Section
quently hydrolysed to the acid. Identical absolute con-
figurations of both products obtained in the same re- General Remarks
action (Table 1, entries 4, 6, 8 and 9) undoubtedly
The enzymes were purchased from BioCatalytics Europe
speak in favour of the first assumption. The cases in
GmbH, Grambach, Austria. NMR spectra were recorded on
which the amide and the acid formed in the same re-
Bruker instruments at 200 MHz with D2O, CDCl3 and
action have opposite absolute configurations (Table 1,
entries 5 and 10) may suggest that the real stereore-
cognition is a result of a kinetic resolution at the
stage of the hydrolysis of the initially formed racemic
amide. However, also in the latter case the concurrent
formation of both products cannot be excluded, as
has been shown by Sheldon et al.,[21] who proposed a
bidirectional mechanism of action of nitrilases leading
to both types of products.
CD3COCD3 as solvents. Optical rotations were measured on
a Perkin–Elmer 241 MC polarimeter. Column chromatogra-
phy was carried out using Merck 60 silica gel. TLC was per-
formed on Merck 60 F254 silica gel plates. The enantiomeric
excess (ee) values were determined by chiral HPLC (Varian
Pro Star 210, Chiralpak AS).
Synthesis of Bis(cyanomethyl) Sulfoxide (2)
To a suspension of anhydrous Na2S (2.0 g, 0.025 mol) in
DME (20 mL) chloroacetonitrile (3.87 g, 0.05 mol) was
added. The mixture was stirred at room temperature and
the reaction was followed by TLC. After 12 h water was
added and the mixture was extracted with chloroform. After
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Desymmetrisation of Prochiral Bis(cyanomethyl) Sulfoxide
FULL PAPERS
drying over anhydrous MgSO4 and evaporation of CHCl3,
(AB, 4H); 5.49 (d, J=4.59 Hz, 2H); 7.51–7.90 (m, 5H); MS
(CI): m/z=266 (M + H).
compound 1 was obtained as a yellow oil; yield: 2.15 g
1
(75%). H NMR (CDCl3): d=3.57 (s, 4H); MS (CI): m/z=
113 (M + H).
Crystallographic Data
The crude compound 1 (TLC pure) was directly oxidised
to sulfoxide 2. Thus, crude 1 (2.15 g, 0.019 mol) was dis-
solved in an ethanol/water mixture (1:1, 20 mL), then NaIO4
(11.3 g, 0.053 mol) was added and the mixture was stirred at
room temperature until TLC indicated completion of the re-
action (ca. 12 h). A white precipitate was filtered off, the fil-
trate was evaporated and the residue was purified by
column chromatography (EtOAc:petroleum ether 40–60,
1:20 to 2:1) to afford 2 as a white powder, which was recrys-
tallized from benzene; yield: 1.47 g (60%); White needles,
See Supporting Information. Crystallographic data for (+)-3
and (+)-8 have been deposited with the Cambridge Crystal-
lographic Data Center, as supplementary publication no.
CCDC 634118 and CCDC 634122, respectively and can be
obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: (internat.) +44–1223/
336–033; e-mail: mailto:deposit@ccdc.cam.ac.uk].
1
mp 110–1118C. H NMR (CD3COCD3): d=4.35 (AB, 4H);
Acknowledgements
13C NMR (CD3COCD3): d=41.67, 113.65; MS (CI): m/z=
129 (M + H); anal. calcd. for C4H4N2OS: C 37.50, H 3.12,
N 21.88, S 25.00; found: C 37.62, H 3.09, N 21.88, S 24.71).
The work was carried out under a Polish-Dutch Joint Re-
search Project: “Dynamic kinetic resolution and desymmetri-
sation of sulfur and phosphorus compounds using enzymes”.
Financial support by the Ministry of Science and Information
Society Technologies (Poland), Grant No 3 T09 A 166 27 for
P.K., is gratefully acknowledged.
Enzymatic Hydrolysis of 2 – General Procedure
Compound 2 (0.10 g, 0.78 mmol) was dissolved in a buffer
solution and an enzyme (10 mg) was added. The reaction
was shaken at 308C during 48 h and monitored by TLC.
After 48 h, water was evaporated and the residue was sepa-
rated using column chromatography. To remove inorganic
substances present in the crude samples, all the products
were dissolved in water and the solutions were eluted
through a strong acidic ion exchange resin (Dowexꢁ 50W).
Lyophilisation of the samples gave the corresponding prod-
ucts 3, 5 and 7. The yields and optical rotations are shown in
Table 1.
Cyanomethylsulfinylacetamide (3): White crystals (from
MeCN), mp 130–1338C; 1H NMR (D2O): d=4.05 (br. s,
4H); 13C NMR (D2O): d=42.7, 56.2, 112.4, 167.8; MS (CI):
m/z=147 (M + H); anal. calcd for C4H4N2O2S: C 32.88, H
4.11, N 19.18, S 21.92; found: C 32.17, H 4.11, N 19.61, S
21.62.
Cyanomethylsulfinylacetic acid (5): Colorless oil;
1H NMR (CD3COCD3): d=4.07 (AB, 2H), 4.25 (AB, 2H);
13C NMR (CD3COCD3): d=39.07, 55.99, 112.43, 165.73; MS
(CI): m/z=148 (M+H), 104 (MÀCO2); anal. calcd. for
C4H5NO3S: C 32.65, H 3.40, N 9.52, S 21.77; found: C 32.82,
H 3.60, N 9.60, S 21.34.
Sulfinylbisacetic acid (7): White powder, mp 154–1578C;
1H NMR (D2O): d=3.92 (AB, 4H); MS (CI): m/z=167
(M+H).
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