Enantioselective Synthesis of α-Thiocarboxylic Acids by Nitrilase Biocatalysed Dynamic Kinetic Resolution of α-Thionitriles

Article abstract of DOI:10.1002/chem.202001108

The enantioselective synthesis of α-thiocarboxylic acids by biocatalytic dynamic kinetic resolution (DKR) of nitrile precursors exploiting nitrilase enzymes is described. A panel of 35 nitrilase biocatalysts were screened and enzymes Nit27 and Nit34 were found to catalyse the DKR of racemic α-thionitriles under mild conditions, affording the corresponding carboxylic acids with high conversions and good-to-excellent ee. The ammonia produced in situ during the biocatalytic transformation favours the racemization of the nitrile enantiomers and, in turn, the DKR without the need of any external additive base.

Full text of DOI:10.1002/chem.202001108

Accepted Article
Accepted Article
Title: Enantioselective synthesis of α-thiocarboxylic acids via nitrilase
biocatalysed dynamic kinetic resolution of α-thionitriles
Authors: Daniele Castagnolo, Kate Lauder, Silvia Anselmi, James D.
Finnigan, Yuyin Qi, and Simon J. Charnock
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001108
Link to VoR: https://doi.org/10.1002/chem.202001108
01/2020

10.1002/chem.202001108
Chemistry - A European Journal
COMMUNICATION
Enantioselective synthesis of α-thiocarboxylic acids via nitrilase
biocatalysed dynamic kinetic resolution of α-thionitriles
Kate Lauder,[a] Silvia Anselmi,[a] James D. Finnigan,[b] Yuyin Qi,[b] Simon J. Charnock[b] and Daniele
Castagnolo[a],*
[a]
[b]
Ms K. Lauder, Ms S. Anselmi, Dr D. Castagnolo
School of Cancer and Pharmaceutical Sciences,
King's College London,
150 Stamford Street, SE1 9NH, London, UK
E-mail: daniele.castagnolo@kcl.ac.uk
Dr J.D. Finnigan, Dr Y. Qi, Dr S. Charnock
Prozomix Limited,
Station Court, Haltwhistle, Northumberland, NE49 9HN, UK.
Supporting information for this article is given via a link at the end of the document.
these methods show some limitations in terms of green chemistry,
such as the need of additives and bases (iPr2NEt or trioctylamine)
to promote the dynamic racemization of the substrates, the use of
ester protecting groups with consequent production of waste in
the chemical process and the need of oxygen-free conditions to
avoid the formation of side products.
Abstract: The enantioselective synthesis of α-thiocarboxylic acids via
biocatalytic dynamic kinetic resolution (DKR) of nitrile precursors
exploiting nitrilase enzymes is described. A panel of 35 nitrilase
biocatalysts were screened and enzymes Nit27 and Nit34 were found
to catalyse the DKR of racemic α-thionitriles under mild conditions,
affording the corresponding carboxylic acids with high conversions
and good-to-excellent ee. The ammonia produced in situ during the
biocatalytic transformation favours the racemization of the nitrile
enantiomers and, in turn, the DKR without the need of any external
additive base.
Enantiomerically pure α-mercapto(thio)carboxylic acids are
ubiquitous structural motifs found in pharmaceutical ingredients,1
peptides,2 metal chelators3 and S,O-ligands4 or used as
precursors in the asymmetric synthesis of drugs like tiopronin 2
and the IMP1 inhibitor 3 (Figure 1).5 Mercapto/thio-carboxylic
acids also belong to the class of volatile sulphur compounds
(VSC) and found application as flavouring and aroma agents in
food and fragrance industries.6 Compounds such as 4 and 5 have
a characteristic meaty-cheese aroma or fruity/tropical flavour
respectively and these organoleptic properties are related to the
configuration of their C-S stereocentre. Despite the key
importance of chiral α-thiocarboxylic acids, the available and
efficient protocols for their asymmetric synthesis are limited. In
fact, the construction of molecules bearing a stereodefined C-S
centre poses a distinct challenge in organic chemistry mainly
because sulphur compounds have different reactivity which can
be incompatible with the type of asymmetric chemistry exhibited
by their O- or N-derivative analogues. The most straightforward
methods to access enantiomerically pure α-thioacids rely on
stereospecific substitution reactions of enantioenriched halo-,7
hydroxy- or aminoacid8 derivatives with sulphur nucleophiles.
Enzymatic and non-enzymatic dynamic kinetic resolution (DKR)
also represents an efficient approach to access enantioenriched
α-thioacids from esters, due to the ability of a C-S stereocentre in
alpha to an electron withdrawing group to racemise under specific
conditions. Examples of DKR of thioesters using hydrolase
enzymes9a-c or thiocarboxylic acids using N. diaphanozonaria
cells9d have been reported by Drueckhammer and Otha
respectively, while Birman described a non-enzymatic DKR of α-
thiocarboxylic acids using homo-benzotetramisole (S)-HBTM10.
However, despite the excellent conversion and selectivity, all
Figure 1. Examples of α-thio carboxylic acids and aim of the work.
1
This article is protected by copyright. All rights reserved.

10.1002/chem.202001108
Chemistry - A European Journal
COMMUNICATION
Due to our interest in the development of green and sustainable
biocatalytic methodologies to access VSC and drug-like
compounds,11 herein we describe a DKR approach to synthesise
enantiomerically pure α-thiocarboxylic acids from racemic
α-thionitriles exploiting nitrilase enzymes. Nitrilases12 catalyse the
hydrolysis of nitriles into the corresponding carboxylic acids with
the formation of ammonia as side product. We hypothesised that
the NH3 produced in situ during the course of the
biotransformation would be sufficient to promote the racemization
of the thionitrile substrates and their DKR, as shown in Figure 1,
without the need of additional external bases to the reaction
mixture or the use of protecting groups.
nitrile hydrolysis reaction by exploiting o-phthalaldehyde (OPA),
was carried out, leading to the identification of 8 hit nitrilases
(blue/dark wells in Scheme 1). The conversion and enantiomeric
excess (ee) for each of the 8 hit enzymes were then analysed by
chiral HPLC and the results are reported in Table 1.
Table 2. Optimization of the DKR of nitrile 8a with Nit27.
Temp.
oC
ee%[c]
8a
ee%[c]
9a
Entry
pH
Time
Conv. %[a],[b]
1
2
3
4
5
6
7
8
9
10
11
12[d]
13[e]
4
7.2
7.2
7.2
7.2
6.5
7.0
7.5
8.0
8.5
8.5
7.2
7.2
7.2
24h
24h
24h
24h
24h
24h
24h
24h
7d
13
41
48
46
7
11
35
68
75[f]
75
90[g]
82
96
9
97
99
99
99
99
93
95
93
94
92
81
87
36
25
30
37
37
37
37
37
37
37
37
37
37
44
56
39
4
6
23
70
60
8
90
72
77
13d
7d
7d
Scheme 1. Synthesis of α-thionitriles and colorimetric screening assay.
7d
[a] All the reactions were carried out on 0.012 mmol of 8a and 1 mg of nitrilase
(Cell Free Extract - CFE). [b] Conversion of the nitrile into the acid is reported.
[c]Determined by HPLC with ChiralPak© IG column. [d] The reaction was carried
out with 0.018 mmol of 8a and 1 mg of nitrilase (CFE). [e] The reaction was
carried out with 0.012 mmol of 8a and 7 mg of nitrilase (CFE). [f] 9a was
obtained in 67% isolated yield. [g] 9a was obtained in 72% isolated yield.
In addition, the enzymatic hydrolysis would allow the efficient
synthesis of α-thiocarboxylic acids under mild conditions. In fact,
even if it is widely documented that nitriles can be hydrolysed into
carboxylic acids under acidic or basic conditions,13 we found that
α-thionitriles decompose into volatile byproducts in the presence
of strong acids/bases,14 thus supporting the need for a mild
methodology to access α-thioacids.
Surprisingly, several of the nitrilases which had been tipped for
activity based on the assay plate demonstrated no conversion of
8a into the carboxylic acid 9a (Table 1, entries 2-3 and 7-8). This
may be due to the sensitivity of the assay which is able to detect
ammonia and lead to a colour change even in the presence of
little enzymatic activity (<1% conversion). On the other hand,
Nit27 afforded 9a with 36% conversion and an excellent 95% ee
(entry 6), while the starting nitrile was recovered with 22% ee.
Higher conversion (89%) was observed with Nit20 (entry 5) but
both the nitrile 8a and the acid 9a were recovered with negligible
selectivity.
Table 1. Colorimetric screening of nitrilases.
Screening
plate
ee%[c]
8a
ee%[c]
9a
Entry
Nitrilase
Conv. %[c],[d]
colour[a],[b]
1
2
3
4
5
6
7
8
Nit02
Nit05
Nit12
Nit14
Nit20
Nit27
Nit28
Nit31
Weak
Weak
20
<1%
<1%
9
2
0
6
ND[e]
ND[e]
4
Medium
Medium
Strong
Strong
Weak
0
2
89
12
22
0
5
36
95
<1%
<1%
ND[e]
ND[e]
Weak
0
[a] The assay was carried out for 18h at 37 oC (see Supp. Info.). [b] Intensity of
the colour is described on the basis of personal perception as shown in Scheme
1. [c] Determined by HPLC with ChiralPak© IG column. [d] The conversion of
the nitrile into the acid, determined by HPLC, is reported. [e] Not determined.
The racemic thionitrile 8a, synthesised from the bromonitrile 7
under microwave irradiation on water (Scheme 1), was chosen as
a model substrate to identify the most appropriate hydrolytic
biocatalysts from a pool of 35 nitrilases, identified and isolated
through a metagenomic approach from Prozomix’s library. A
colourimetric assay,12d,15 able to detect the NH3 formed during the
Scheme 2. Plausible mechanism for the DKR of nitrile 8a and racemization
experiments
2
This article is protected by copyright. All rights reserved.

10.1002/chem.202001108
Chemistry - A European Journal
COMMUNICATION
The absolute configuration of the acid was assigned as (S)-9a by
comparison of its alpha value with that described in the
literature.16 Due to the promisingly high ee, Nit27 was selected for
further optimization of the reaction conditions. The effects of
temperature and pH on the biocatalytic transformation were then
investigated (Table 2). When the biotransformation was carried
out for a longer time (24h) an increase in the conversion and the
ee was observed. Similar data were obtained at 25 oC, 30 oC or
37 oC (entries 2-4), while a drop in the conversion was observed
mechanism of the biotransformation. From previous experiments,
it was clear that while the conversions of the biocatalytic
transformation increase at lower pH, the best conv./ee ratio were
obtained at higher pH.
Table 3. DKR of α-thionitriles 8b-m with Nit27
o
at 4 C (entry 1), likely due to a loss in enzyme activity at this
temperature. On the other hand, the pH of the buffer solution had
a more incisive effect on the biocatalytic hydrolysis. In fact, at pH
= 8 the acid (S)-9a was obtained with 68% conversion (entry 8),
clearly indicating that a DKR reaction was occurring. The rate of
substrate racemization of 8a is faster at higher pH due to a higher
proportion of the generated ammonia being present in the non-
protonated form rather than as an ammonium ion. The increased
nucleophilicity of the non-protonated NH3 may lead to a greater
proton exchange for the α-thionitrile substrate favouring, in turn,
the formation of (S)-9a with conv. > 50%, as shown in Scheme 2a.
When the reaction was carried out at a higher pH (pH = 8.5), a
decrease in the conversion was initially observed, likely due to a
reduced activity of the enzyme at this pH. However, when the
reaction was left stirring for a longer time (7 days, Table 2, entry
9), excellent (75%) conversion and 94% ee of (S)-9a were
obtained. Interestingly, under these conditions, the unreacted
nitrile 8a was recovered with 60% ee, while, when the reaction
was left for 13 days, 8a was obtained with only 8% ee (Table 2,
entry 10). It is plausible that the maximum conversion of the
biotransformation at pH = 8.5 is reached in 7 days due to the loss
of enzyme activity after this time in the presence of ammonia. Due
to the basic environment, the unreacted nitrile 8a racemises
affording, after 13 days, 8a with 8% ee, thus indirectly confirming
the proposed mechanism for the DKR reaction. Noteworthy, no
racemization of the acid 9a occurs after 13 days. Finally, 8a was
treated at pH = 7.2 for 7 days to investigate if longer reaction times
at lower pH could also lead to improved conversions and ee.
Under these conditions, an excellent conversion of 90% was
observed but the ee of 9a slightly dropped down to 81% (entry
11).17 It is likely that after some hours at pH = 7.2 the Nit27 begins
to also hydrolyse the (R)-8a enantiomer, thus resulting in a lower
ee of 9a. Increasing the loading of the biocatalysts proved
detrimental leading to 9a with poor (36%) ee (Table 2, entry 13).
On the other hand, the lower conversions observed at pH = 8.5
after 7 days must be ascribable to the reduced activity of the
enzyme under these conditions, which counterbalances the
racemization process. Even if at pH = 7.2 the amount of non-
protonated NH3 is low, it appears to be sufficient to catalyse the
racemization of nitrile 8a, in agreement with previous literature
data.9 In order to confirm our hypothesis on the role of ammonia
in the racemization of 8a, two additional experiments were carried
out (Scheme 2b). The enantioenriched nitrile 8a (ee = 72%, Table
2, entry 12) was suspended in a buffer solution (pH 7.2) and
treated with 1 eq of aqueous ammonia. After 48h, the full
racemization of 8a was observed. On the contrary, when the
enantioenriched 8a was left stirring for 48h in buffer solution (pH
7.2) without any base, no racemization occurred and only a small
decrease in the ee was observed.18 These experiments show that,
even if ammonia at pH 7.2 mainly exists in its protonated form,
the amount of free base is sufficient to favour the racemization of
the α-thionitrile substrates, thus supporting the hypothesised
Cmpd[a]
pH
7.2
Conv.[b]
%
ee%[b]
Yield %[c],[d]
Entry
65
92
93
89
46
39
53
1
9b
9c
8.5
7.2
50
92
2
3
4
5
8.5
7.2
8.5
93
73
52
94
99
93
57
66
41
9d
9e
9f
8.5
40
99
24
7.2
8.5
0
6
-
-
99
ND[e]
6
7
9g
9h
8.5
7.2
>99
13
99
94
83
ND[e]
8
9i
7.2
3
99
ND[e]
7.2
8.5
0
0
-
-
-
-
9
9j
9k
9l
7.2
8.5
0
0
-
-
-
-
10
11
8.5
8.5
0
0
-
-
-
-
12
9m
[a] The absolute configuration of acids 9 was assigned as S, by comparison of
the αD values with that of 9a and those reported in the literature.16 [b] Determined
by HPLC with ChiralPak© IG column. [c] Isolated yields are reported. [d] The low
yields sometimes observed may be ascribable to the volatility of some products.
[e] Not determined due to low conversion.
3
This article is protected by copyright. All rights reserved.

10.1002/chem.202001108
Chemistry - A European Journal
COMMUNICATION
The scope of the biocatalysed hydrolysis of α-thionitriles 8a-m
with nitrilase Nit27 was thus investigated under different pH
conditions. The thionitrile substrates 8a-m were synthesised from
α-bromonitrile 7 or from the appropriate cyanohydrins 10a-d as
shown in Table 3. The α-thiopropanoic acids 9b-d (Table 3,
entries 1-3) were obtained from the corresponding nitriles with
high to excellent conversions (50 to 93%) and excellent ee (89-
99%) regardless of the pH at which the reaction was carried out.
The unreacted nitriles are generally recovered with good ee when
the reaction was performed at pH = 7.2.19,17 The thioacids 9e-f
were also obtained with excellent ee but lower conversions (Table
3, entries 4-5), most likely due to presence of bulkier substituents
on the S-phenyl ring that may affect their interaction with the
nitrilase catalytic site, while the allyl nitrile 8g was fully and
enantioselectively converted into 9g (ee 99%, Table 3, entry 6).
All the α-thiopropanoic acids were obtained with absolute
configuration S, thus confirming the selectivity of the nitrilase
Nit27 for the enantiomers (S)-8.16 When the methyl substituent on
the stereocentre was replaced with bulkier groups in derivatives
8h-m, low or no conversion into the corresponding acids 9 was
observed. The α-thiobutanoic acids 9h-i (Table 3, entries 7-8)
were obtained with excellent ee but poor conversions, while no
conversion was observed for derivatives 9j-m bearing bulkier
groups. Since the reason for the poor activity of Nit27 on nitriles
8h-m was ascribable to their steric hindrance, a second screening
of the nitrilase pool from Prozomix’s library was carried out. For
the substrate 8h only the nitrilase Nit34 gave a positive response
in the colorimetric assay, while 4 nitrilases (Nit2, Nit6, Nit20 and
Nit34) were identified to be active for the phenylacetonitrile
derivative 8j (Table 4). As expected, Nit27 was not highlighted in
the colorimetric assay, which was consistent with the lack of
conversion observed in the initial reaction scope experiments.
Nit20 and Nit34 fully converted the phenylacetonitrile 8j into the
acid 9j although with poor ee (22% and 5% respectively, Table 4,
entry 3), while irrelevant conversion was observed with Nit02 and
Nit06. While both Nit20 and 34 are active upon the α-phenyl-
substituted thionitrile 8j, the lack of enantioselectivity indicates
that these nitrilases are less able to differentiate the two
enantiomers. Surprisingly, even if the nPr and the allyl groups are
similar in terms of size, the derivative 8p was not converted by
any of the four nitrilases (Table 4, entry 4). It is plausible that the
shorter length of the double bond together with the different
electronic properties of the allyl and nPr groups may affect the
binding of 8j and 8p to the catalytic site of Nit20 and thus account
for the different conversions observed. On the other hand,
nitrilase Nit34 proved to be effective on derivatives 8h and 8k
leading to the corresponding acids 9 with good ee and
conversions >50%, clearly indicating a DKR of the substrate.
Compounds 9i and 9l-o were obtained with good-excellent ee (up
to 93%) but with poor conversions and yields most likely due to
steric factors which may prevent the substrates 8 to interact with
the enzyme catalytic site.20
Table 4. DKR of α-thionitriles 8h-p with Nit20 and Nit34
Conv.
%
ee
%
Yield
%
Entry
Cmpd
Nitrilase
[a]
[a]
[b]
1
34
84
27
61
79
63
9h
2
34
31
9i
9j
02
06
20
1
1
-
-
ND[c]
ND[c]
ND[c]
3
4
>99
22
34
>99
5
ND[c]
34
72
81
63
9k
5
6
7
34
34
34
25
1
93
99
71
24
ND[c]
26
9l
9m
9n
32
8
9
34
20
1
99
ND[c]
9o
9p
<1
-
ND[c]
[a] Determined by HPLC with ChiralPak© IG column. [b] Isolated yields are
reported. [c] Not determined due to low conversion.
The main advantage of the methodology relies in the exploitation
of the ammonia formed during the biocatalytic hydrolysis to
catalytically facilitate the in-situ racemization of nitrile 8 (R)-
enantiomers, avoiding the need of external bases, while the
nitrilase selectively hydrolyses the (S)-enantiomers. To the best
of our knowledge, this is the first example of base-free DKR of
α-thionitriles via nitrilase biocatalysed hydrolysis and represents
In conclusion, nitrilase enzymes proved to be efficient biocatalysts
for the enantioselective synthesis of α-thiocarboxylic acids 9
through base-free dynamic kinetic resolution of the corresponding
racemic nitriles. Within this work two biocatalysts, namely Nit27
and Nit 34, have been identified from a panel of 35 nitrilases and
have shown the ability to catalyse the enantioselective synthesis
of α-thiocarboxylic acids 9 with good to excellent conversion and
ee via DKR reaction under mild conditions.
a
mild, greener and straightforward means to access
enantiomerically pure α-thioacids such as the flavouring
compounds 4 and 5.
4
This article is protected by copyright. All rights reserved.

10.1002/chem.202001108
Chemistry - A European Journal
COMMUNICATION
racemization of 8a would be completed due to the formed NH3. We
hypothesised that different factors could explain the observed ee, not the
least the formation of the carboxylic acid 9a which may affect the
racemization process in the longer period.
Acknowledgements
BBSRC (iCASE Studentship BB/M016218/1) and Royal Society
(RG160870) are gratefully acknowledged for financial support. Dr
Francesca Mazzacuva at Mass Spectrometry Facility at King’s
College London is gratefully acknowledged for HRMS
experiments.
[18] It cannot be excluded that the little racemization of 8a might be partially
favoured by the vacant d-orbital on sulfur atom.
[19] The ee of the recovered nitriles 8 are reported in the Supporting Information.
[20] Since the conversions for compounds 9i and 9l-o are lower than 50% it
cannot be excluded that an enzymatic kinetic resolution rather than a DKR
is occurring for these substrates
Keywords: nitrilase • biocatalysis • thiocarboxylic acid • thionitrile
• dynamic kinetic resolution
[1] R. Sasaki, Y. Tanabe, Chirality 2018, 30, 816–827.
[2] a) D.S. Cohen, C.A. Fink, A.J. Trapani, R.L. Webb, P.A. Zane, R.E.
Chatelain, Cardiovasc. Drug Rev. 1999, 17, 16; b) R. Bohacek, S.D.
Lombaert, C. McMartin, J. Priestle, M. Grütter, J. Am. Chem. Soc. 1996,
118, 8231-8249.
[3] J. Wagner, P. Vitali, J. Schoun, E. Giroux, Can. J. Chem. 1977, 9, 4028–
4036.
[4] a) K. Naksomboon, C. Valderas, M. Gómez-Martínez, Y. Álvarez-Casao,
M. Á. Fernández-Ibáñez, ACS Catal. 2017, 7, 6342–6346. b) Y. Álvarez‐
Casao, M.Á. Fernández‐Ibáñez, Eur. J. Org. Chem. 2019, 8,1842-1845.
[5] a) J. Clayden, P. MacLellan, Beil. J. Org. Chem. 2011, 7, 582‐595; b) H.
Wang, C. Ma, J. Zhou, X.Q. Liu., Chirality 2009, 21, 531‐538.
[6] http://www.thegoodscentscompany.com/
[7] a) S.-K. Lee, S.Y. Lee, Y.S. Park, Synlett 2001, 1941-1943; b) H. Schedel,
P.J.L.M. Quaedflieg, Q.B. Broxtermann, W. Bisson, A.L.L. Duchateau,
I.C.H. Maes, R. Herzschuh, K. Burger, Tetrahedron: Asymmetry 2000, 11,
2125; c) R.L. Harding, T.D.H. Bugg, Tetrahedron Lett. 2000, 41, 2729-
2731; d) B.-C. Chen, M.S. Bednarz, O.R. Kocy, J.E. Sundeen,
Tetrahedron:Asymmetry, 1998, 9,1641-1644; e) S.E. Kelly, T.G. LaCour,
Tetrahedron:Asymmetry 1992, 3, 715-718; f) B. Strijtveen, R.M. Kellogg, J.
Org. Chem. 1986, 51, 3664-3671.
[8] B. Strijtveen, R.M. Kellogg, Tetrahedron 1987, 43, 5039–5054
[9] a) D. S.Tan, M. M. Guenter, D. G. Drueckhammer, J. Am. Chem. Soc. 1995,
117, 9093–9094; b) P.-J. Um, D. G. Drueckhammer, J. Am. Chem. Soc.
1998, 120, 5605–5610; c) J. Nam, S. Lee, K.Y. Kim, Y.S. Park, Tetrahedron
Lett. 2002, 43, 8253-8255; d) D. Kato, K. Miyamoto, H. Otha,
Tetrahedron:Asymmetry 2004, 15, 2965–2973.
[10] a) X. Yang, V. B. Birman, Angew. Chem., Int. Ed. 2011, 50, 5553–5555; b)
V.B. Birman, Chem. Eur. J. 2011, 17, 11296-11304; c)
[11] K. Lauder, A. Toscani, Y. Qi, J. Lim, S.J. Charnock, K. Korah, D.
Castagnolo, Angew. Chem., Int. Ed. 2018, 57, 5803-5807.
[12] a) E. Liardo, N. Ríos-Lombardía, F. Morís, J. González-Sabín, F. Rebolledo,
Org. Lett. 2016, 18, 3366–3369; b) K. Yasukawa, R. Hasemi, Y. Asano,
Adv. Synth. Catal. 2011, 353, 2328–2332; c) J. Qiu, E. Su, W. Wang, D.
Wei, Tetrahedron Lett. 2014, 55, 1448–1451; d) G.W. Black, N.L. Brown,
J.J.B. Perry, P.D. Randall, G. Turnbull, M. Zhang, Chem. Commun. 2015,
51, 2660–2662; e) P. Kiełbasiński, M. Rachwalski, M. Mikołajczyk, F.P.J.T.
Rutjes, Tetrahedron:Asymmetry 2008, 19, 562–567; f) M. Kobayashi, M.
Goda, S. Shimizu, Biochem. Biophys. Res. Commun. 1998, 253, 662–666;
g) C. Brenner, Curr. Opin. Struct. Biol. 2002, 12, 775–782.
[13] a) V.K. Krieble, C.I. Noll, J. Am. Chem. Soc. 1939, 61, 560-563; b) C.P.
Wilgus, S. Downing, E. Molitor, S. Bains, R.M. Pagni, G.W. Kabalka,
Tetrahedron Lett. 1995, 36, 3469-3472; c) V. Theodorou, G.
Paraskevopoulos, K. Skobridis, ARKIVOC 2015, vii, 101-112.
[14] When compound 8a was treated with HCl 1N or NaOH 1N under reflux, no
traces of the acid 9a were detected after few hours. We hypothesised that
the acid or the base can promote the elimination of the thiol group leading
to volatile side products.
[15] Details on the colourimetric assay are reported in the Supporting
Information.
[16] S. Koul, J.L. Koul, B. Singh, M. Kapoor, R. Parshad, K.S. Manhas, S.C.
Taneja, G.N. Qazi, Tetrahedron:Asymmetry 2005, 16, 2575–2591.
[17] The high ee observed for the recovered 8a is surprising if compared with
that in entry 4 of Table 2. In fact, it should be expected that after 7 days the
5
This article is protected by copyright. All rights reserved.

10.1002/chem.202001108
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
Nitrilase biocatalysts catalyse the dynamic kinetic resolution (DKR) of α-thionitriles affording enantiomerically pure α-thiocarboxylic
acids under mild conditions. The DKR is favoured by the ammonia formed in situ during the biotransformation.
Institute and/or researcher Twitter usernames: @d_castagnolo
@KingsCollegeLon @KCLcancer_pharm
6
This article is protected by copyright. All rights reserved.