Dynamic thiol exchange with β-sulfidor-α,β-unsaturated carbonyl compounds and dithianes

Article abstract of DOI:10.1021/ol301781u

A reversible covalent bond exchange of thiols, β-sulfido-α, β-unsaturated carbonyls, and dithianes has been studied in DMSO and D ₂O/DMSO mixtures. The equilibrium between thiols and β-sulfido-α,β-unsaturated carbonyls is obtained within a few hours, while the equilibration starting with the β-dithiane carbonyls and thiols requires a few days. This time scale makes the system ideal for utilization in dynamic combinatorial chemistry.

Full text of DOI:10.1021/ol301781u

ORGANIC
LETTERS
2012
Vol. 14, No. 18
4714–4717
Dynamic Thiol Exchange with β‑Sulfido-
r,β-Unsaturated Carbonyl Compounds
and Dithianes
Gururaj Joshi and Eric V. Anslyn*
Department of Chemistry and Biochemistry, University of Texas at Austin, Austin,
Texas 78712, United States
anslyn@austin.utexas.edu
Received June 28, 2012
ABSTRACT
A reversible covalent bond exchange of thiols, β-sulfido-r,β-unsaturated carbonyls, and dithianes has been studied in DMSO and D2O/DMSO
mixtures. The equilibrium between thiols and β-sulfido-r,β-unsaturated carbonyls is obtained within a few hours, while the equilibration starting
with the β-dithiane carbonyls and thiols requires a few days. This time scale makes the system ideal for utilization in dynamic combinatorial
chemistry.
The field of Dynamic Combinatorial Chemistry (DCC)
has expanded dramatically since it was first established
over a decade ago.1 The popularity of DCC is due to the
simplicity with which hosts and ligands,2 or enzyme
inhibitors,3 can be discovered withminimal syntheticeffort
as compared to traditional approaches. There are, however,
only a handful of dynamic covalent motifs that exchange
components within a few hours or days, the most common
being disulfide,4 transimination,5 and hydrazone6 as well as
thioester exchange.7 Thus, there is interest in expanding the
‘tool kit’ available to the DCC community by developing
new dynamic covalent chemistry.
A lesser used reaction for DCC is the thia-Michael addi-
tion, which takes approximately a week to achieve equilibria
in slightly basic conditions. For example, Greaney used this
(4) (a) Otto, S.; Furlan, R. L. E.; Sanders, J. K. M. J. Am. Chem. Soc.
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Chem., Int. Ed. 2003, 42, 2171. (c) Otto, S.; Kubik, S. J. Am. Chem. Soc.
2003, 125, 780. (d) West, K. R.; Bake, K. D.; Otto, S. Org. Lett. 2005, 7,
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Soc. Rev. 2012, 41, 1031. (b) Cougnon, F. B. L.; Sanders, J. K. M. Acc.
Chem. Res. 2012ASAP. (c) Corbett, P. T.; Vial, L.; West, K. R.; Wietor, J.-L.;
Sanders, J. K. M.; Otto, S. Chem. Rev. 2006, 106, 3652. (d) Ladame, S. Org.
Biomol. Chem. 2008, 6, 219. (e) Lehn, J.-M. Chem. Soc. Rev. 2007, 36,
151. (f) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.;
Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 898.
(2) (a) Reek, J. N. H.; Otto, S. Dynamic Combinatorial Chemistry;
Wiley-VCH: 2010, Weinheim. (b) Bugaut, A.; Jantos, K.; Wietor, J. L.;
Rodrigues, R.; Sanders, J. K. M.; Balasubramanian, S. Angew. Chem., Int.
Ed. 2008, 47, 2677. (c) Ludlow, R. F.; Otto, S. J. Am. Chem. Soc. 2008,
130, 12218. (d) Milanesi, L.; Hunter, C. A.; Sedelnikova, S. E.; Waltho,
J. P. Chem.;Eur. J. 2008, 12, 1081.
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R. M.; Gordon, E. M.; Wells, J. A. Proc. Natl. Acad. Sci. U.S.A. 2000,
97, 9367. (c) Krasinski, A.; Radic, Z.; Manetsch, R.; Raushel, J.; Taylor,
P.; Sharpless, K. B.; Kolb, H. C. J. Am. Chem. Soc. 2005, 127, 6686.
(d) Shi, B.; Stevenson, R.; Campopiano, D. J.; Greaney, M. F. J. Am. Chem.
Soc. 2006, 128, 8459.
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2106. (b) Hochgurtel, M.; Kroth, H.; Piecha, D.; Hofmann, M. W.;
Nicolau, C.; Krause, S.; Schaaf, O.; Sonnenmoser, G.; Eliseev, A. V.
Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 3382. (c) Gerber-Lemaire, S.;
Popowycz, F.; Rodriguez-Garcia, E.; Asenjo, A. T. C.; Robina, I.;
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43, 3144. (f) Zameo, S.; Vauzeilles, B.; Beau, J.-M. Angew. Chem., Int.
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3804. (b) Simpson, M. G.; Pittelkow, M.; Watson, S. P.; Sanders,
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r
10.1021/ol301781u
Published on Web 08/30/2012
2012 American Chemical Society

reaction for the discovery of inhibitors for glutathione-S-
transferase.3d Recently, Taunton used R-cyano acrylamides
as the conjugate acceptor to target noncatalytic cysteines,
thereby creating kinase inhibitors.8 In a slightly different
approach, Che has used thia-Michael additions to β-sulfido-
R,β-unsaturated carbonyls (Vinyl Sulfides Carbonyls, VSCs)
to modify cysteines,9 and as sensors for cysteine.10 He found
the reaction to be dynamic, but he did not explore the
mechanistic details of the exchange.
Scheme 2. Exchange Studies with Vinyl Sulfide 1a and Thiols
2c and 2b
Herein, we present experiments aimed at uncovering the
time scale and mechanism of thia-Michael additions to
VSCs, thereby creating new VSCs, and we report the
formation of β-dithiane carbonyls. We find that the thia-
conjugate addition is kinetically fast to exchange thiols and
VSCs, typically complete in less than an hour, while
equilibration with the β-dithiane carbonyls (BdTCs) can
require a few days. Importantly, the reactions proceed in
DMSO and DMSO water mixtures at physiological pH.
To examine the reversible covalent bond exchange, a
series of β-sulfido-R,β-unsaturated ketones (i.e., VSCs)
were required. The syntheses of 1aÀc were achieved by
treating the butynone 3 with 1 equiv of thiols 2aÀc in good
yields (Scheme 1). The conjugate addition yields both E
and Z isomers, but the Z isomer dominates, as has been
reported earlier.11
Thiol exchange with the VSCs was first carried out in
DMSO-d6 and monitored by 1H NMR spectroscopy. The
addition of 1 equiv of 2c with 1a in the presence of Et3N
(Scheme 2) quickly showed an equilibrating mixture of
VSCs 1a and 1c (doublets between 5.8 and 6.8 ppm), while
within 1 h the BdTCs 4, 5, and 6 (triplets at 4.7, 4.4, and 5.1
ppm respectively)12 were present, with all five species in a
The subsequent addition of 4-mercaptobenzoic acid (2b)
to the equilibrated mixture further generated 1b, 7, 8, and 9
as depicted in Scheme 2, while the first products decreased
but remained. This confirmed that all the species are
interconverting. Interestingly, we find an equilibrating
mixture of VSCs and BdTCs, even though excess thiols
are present in the solution. Thus, the thermodynamic
stability of vinyl sulfide carbonyls and β-dithiane carbo-
nyls are comparable under the experimental conditions.
We sought to confirm the dynamic nature of the BdTCs
by treating pure 6 with several equivalents of 2c in DMSO-
d6 and Et3N, monitoring the reaction by 1H NMR spectro-
scopy (Scheme 3). Within 1 h, the formation of 1a, 1c, 4,
and 5 was confirmed by the appearance of proton reso-
nances at δ = 5.9, 6.1, 4.7, 4.4 ppm, respectively.14 While
acid catalyzed dithiane exchange was previously reported
by Sutton and co-workers,15 to our knowledge this is the
first report of a base catalyzed dithiane exchange, which is
undoubtedly due to the β-keto group.
1
ratio of 3:33:19:8:37, respectively, as determined by H
NMR integration (for the VSCs, the ratios are inclusive of
both E and Z isomers). The spectra did not further change
with time, indicative that the mixture had reached equili-
brium (Figure 1). Although, thiols are known to oxidize in
DMSO under basic conditions, we found that the equili-
brium between the VSC’s and the thiols is attained faster
(hours) than the disulfide-thiol formation (days).13
Scheme 1. Synthesis of 1aÀc
(10) Shiu, H.-Y.; Chong, H.-C.; Leung, Y.-C.; Wong, M. K.; Che,
C.-M. Chem.;Eur. J. 2010, 16, 3308.
(11) (a) Truce, W. E.; Tichenor, G. J. W. J. Org. Chem. 1972, 37,
2391. (b) Halphen, P. D.; Owen, T. C. J. Org. Chem. 1973, 38, 3507.
(12) The assignments were made based on the 1H NMR of 5 and 6.
The dithiane 6 was synthesized and purified, while the dithiane 5 was
synthesized in the NMR tube from 1c and 2c in DMSO-d6 and Et3N. See
Supporting Information for the 1H NMR spectra.
(13) Singh, R.; Whitesides, G. M. J. Am. Chem. Soc. 1990, 112, 1191.
(14) See Supporting Information, Figure S7.
(15) Sutton, L. R.; Donaubauer, W. A.; Hampel, F.; Hirsch, A.
Chem. Commun. 2004, 1758.
(8) Serafimova, I. M.; Pufall, M. A.; Krishnan, S.; Duda, K.; Cohen,
€
M. S.; Maglathlin, R. L.; McFarland, J. M.; Miller, R. M.; Frodin, M.;
Taunton, T. Nat. Chem. Biol. 2012, 8, 471.
(9) Shiu, H.-Y.; Chan, T.-Y.; Ho, C.-M.; Liu, Y.; Wong, M.-K.; Che,
C.-M. Chem.;Eur. J. 2009, 15, 3839.
Org. Lett., Vol. 14, No. 18, 2012
4715

Figure 1. 1H NMR monitoring of thiol scrambling in DMSO-d6
and Et3N; (a) Partial spectra of 1a. (b) 5 min after the addition of
2c. (c) 50 min after the addition of 2c. (d) 5 min after the addition
of 2b. (e) 14 h after the addition of 2b. The labels correspond to
the H’s in Scheme 2.
Figure 2. 1H NMR monitoring of thiol scrambling in D2O:
DMSO-d6 (4:1) in PBS at pD = 7.7; (a) Partial spectra of 1b.
(b) 2 min after the addition of 2c. (c) 10 min after the addition of
2c. (d) 1 h after the addition of 2c. (e) 2 h after the addition of 2c.
The labels correspond to H’s in Scheme 4.
Scheme 3. Base Catalyzed Thioacetal Exchange
Scheme 5. Mechanism for the Thiol Exchange with 1b.
Scheme 4. Thiol Scrambling in D2O/DMSO-d6 (4:1) in PBS and
Deuteration of the R-Carbon
(Scheme 4), and the reaction was monitored by 1H NMR
spectroscopy and LCMS.16 This solvent system allows one
to follow the formation of any enol or enolate intermedi-
ates because they will necessarily be deuterated. When 1
equiv of 2c was added to a solution of 1b, a proton resonance
at δ = 6.1 ppm appeared in less than 2 min, which is
indicative of formation of the product 1c (Figure 2). A
resonance at δ = 5.2 ppm appeared a few minutes later,
assigned to the methine proton of product 8a (R-deuterio-8),
To elucidate the mechanism of the dynamic exchange of
thiols, VSCs, and BdTCs, we carried out studies using
D2O/DMSO-d6 (4:1) at pD 7.7 in phosphate buffer
(16) The reaction for LCMS was carried out in H2O/DMSO (4:1) and
diluted with methanol. The LCMS traces are included in the Supporting
Information.
4716
Org. Lett., Vol. 14, No. 18, 2012

first formed neutral intermediate. The enol can regenerate
the enolate leading to a thiolate leaving group departure,
which can rapidly scramble thiols and the stereochemistry
of the alkene as observed. Any other VSCs would undergo
the same fate, leading to enols 15 and 17. However,
ultimately protonation at the R-carbon gives 12, 5a, and
8a, which must revert slowly to the enolates because BdTC
scrambling is the slowest process.
From the conclusions above, a qualitative energy profile
diagram can be derived as shown in the Figure 3. In order
for the BdTCs to be the slowest species to exchange thiols,
one must conclude that the deprotonation of the R-carbon
is the slowest step in the entire sequence, including the
original conjugate addition of the thiols. The relative
energy levels of the enolates and enols fall into place as
shown. Future computational studies can put quantitative
numbers on this qualitative diagram.
In summary, we report that β-sulfido-R,β-unsaturated
carbonyls will reversibly and rapidly exchange thiols in
organic and aqueous media, ultimately with an equili-
brated mixture of thiols, vinyl sulfide carbonyls, and
β-dithiane carbonyls. These latter species equilibrate the
slowest, leading to the conclusion that enols are the neutral
intermediates through which vinyl sulfide carbonyls ex-
change thiols rather than dithiane carbonyls.
Figure 3. Reaction coordinate diagram showing relative ener-
gies of reactions, intermediates, products, and barriers that are
consistent with the experimental observations for rates of con-
jugate additions, thiol scrambling, and dithiane exchange.
and exists as a doublet rather than a triplet due to mono-
deuteration of the R-carbon. After 2 h, products 7a and 5a
were observed (see Scheme 4); the corresponding β-proton
resonances were at δ = 4.8 and 4.3 ppm, respectively.
The results of Figures 1 and 2 show that the scrambling
ofthiolsand VSCsoccurs faster thanthe formation, aswell
as the scrambling, of BdTCs. We expected the BdTCs to be
the intermediates through which the thiols and VSCs
would exchange. Hence, we wondered - How can VSCs
exchange thiols faster than BdTCs if BdTCs are the
intermediates? Therefore, the results show that these spe-
cies must not be the correct intermediates.
Acknowledgment. Support for this work from the Na-
tional Institute of Health (R01GM065515) and the Welch
Foundation (F-1151), and recently the NSF (CHE-1212971),
is gratefully acknowledged. Karin Keller and Zhong Ye are
thanked for the assistance with LCMS data. Unai Cossio is
acknowledged for reproducing the experiments.
Supporting Information Available. General experimen-
tal procedures and spectroscopic data for all the com-
pounds, experimental protocol for thiol exchange, and
LCMS traces are mentioned in the Supporting Informa-
tion. This material is available free of charge via the
Internet at http://pubs.acs.org.
Instead, the results force us to the following conclusions
(Scheme 5). The attack of thiolate 2c at the β-position of 1b
is a slow step. The attack generates an enolate 10, which is
ultimately deuterated at the R-carbon to create 12. How-
ever, it is well-known that protonation of enolates, such as
10, occurs faster on oxygen.17 Hence, enol 11 would be the
The authors declare no competing financial interest.
(17) Bernasconi, C. F.; Ni, J. X. J . Org. Chem. 1994, 59, 4910.
Org. Lett., Vol. 14, No. 18, 2012
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