Synthesis of thioesters by simultaneous activation of carboxylic acids and alcohols using PPh3/NBS with benzyltriethylammonium tetrathiomolybdate as the sulfur transfer reagent

Article abstract of DOI:10.1002/ejoc.200900956

A new and simple route for the synthesis of thioesters starting from carboxylic acids and alcohols is reported by using tetrathiomolybdate as the key sulfur transfer reagent, Triphenylphosphane and N-bromosuccinimide were used for the activation of the ca

Full text of DOI:10.1002/ejoc.200900956

FULL PAPER
DOI: 10.1002/ejoc.200900956
Synthesis of Thioesters by Simultaneous Activation of Carboxylic Acids and
Alcohols Using PPh3/NBS with Benzyltriethylammonium Tetrathiomolybdate
as the Sulfur Transfer Reagent
Purushothaman Gopinath,[a] Ravindran Sasitha Vidyarini,[a] and
Srinivasan Chandrasekaran*[a][‡]
Keywords: Carboxylic acids / Alcohols / Synthetic methods / Sulfur / Molybdenum
A new and simple route for the synthesis of thioesters start-
ing from carboxylic acids and alcohols is reported by using
tetrathiomolybdate as the key sulfur transfer reagent. Tri-
phenylphosphane and N-bromosuccinimide were used for
the activation of the carboxylic acid and alcohol in the same
mary alcohols show excellent reactivity and gave good yields
of the corresponding thioesters, whereas secondary alcohols
gave moderate yields and tertiary alcohols were very less re-
active and gave poor yields of the corresponding thioesters.
pot followed by the transfer of sulfur from tetrathiomolybd- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
ate. Thioesters were obtained in good to moderate yields. Pri-
Germany, 2009)
activating the carboxylic acid[13] and benzyl triethylammo-
nium tetrathiomolybdate[14] (1), as the sulfur transfer rea-
gent. Because alkyl halides are generally synthesized from
the corresponding alcohols, we speculated that an alterna-
tive protocol with the use of alcohols as starting materials
would be an important and more versatile route for the syn-
thesis of thioesters (Scheme 1).
Introduction
The thioester unit is an integral component of many nat-
ural products[1] and bioactive compounds.[2] The recent ad-
vent of organometallic coupling[3] reactions with thioesters
as coupling partners has increased the quest for newer
methods of thioester synthesis. Thioesters also serve as im-
portant intermediates in the biosynthesis of polyketides and
nonribosomal polypeptides from fatty acids and amino ac-
ids.[4] Additionally, thioesters are used as building blocks
for the synthesis of heterocyclic compounds,[5] as substrates
in stereoselective aldol reactions,[6] and as intermediates in
the synthesis of various natural products[7] and drugs of
clinical interest (antihypertensive agents).[8]
Due to these widespread applications, a number of chem-
ical and enzymatic methods have been developed for syn-
thesis of thioesters. General synthetic methods include the
direct coupling of an activated carboxylic acid with a thiol[9]
or the coupling of thiocarboxylates with arenediazonium
salts[10] or alkyl halides.[11] With issues related to handling
of thiols and thioacids and the availability of starting mate-
rials, a simple and alternative protocol for the synthesis of
thioesters is needed.
Scheme 1. General reaction scheme.
To the best of our knowledge this is the first report of the
synthesis of thioesters starting from carboxylic acids and
alcohols without the use of thiols or thioacids as starting
materials. The results of this investigation are presented in
this paper.
Recently, we reported a one-pot protocol for the synthe-
sis of thioesters directly from carboxylic acids and alkyl ha-
lides[12] by using PPh3 and N-bromosuccinimide (NBS) for
Results and Discussion
It is known from the literature that alcohols can also be
activated by using PPh3 and NBS[15] to form the corre-
sponding bromide. So, we conceived that the simultaneous
activation of both the carboxylic acid and the alcohol by
using PPh3 and NBS in the same pot, followed by treatment
with reagent 1 could form the corresponding thioester. Ac-
cordingly, benzoic acid (2a; 1.0 equiv.) and ethanol
(1.2 equiv.) were treated with PPh3 and NBS to form the
[a] Department of Organic Chemistry, Indian Institute of Science,
Bangalore 560012, India
Fax: +91-80-2360-2423
E-mail: scn@orgchem.iisc.ernet.in
[‡] Honorary Professor, Jawaharlal Nehru Center for Advanced
Scientific Research, Jakkur, Bangalore
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200900956.
Eur. J. Org. Chem. 2009, 6043–6047
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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P. Gopinath, R. S. Vidyarini, S. Chandrasekaran
FULL PAPER
Scheme 2. Synthesis of ethyl thiobenzoate by simultaneous activation protocol.
corresponding activated intermediates followed by the ad- Table 1. Reaction of benzoic acid with various alcohols.
dition of reagent 1 (2.0 equiv.) to give the corresponding
thioester 8 in 74% yield (Scheme 2). We speculated that tet-
rathiomolybdate 1 would react with benzoyloxy phospho-
nium salt 4a (as it is more reactive than 5a or 6a) to form
the corresponding dibenzoyl disulfide (7a), followed by re-
ductive cleavage to generate the thiobenzoate ion. The
thiobenzoate ion may then react with the alkoxy phospho-
nium intermediate 5a or bromide 6a to give ethyl thioben-
zoate (8).
In order to test the generality of this methodology, the
reaction was then carried out with other alcohols and ben-
zoic acid as the standard. The results of this study are sum-
marized in Table 1. Under these conditions the reaction of
primary alcohols (Table 1, Entries 1–3) gave very good
yields of products in short reaction times (1–3 h). However,
the reaction of allylic alcohols and secondary alcohols
(Table 1, Entries 4–6) gave only moderate yield of the prod-
uct. The reaction of a tertiary alcohol (Table 1, Entry 7)
gave very poor yield and required a longer reaction time
(12 h). These results could be rationalized on the basis of
the steric crowding around the reaction center (carbon at-
tached to the OH group), which offers hindrance to nucleo-
philic substitution by the thioaroylate ion.
Interestingly, the reaction of benzyl alcohol (Table 1, En-
try 8) gave the corresponding disulfide instead of the ex-
pected thioester. This is because benzyloxy phosphonium
intermediate 5h or benzyl bromide (6h) reacts with reagent
1 in preference to benzoyloxy phosphonium intermediate
4a (Scheme 3).
When the reaction was carried out with other carboxylic
acids, the reactivity profile and the yields were very similar
to those obtained earlier. Phenyl acetic acid (aliphatic acid)
and p-methoxy benzoic acid (aromatic acid) both gave al-
most similar yields of their corresponding thioesters. The
results of this study with various carboxylic acids and pri-
mary alcohols are summarized in Table 2.
sponding β-thioglycoside 25 in 60% yield (Scheme 4). Simi-
Thioester derivatives of carbohydrates also play a key larly, phenyl acetic acid gave the corresponding β-thioglyco-
role in several synthetic transformations. We first studied side 26 in 55% yield.
the reactivity of anomeric alcohols to show the generality of
In pursuing these studies further, we decided to study the
the methodology. Thus, anomeric alcohol[16] 24, synthesized reactivity of 1,2,3,4-tetra-O-acetyl-β--glucopyranuronic
from -(+)-mannose, iodine, and acetone, was treated with acid[17] (27), a sugar carboxylic acid synthesized from gluc-
benzoic acid, PPh3, NBS, and reagent 1 to form the corre- uronic acid by acetylation with the use of acetic anhydride,
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Synthesis of Thioesters by Using a Sulfur Transfer Reagent
Scheme 3. Reaction of benzyl alcohol with reagent 1.
Table 2. Reaction of various carboxylic acids with alcohols.
iodine, and methanol. Carboxylic acid 27, when treated
with PPh3, NBS, reagent 1, and ethanol (CHCl3, 28 °C, 2 h)
afforded the corresponding ethyl thioester 28 in 63% yield
(Scheme 5).
Scheme 5. Synthesis of glucuronic acid based thioester 28.
Cysteine is an important structural and functional com-
ponent of many proteins and enzymes. Although it is more
commonly synthesized through biotransformation, the
chemical synthesis is still a challenge to the synthetic or-
ganic chemist. One of the most common methods for its
preparation is starting from N-Boc serine ester, which is tos-
ylated first followed by nucleophilic displacement with the
corresponding thioacid or thioacetate.[18] It seemed logical
to attempt a one-pot conversion of N-Boc serine into S-
protected cysteine by using our methodology involving rea-
gent 1. Accordingly, the reaction of N-Boc--serine methyl
ester[19] (29) with benzoic acid and reagent 1 under the reac-
tion conditions (CHCl3, 28 °C, 2 h) led to the formation of
cysteine derivative 30 in 70% yield (Scheme 6).
Scheme 6. Synthesis of N-Boc--cysteine derivative 30.
Conclusions
In conclusion, a variety of substituted thioesters and
thioglycosides were synthesized from carboxylic acids and
alcohols by using benzyltriethylammonium tetrathiomo-
lybdate as a sulfur transfer reagent via acyloxy and alkoxy
phosphonium salts as intermediates. We have also shown a
one-pot protocol for the synthesis of a cysteine derivative
from serine.
Scheme 4. Synthesis of β-thioglycosides 25 and 26.
Eur. J. Org. Chem. 2009, 6043–6047
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6045

P. Gopinath, R. S. Vidyarini, S. Chandrasekaran
FULL PAPER
The reaction mixture was stirred for 2 h at room temperature
(28 °C). Diethyl ether (30 mL) was added to the reaction mixture,
and it was filtered through a pad of Celite. The residue was again
extracted with CH2Cl2 (8 mL) followed by extraction with diethyl
ether (30 mL) and filtered again through a pad of Celite. The com-
bined extract was evaporated, and the residue was purified by col-
umn chromatography on silica gel to give 25 as a colorless liquid
Experimental Section
General Procedure for the Synthesis of Thioesters 8–23: To a well-
stirred solution of the corresponding carboxylic acid (1.0 mmol),
PPh3 (2.8 mmol), alcohol (1.2 mmol), and NBS (2.8 mmol) in
CHCl3 (5 mL) (stirred for 10 min) was added benzyltriethylammo-
nium tetrathiomolybdate (1; 2.0 mmol). After completion of the
reaction, diethyl ether (20 mL) was added to the reaction mixture,
and it was filtered through a pad of Celite. The residue was again
extracted with CH2Cl2 (5 mL) followed by extraction with diethyl
ether (20 mL) and filtered again through a pad of Celite. The com-
bined extract was evaporated, and the residue was purified by col-
umn chromatography on silica gel to give the corresponding thioes-
ters.
(0.209 g, 60%). [α]2D6 = +10.8 (c = 2.5, CHCl ). IR (neat): ν = 2915,
˜
3
1682, 1235, 868 cm–1. H NMR (300 MHz, CDCl3): δ = 8.00 (d, J
1
= 8.0 Hz, 2 H), 7.62–7.43 (m, 3 H), 5.64 (d, J = 3.6 Hz, 1 H), 4.94–
4.86 (m, 2 H), 4.89–4.23 (m, 1 H), 4.12–4.10 (m, 2 H), 3.66 (dd, J
= 3.3, 5.4 Hz, 1 H), 1.56 (s, 3 H), 1.47 (s, 3 H), 1.39 (s, 6 H) ppm.
13C NMR (75 MHz, CDCl3): δ = 189.3, 136.3, 133.8, 128.6, 127.5,
113.4, 109.3, 83.3, 81.9, 80.7, 80.2, 72.8, 67.1, 27.0, 25.8, 25.2,
24.8 ppm. HRMS: calcd. for C19H24O6SNa+ [M + Na+] 403.1191;
found 403.1197.
Compound 10: Colorless liquid. Yield: 0.218 g, 90%. IR (neat): ν =
˜
1
1662, 1207, 912 cm–1. H NMR (400 MHz, CDCl3): δ = 7.98–7.96
(m, 2 H), 7.57 (t, J = 7.2 Hz, 1 H), 7.45 (t, J = 8.0 Hz, 2 H), 7.34–
7.22 (m, 5 H), 3.32 (t, J = 8.0 Hz, 2 H), 2.98 (t, J = 8.0 Hz, 2
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 191.8, 140.1, 137.1,
133.3, 128.6, 128.5, 127.2, 126.5, 35.9, 30.4 ppm. HRMS: calcd. for
C15H14OSNa+ [M + Na+] 265.0663; found 265.0662.
2,3:5,6-Di-O-isopropylidene-(1S)-phenylacetyl-1-thio-β-D-mannofur-
anose (26): The same procedure as that used for compound 25 was
followed. Colorless liquid. Yield: 0.200 g, 55%. [α]2D7 = –9.9 (c =
1
1.1, CHCl ). IR (neat): ν = 2920, 1690, 1230, 862 cm–1. H NMR
˜
3
(400 MHz, CDCl3): δ = 7.36–7.27 (m, 5 H), 5.38 (d, J = 3.2 Hz, 1
H), 4.82–4.77 (m, 2 H), 4.42–4.38 (m, 1 H), 4.10–4.03 (m, 2 H),
3.87 (s, 2 H), 3.55 (dd, J = 2.8, 8.4 Hz, 1 H), 1.49 (s, 3 H), 1.44 (s,
3 H), 1.36 (s, 3 H), 1.34 (s, 3 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 194.9, 132.7, 129.6, 128.7, 127.6, 113.3, 109.4, 83.3,
81.7, 81.6, 80.2, 72.7, 67.1, 50.5, 27.1, 25.8, 25.1, 24.7 ppm. HRMS:
calcd. for C20H26O6SNa+ [M + Na+] 417.1348; found 417.1331.
Compound 17: Colorless liquid. Yield: 0.184 g, 72%. IR (neat): ν =
˜
1
1686, 1163 cm–1. H NMR (400 MHz, CDCl3): δ = 7.36–7.17 (m,
10 H), 3.82 (s, 2 H), 3.10 (t, J = 8.0 Hz, 2 H), 2.84 (t, J = 8.0 Hz,
2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 197.2, 139.9, 133.6,
129.5, 128.7, 128.5, 127.4, 126.5, 50.6, 35.7, 30.6 ppm. HRMS:
calcd. for C16H16OSNa+ [M + Na+] 279.0820; found 279.0807.
Compound 18: Colorless liquid. Yield: 0.190 g, 70%. IR (neat): ν =
˜
S-Ethyl 1,2,3,4-Tetra-O-acetyl-β-D-glucopyranoside Uronthioate
1
1658, 1601, 1167 cm–1. H NMR (300 MHz, CDCl3): δ = 7.95 (d,
(28): To a well-stirred mixture of 1,2,3,4-tetra-O-acetyl-β--gluco-
pyranuronic acid (0.362 g, 1.0 mmol), PPh3 (0.734 g, 2.8 mmol),
ethanol (0.05 g, 1.2 mmol), and NBS (0.498 g, 2.8 mmol) in CHCl3
(8 mL) was added tetrathiomolybdate (1; 1.2 g, 2.0 mmol). The re-
action mixture was stirred for 2 h at room temperature (28 °C).
Diethyl ether (30 mL) was added to the reaction mixture, and it
was filtered through a pad of Celite. The residue was again ex-
tracted with CH2Cl2 (8 mL) followed by extraction with diethyl
ether (30 mL) and filtered again through a pad of Celite. The com-
bined extract was evaporated, and the residue was purified by col-
umn chromatography on silica gel to give 28 as a colorless liquid
J = 9.0 Hz, 2 H), 7.35–7.21 (m, 5 H), 6.92 (d, J = 9.0 Hz, 2 H),
3.86 (s, 3 H), 3.29 (t, J = 7.8 Hz, 2 H), 2.96 (t, J = 7.8 Hz, 2
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 190.3, 163.7, 140.2,
129.4, 128.6, 113.7, 55.5, 36.1, 30.3 ppm. HRMS: calcd. for
C16H16O2SNa+ [M + Na+] 295.0769; found 295.0769.
Compound 19: Colorless liquid. Yield: 0.189 g, 66%. IR (neat): ν =
˜
1
1654, 1597, 1163 cm–1. H NMR (300 MHz, CDCl3): δ = 7.95 (d,
J = 8.4 Hz, 2 H), 7.18–7.11 (m, 4 H), 6.92 (d, J = 8.4 Hz, 2 H),
3.86 (s, 3 H), 3.27 (t, J = 7.5 Hz, 2 H), 2.92 (t, J = 7.5 Hz, 2 H),
2.33 (s, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 190.4, 163.7,
137.1, 136.0, 130.1, 129.4, 129.2, 128.5, 113.7, 55.5, 35.6, 30.4,
21.0 ppm. HRMS: calcd. for C17H18O2SNa+ [M + Na+] 309.0925;
found 309.0903.
(0.256 g, 63%). [α]2D7 = +5.7 (c = 0.6, CHCl ). IR (neat): ν = 1760,
˜
3
1682, 1216, 1040 cm–1. H NMR (400 MHz, CDCl3): δ = 5.79 (d,
1
J = 7.6 Hz, 1 H), 5.31–5.24 (m, 2 H), 5.17–5.13 (m, 1 H), 4.18 (dd,
J = 2.5, 6.5 Hz, 1 H), 2.87 (q, J = 7.4 Hz, 2 H), 2.14 (s, 3 H), 2.05
(s, 6 H), 2.03 (s, 3 H), 1.24 (t, J = 7.4 Hz, 3 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 195.0, 170.0, 169.2, 168.8, 91.3, 77.9, 72.1,
68.9, 23.0, 20.8, 20.6, 20.5, 14.1 ppm. HRMS: calcd. for
C16H22O10SNa+ [M + Na+] 429.0831; found 429.0835.
Compound 21: Colorless liquid. Yield: 0.172 g, 82%. IR (neat): ν =
˜
1657, 1603, 1259, 1167 cm–1. 1H NMR (300 MHz, CDCl3): δ =
7.95 (d, J = 8.7 Hz, 2 H), 6.92 (d, J = 8.7 Hz, 2 H), 3.86 (s, 3 H),
3.04 (t, J = 7.2 Hz, 2 H), 1.76–1.64 (m, 2 H), 1.03 (t, J = 7.5 Hz,
3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 190.6, 163.6, 130.2,
129.3, 113.7, 55.5, 30.8, 23.1, 13.4 ppm. HRMS: calcd. for
C11H14O2SH+ [M + H+] 211.0793; found 211.0793.
S-Benzoyl,N-Boc-L-Cysteine Methyl Ester (30): To a well-stirred
mixture of benzoic acid (0.122 g, 1.0 mmol), PPh3 (0.734 g,
2.8 mmol), N-Boc--Serine methyl ester (0.438 g, 1.2 mmol), and
NBS (0.498 g, 2.8 mmol) in CHCl3 (8 mL) was added tetrathiomo-
lybdate (1; 1.2 g, 2.0 mmol). The reaction mixture was stirred for
2 h at room temperature (28 °C). Diethyl ether (30 mL) was added
to the reaction mixture, and it was filtered through a pad of Celite.
The residue was again extracted with CH2Cl2 (8 mL) followed by
extraction with diethyl ether (30 mL) and filtered again through a
pad of Celite. The combined extract was evaporated, and the resi-
due was purified by column chromatography on silica gel to give
30 as a colorless liquid (0.237 g, 70%). [α]2D6 = +38.2 (c = 2.0,
Compound 23: Colorless liquid. Yield: 0.155 g, 80%. IR (neat): ν =
˜
1
1661, 1206, 914 cm–1. H NMR (400 MHz, CDCl3): δ = 7.87 (d, J
= 8.0 Hz, 2 H), 7.23 (d, J = 8.0 Hz, 2 H), 3.04 (t, J = 7.2 Hz, 2 H),
2.40 (s, 3 H), 1.73–1.64 (m, 2 H), 1.03 (t, J = 7.2 Hz, 3 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 191.7, 144.0, 134.7, 129.2, 127.2,
30.8, 23.0, 21.6, 13.4 ppm. HRMS: calcd. for C11H14OSNa+ [M +
Na+] 217.0663; found 217.0661.
(1S)-Benzoyl-2,3:5,6-di-O-isopropylidene-1-thio-β-D-mannofuranose
(25): To a well-stirred mixture of benzoic acid (0.122 g, 1.0 mmol),
PPh3 (0.734 g, 2.8 mmol), 2,3:5,6-di-O-isopropylidene-α--mannof-
CHCl ). IR (neat): ν = 3575, 1747, 1718, 1670, 1165 cm–1. 1H
˜
3
uranose (0.312 g, 1.2 mmol), and NBS (0.498 g, 3.0 mmol) in NMR (300 MHz, CDCl3): δ = 7.97–7.95 (m, 2 H), 7.62–7.57 (m, 1
CHCl3 (8 mL) was added tetrathiomolybdate (1; 1.2 g, 2.0 mmol). H), 7.49–7.43 (m, 2 H), 5.35 (d, J = 7.5 Hz, 1 H), 4.66–4.60 (m, 1
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Synthesis of Thioesters by Using a Sulfur Transfer Reagent
H), 3.78 (s, 3 H), 3.58–3.54 (m, 2 H), 1.43 (s, 9 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 190.8, 171.1, 155.1, 136.4, 133.7, 128.6,
127.4, 80.2, 53.2, 52.7, 31.1, 28.2 ppm. HRMS: calcd. for
C16H21NO5SNa+ [M + Na+] 362.1038; found 362.1025.
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Received: August 23, 2009
Published Online: October 21, 2009
Eur. J. Org. Chem. 2009, 6043–6047
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6047