Direct Access to 2-Thioxooxazolidin-4-ones and Oxazolidine-2,4-diones from α-Keto Thioesters through Thiolate Transfer

Article abstract of DOI:10.1002/adsc.201701020

We report a direct synthesis of 2-thioxooxazolidin-4-ones and oxazolidine-2,4-diones from α-keto thioesters with sodium thiocyanate or potassium cyanate, respectively. The reactions proceed through nucleophilic substitution of a thioester with thiocyanate/cyanate anion, thiolate anion addition to carbonyl carbon, and subsequent intramolecular C?O heterocyclization. (Figure presented.).

Full text of DOI:10.1002/adsc.201701020

Title: Direct Access to 2-Thioxooxazolidin-4-ones and Oxazolidine-2,4-
diones from α-Keto Thioesters through Thiolate Transfer
Authors: Rajib Maity, Sandip Naskar, Kanchan Mal, Sandipa Biswas,
and INDRAJIT DAS
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201701020
Link to VoR: http://dx.doi.org/10.1002/adsc.201701020

10.1002/adsc.201701020
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Direct Access to 2-Thioxooxazolidin-4-ones and Oxazolidine-
2,4-diones from α-Keto Thioesters through Thiolate Transfer
Rajib Maity,a,‡ Sandip Naskar,a,‡ Kanchan Mal,a,‡ Sandipa Biswas,a and Indrajit Dasa,*
a
Organic and Medicinal Chemistry Division, CSIR-Indian Institute of Chemical Biology, 4, Raja S. C. Mullick Road,
Jadavpur, Kolkata 700032, India
Fax: (+91)-33-2473-5197; e-mail: indrajit@iicb.res.in
Equal contribution
‡
Received:
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
synthesis
of
2-thioxooxazolidin-4-ones
and
Abstract. We report
a
direct synthesis of 2-
thioxooxazolidin-4-ones and oxazolidine-2,4-diones from oxazolidine-2,4-diones from readily accessible
α-keto thioesters with sodium thiocyanate or potassium
cyanate, respectively. The reactions proceed through
starting materials will be a valuable contribution to
this field.
nucleophilic
substitution
of
a
thioester
with
thiocyanate/cyanate anion, thiolate anion addition to
carbonyl carbon, and subsequent intramolecular C-O
heterocyclization.
Keywords: α-keto thioesters; oxazolidine-2,4-diones;
rearrangement; synthetic methods; 2-thioxooxazolidin-4-
ones;
2-Thioxooxazolidin-4-one[1] and oxazolidine-2,4-
dione[2] derivatives are important synthetic
intermediates
and
have
found widespread
applications in medicinal chemistry. For example, 2-
thioxooxazolidin-4-one derivatives were used for the
synthesis
of
(5-phenyl-4-oxo-2-oxazolin-2-
yl)piperazine derivatives, which show promising
antimalarial activity against Plasmodium berghei in
mice.[1a] On the other hand,
a
variety of
anticonvulsant drugs including dimethadione,
ethadione, paramethadione, and trimethadione have
the oxazolidine-2,4-dione skeleton as the core
structure.[3] However, there is a lack of general
methods to access them. Among the few methods
available for synthesizing 2-thioxooxazolidin-4-ones,
the reaction of a carbonyl compound with an aqueous
solution of potassium thiocyanate and potassium
cyanide or sodium cyanide in acetone, followed by
hydrolysis with hydrochloric acid is more
common.[1,4] However, this method suffers from
certain drawbacks, such as utilization of the highly
toxic KCN or NaCN, and limited substrate scope
(Scheme 1a). On the other hand, the synthesis of
oxazolidine-2,4-diones mainly depends upon the
condensation of dialkyl carbonates with an amide of
the appropriately substituted α-hydroxy acids in the
presence of a base (Scheme 1b).[2,5] However, this
method requires preformed α-hydroxy amides as a
substrate which are sometimes difficult to make.
Thus, the development of an efficient route for the
Scheme 1. Synthesis of 2-thioxooxazolidin-4-ones and
oxazolidine-2,4-diones.
Thiocyanate and cyanate salts have been
recognized as one of the most efficient and versatile
reagents for accessing diverse heterocycles.[6] In the
course of our continuing efforts,[7] the reactivity of α-
keto thioesters with these salts was investigated. We
observed that α-keto thioesters underwent
nucleophilic displacement with thiocyanate/cyanate
on the thioester carbonyl, followed by intermolecular
thiolate addition to the carbonyl carbon, and
subsequent intramolecular C-O heterocyclization to
1
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10.1002/adsc.201701020
Advanced Synthesis & Catalysis
afford 2-thioxooxazolidin-4-one and oxazolidine-2,4-
dione derivatives, respectively (Scheme 1c).
Co(SCN)2 provided inferior results (entries 8-12,
Table 1). Different silver salts such as AgOAc,
AgBF4, AgPF6, AgOTf, and Ag2CO3 were also
screened (entries 13-18), but without success. The
amount of NaSCN appeared to be crucial. Increasing
the amount to 3.0 equiv or decreasing to 1.2 equiv
only lowered the yield (entries 19-20). But in the
absence of any solvent the reaction did not proceed at
all (entry 21).
To optimize the reaction conditions for 2-
thioxooxazolidin-4-ones, we initiated model studies
o
with α-keto thioester 1a and NaSCN 2a at 80 C in
various solvents such as DMSO, CH3CN,
trifluoroacetic acid, formic acid, and water (entries 1-
5, Table 1). However, the product 3a was formed
With the optimized reaction conditions, we started
to explore the substrate scope and generality for this
transformation (Table 2). In general, the reaction is
Table 1. Optimization of the reaction conditions for 2-
thioxooxazolidin-4-ones.[a]
Table 2. Substrate scope for α-keto thioesters and
thiocyanates.[a]
[a]
Reaction conditions: 1a (0.03 g, 0.124 mmol) and
MSCN 2 (2.0 equiv/mmol) were heated in 0.3 mL
o
solvent at 80 C, employing time as noted; yields are of
isolated products after purification.
[b] n.d. = product not detected; Starting materials isolated.
[c]
o
Heated to 60 C; n.d. = product not detected; isolated
PhCOCO2Me in 79% yield.
[d] AgOAc (10 mol %) was used.
[e] AgBF4 (10 mol %) was used.
[f] AgPF6 (10 mol %) was used.
[g] AgOTf (10 mol %) was used.
[h] Ag2CO3 (10 mol %) was used.
either in low to moderate yields or only starting
materials were recovered. The use of MeOH as a
solvent also failed to deliver the required product 3a
(entry 6); methyl benzoylformate (PhCOCO2Me) was
obtained instead in good yield. To our delight, the
utilization of AcOH as a solvent could significantly
improve the yield to 84% (entry 7). A screening of
other commercially available thiocyanate sources
such as KSCN, NH4SCN, AgSCN, CuSCN, and
[a]
Reaction conditions: α-Keto thioeseter 1 (0.05 g, 1
equiv) and NaSCN 2a (2 equiv/mmol) in AcOH (0.5
mL) at 80 oC; yields are of isolated products.
[b]
The thermal ellipsoids are shown in 50% probability
level.
2
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10.1002/adsc.201701020
Advanced Synthesis & Catalysis
[c]
With unreacted starting materials, minor unidentified
spots were detected.
[d] n.d. = not detected
[e] Starting materials were present.
tolerant to variation in substituents in the aryl group
at the α-position of the α-keto thioesters. Substrates
containing an aromatic ring with electron-donating
(1b-g) or withdrawing substituents (1h-l) worked
well and delivered the corresponding products in
moderate to good yields (3a-l). Furthermore,
replacement of the aryl group with a biphenyl (1m) or
fused aryl ring (1n-o) delivered the corresponding
products in moderate yields (3m-o). It is noteworthy
that the α-keto thioester having an alkyl group at the
α-position (1p-r) were also found to be reactive under
the reaction conditions and furnished the products in
moderate yields (3p-r).
We next turned our attention to assess the scope of
this reaction by varying the thiol substituents in α-
keto thioesters from –SPh to substituted aromatic
thiols or aliphatic thiols. As indicated in Table 2,
electron-donating (1s-t) or electron-withdrawing (1u-
v) substituents in the aromatic ring or a naphthyl
substituent (1w) were well-tolerated and produced the
expected products in moderate to good yields (3s-w).
However, α-keto thioesters bearing short-chain
aliphatic thiols such as benzyl and ethyl (1x-y) or
longer alkyl chain such as hexyl (1z) did not undergo
any such cyclization, perhaps due to the lower
stability of the in-situ generated thiolate anion in
acidic solution. The structure of 3s was established by
single crystal X-ray diffraction analysis (Table 2).[8]
Replacement of the α-keto thioester with α-keto
oxoester did not result in any expected product,
ostensibly owing to the stronger C-O bond in
oxoester and hence less reactive compared to the C-S
bond in thioesters.
produce oxazolidine-2,4-diones (entries 1-2, Table 3).
This might be ascribed to the nitrogen atom of a
cyanate anion being less nucleophilic than that of a
thiocyanate anion. However, in the presence of Ag-
catalyst, which may form a complex by coordinating
with sulphur atom, nucleophilic attack of a cyanate
anion on the thioester carbonyl will be facilitated,
resulting in the formation of the desired oxazolidine
product. After optimization with different cyanate
salts, catalysts, and solvents (Table 3), use of KOCN
o
and 20 mol% AgOTf in acetonitrile at 70 C proved
to be optimal (see also the Supporting Information for
optimization studies).
As indicated in Table 4, the reactions worked well
with electron-poor, electron-rich, or naphthyl
substituted α-keto thioesters and produced the
oxazolidine-2,4-dione derivatives in moderate to
good yields (5b-h). An alkyl group at the α-position
Table 4. Substrate scope for α-keto thioesters and
cyanates.[a]
Next, we examined the scope of the reaction for α-
keto thioesters with cyanate salts 4 (Table 3). Under
the standard reaction conditions described in Table 2
for the formation of 2-thioxooxazolidin-4-ones, 1a
did not undergo any cyclization with cyanate salts to
Table 3. Optimization of the reaction conditions for
oxazolidine-2,4-diones.[a]
[a]
Reaction conditions: α-Keto thioeseter 1 (0.05 g, 1
[a] Reaction conditions: 1a (0.03 g, 0.124 mmol), MOCN 4
(equiv/mmol), and catalyst (if needed) were heated in 1.0
mL solvent at 70 oC, employing time as noted.
[b]Yields are of isolated products after purification.
[c] Heated to 80 oC with 0.3 mL AcOH.
equiv), KOCN 4a (2 equiv/mmol), AgOTf (20 mol %) in
acetonitrile (1.5 mL) at 70 C, employing time as noted;
yields are of isolated products.
The thermal ellipsoids are shown in 50% probability
o
[b]
level.
3
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10.1002/adsc.201701020
Advanced Synthesis & Catalysis
[c] Some starting materials were recovered.
rapid tautomerization to give the desired product
(3,5).
of keto thioesters also furnished the products (5i-k).
Substrates bearing aliphatic thiols were also found to
react under these conditions, delivering the products
in moderate yields (5l-o, Table 4). It is interesting to
note that α-keto thioesters 1x-z did not give the
desired products with NaSCN/AcOH, with or without
catalyst (Table 2). However, similar thioesters
afforded the corresponding products 5l-o with KOCN
in presence of catalytic AgOTf (Table 4). It is
possible that AgOTf is responsible for stabilizing the
aliphatic thiolate anion generated in situ, though the
exact reason is not very clear at this moment. The
structure of 5f was established by a single crystal X-
ray diffraction analysis (Table 4).[8]
To gain some mechanistic insight, crossover
experiment for 2-thioxooxazolidin-4-one derivatives
were carried out (Scheme 2). An equimolar mixture
of 1s and 1g was treated with NaSCN in AcOH at 80
oC under standard conditions. It gave crossover
products (3a and 3am) in addition to the expected
Scheme 3. Proposed mechanistic pathway.
To highlight the practical usefulness of these
syntheses, we successfully scaled up the reaction to
0.5 g of 1 and isolated 3a, 3b, 5a, and 5f respectively,
without any significant diminution in the yields
(Table 5).
Table 5. Scale-up batches.[a]
Scheme 2. Crossover experiment.
products (3s and 3g). This result suggested that the
thiolate anion transfer[9] was intermolecular, thus
supporting a stepwise mechanism for this reaction.
The crossover experiment for oxazolidine-2,4-dione
derivatives also confirmed the stepwise mechanism
(see the Supporting Information).
Based on the experimental results and control
experiments, a plausible reaction mechanism is
proposed in Scheme 3. We presume that initial
nucleophilic attack of thiocyanate anion on the
thioester carbonyl with simultaneous elimination of
the thiolate anion could lead to intermediate II. On
the other hand, in a cyanate reaction, Ag(I) may
coordinate with sulphur atom of the α-keto thioester
to form a complex I,[10] resulting in increase of the
electrophilicity of thioester carbonyl centre, which
undergoes substitution easily with less nucleophilic
cyanate anion and elimination to generate II.
Subsequent nucleophilic attack by the thiolate to the
carbonyl carbon followed by intramolecular C-O
heterocyclization would furnish intermediate IV, via
intermediate III. Finally, intermediate IV undergoes
[a]
Unless otherwise noted, all the reactions were carried
out under the conditions in Table 2 and Table 4; Yields
of 3,5 are of isolated products after purification.
The synthetic versatility of this work could be
demonstrated by treating 2-thioxooxazolidin-4-one
derivatives with alkyl bromide/iodide in the presence
of K2CO3 as a base to isolate S-alkylated oxazol-
4(5H)-one derivatives 6a-e in excellent yields
(Scheme 4).[11] The structure of 6a was established by
a single crystal X-ray diffraction analysis (Scheme
4).[8]
In summary, we have developed a general
approach to 2-thioxooxazolidin-4-one derivatives
from α-keto thioesters, NaSCN, and acetic acid.
Direct access to oxazolidine-2,4-dione derivatives has
4
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10.1002/adsc.201701020
Advanced Synthesis & Catalysis
also been realized by AgOTf-catalyzed reaction of α-
keto thioesters and KOCN in acetonitrile. Both the
reactions proceed through the nucleophilic
substitution, thiolate addition, and subsequent
CH3CN (1.5 mL) at 70 oC under argon atmosphere for 11 h.
After completion of the reaction (TLC), ethyl acetate was
added into it. The solution was filtered through a Buchner
funnel, saturated NH4Cl solution was added into the filtrate,
and the mixture was extracted with ethyl acetate (10 mL).
The combined organic layers were dried over anhydrous
Na2SO4 and filtered, and the filtrate was concentrated
under reduced pressure to get a residue. The crude residue
was purified by silica gel column chromatography [230–
400; eluent:ethyl acetate/n-hexane] to obtain 5a; Rf = 0.3;
eluent, EtOAc/n-hexane (17.5%); isolated yield = 0.041 g,
70%; white solid; mp 129-131 oC. 1H NMR (600 MHz, d6-
DMSO): δ = 12.23 (br. s., 1 H), 7.68 (d, J = 7.8 Hz, 2 H),
7.57 (d, J = 7.8 Hz, 2 H), 7.54 (d, J = 7.2 Hz, 1 H), 7.46 -
7.51 ppm (m, 5 H); 13C NMR (150 MHz, d6-DMSO): δ =
171.4, 153.2, 136.9 (2 CH), 133.3, 131.6, 130.4, 130.2 (2
CH), 129.4 (2 CH), 127.5, 125.8 (2 CH), 94.5 ppm; IR
(KBr): ṽmax = 3318, 1824, 1732, 1351, 1185, 1020, 971,
747, 691, 622 cm-1; HRMS (EI): m/z calcd for C15H11NO3S
[M]+: 285.0460; found: 285.0464.
Scheme 4. Synthetic applications.
intramolecular thiolate addition, and subsequent
2-(Benzylthio)-5-(phenylthio)-5-(p-tolyl)oxazol-4(5H)-
one 6a: Dry K2CO3 (0.044 g, 0.316 mmol, 2.0 equiv) was
added to a well-stirred solution of 2-thioxooxazolidin-4-
one 3b (0.05 g, 0.158 mmol, 1.0 equiv) in dry DMF (1.0
intramolecular
C-O
heterocyclization.
To
demonstrate the potentiality of 2-thioxooxazolidin-4-
one derivatives, a base-mediated alkylation reaction
of these derivatives has been performed, which shows
that they offer efficient access to S-alkylated oxazol-
4(5H)-ones. Further investigations with α-keto
thioesters as synthetically useful building block are
ongoing and will be reported in due course.
o
mL) under argon atmosphere at 0 C. The reaction mixture
was stirred for 10 min. Benzyl bromide (0.04 mL, 0.316
mmol, 2.0 equiv) was then added dropwise via syringe at
that same temperature. After completion of the reaction
(TLC), saturated NH4Cl solution was added, and the
mixture was extracted with ethyl acetate (10 mL). The
combined organic layers were dried over anhydrous
Na2SO4 and filtered, and the filtrate was concentrated
under reduced pressure to get a residue. The crude residue
was purified by silica gel column chromatography [230–
400; eluent:ethyl acetate/n-hexane] to obtain 6a; Rf = 0.3;
eluent, EtOAc/n-hexane (10%); isolated yield = 0.062 g,
97%; white solid; mp 119-121 oC. 1H NMR (600 MHz, d6-
DMSO): δ = 7.46 (t, J = 7.2 Hz, 1 H), 7.43 (d, J = 7.8 Hz,
2 H), 7.35 - 7.37 (m, 3 H), 7.32 - 7.34 (m, 6 H), 7.24 (d, J
= 7.8 Hz, 2 H), 4.43 (q, J = 13.8, 18.0 Hz, 2 H), 2.30 ppm
(s, 3 H); 13C NMR (150 MHz, d6-DMSO): δ = 193.8, 182.7,
140.1, 136.5 (2 CH), 136.4, 131.4, 130.1, 130.0 (4 CH),
129.4 (2 CH), 129.2 (2 CH), 128.3, 127.5, 125.7 (2 CH),
97.5, 35.6 (CH2), 21.2 ppm; IR (KBr): ṽmax = 2924, 1753,
1488, 1254, 1212, 1013, 951, 807, 753, 698 cm-1; HRMS
(ESI): m/z calcd for C23H19NO2S2Na [M + Na]+: 428.0755;
found: 428.0749.
Experimental Section
Representative examples
5-Phenyl-5-(phenylthio)-2-thioxooxazolidin-4-one 3a: α-
Keto thioester 1a (0.05g, 0.206 mmol, 1.0 equiv) and
sodium thiocyanate 2a (0.033 g, 0.41 mmol, 2.0 equiv)
o
was heated in acetic acid (0.5 mL) at 80 C for 4 h. After
completion of the reaction (TLC), saturated NaHCO3
o
solution was added at 0 C, and the mixture was extracted
with ethyl acetate (10 mL). The combined organic layers
were dried over anhydrous Na2SO4 and filtered, and the
filtrate was concentrated under reduced pressure to get a
residue. The crude residue was purified by silica gel
column chromatography [230–400; eluent:ethyl acetate/n-
hexane] to obtain 3a; Rf = 0.3; eluent, EtOAc/n-hexane
(15%); isolated yield = 0.052 g, 84%; white solid; mp 139-
o
1
141 C. H NMR (600 MHz, d6-DMSO): δ = 13.54 (br. s.,
1 H), 7.65 (d, J = 7.8 Hz, 2 H), 7.57 (d, J = 7.8 Hz, 2 H),
7.50 - 7.55 (m, 4 H), 7.46 ppm (t, J = 7.2 Hz, 2 H); 13C
NMR (150 MHz, d6-DMSO): δ = 188.5, 171.7, 136.9 (2
CH), 132.7, 131.8, 130.7, 130.2 (2 CH), 129.7 (2 CH),
127.1, 125.7 (2 CH), 97.8 ppm; IR (KBr): ṽmax = 3112,
2901, 1777, 1474, 1321, 1207, 1000, 923, 871, 746, 690
cm-1; HRMS (EI): m/z calcd for C15H11NO2S2 [M]+:
301.0231; found: 301.0236.
Acknowledgements
I.D. thanks DST-SERB (EMR/2016/001720) for financial support
and Dr. Basudeb Achari for valuable discussions. R.M. and S.N.
thank UGC-India for fellowships. K.M. thanks CSIR-India for a
fellowship. I.D. thanks Ms. Upasana Ganguly for carrying out
some preliminary works in this project.
References
5-Phenyl-5-(phenylthio)oxazolidine-2,4-dione 5a: α-
Keto thioester 1a (0.05g, 0.206 mmol, 1.0 equiv),
potassium cyanate 4a (0.033 g, 0.41 mmol, 2.0 equiv), and
AgOTf (0.01 g, 0.041 mmol, 0.2 equiv) was heated in
[1] 2-Thioxooxazolidin-4-ones:
For
application
in
medicinal chemistry see: a) T. R. Herrin, J. M. Pauvlik,
5
This article is protected by copyright. All rights reserved.

10.1002/adsc.201701020
Advanced Synthesis & Catalysis
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contain
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via
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Advanced Synthesis & Catalysis
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