Exploration of the "traceless" reductive ligation of S-nitrosothiols

Article abstract of DOI:10.1021/ol802663q

(Chemical Equation Presented) The first "traceless" reductive ligation of S-nitrosothiols using phosphine ester/thioester conjugates is reported. Experiments also show that stable thiolmidate compounds could be formed in the reaction between S-nitrosothio

Full text of DOI:10.1021/ol802663q

ORGANIC
LETTERS
2009
Vol. 11, No. 2
477-480
Exploration of the “Traceless”
Reductive Ligation of S-Nitrosothiols
Jiming Zhang, Hua Wang, and Ming Xian*
Department of Chemistry, Washington State UniVersity, Pullman, Washington 99164
mxian@wsu.edu
Received November 18, 2008
ABSTRACT
The first “traceless” reductive ligation of S-nitrosothiols using phosphine ester/thioester conjugates is reported. Experiments also show that
stable thioimidate compounds could be formed in the reaction between S-nitrosothiols and some phosphine-thioester substrates.
S-Nitrosothiol (RSNO) formation on cysteine residues rep-
resents an important post-translational modification that
transduces nitric oxide (NO)-dependent signals.1 However,
the detection of RSNOs in biological systems is still a
challenge due to the lability of the SNO moiety.2 The
analytical deficiencies become evident when it is observed
that reported values of the analysis of the same tissue or
biological fluid by different research groups cover some
orders of magnitude.3 In our opinion, SNO is a unique
functional group that should have distinct reactivity from
other biological functional groups. If we can develop new
bioorthogonal reactions specifically targeting SNO and
converting unstable S-nitrosothiols to stable products, such
a reaction would hold considerable promise in applications
for RSNO detection.
mechanism is similar to the Staudinger ligation of azides.5
RSNO 1 reacts with 2 equiv of ligation substrate 2 to
generate the corresponding phosphine oxide and aza-ylide
3.4,6 Then, an intramolecular reaction between the ester and
aza-ylide may occur to provide phosphorane 5. Finally,
hydrolysis of 5 in the presence of water should furnish the
final product 6. This reductive ligation is quite selective for
SNO moieties and could potentially be applied for RSNO
detection.4
Although the ligation reaction using the phosphine-ester
substrate shown in Scheme 1 has been demonstrated, a
modification in which an amide bond is formed between the
two coupling partners to give a product without the phosphine
oxide moiety might be even more attractive. It might provide
a new way to prepare useful sulfenamide derivatives. Similar
to the well-studied traceless Staudinger ligation pioneered
by Raines and Bertozzi,7 we proposed that compounds like
7 should undergo a traceless reductive ligation with RSNOs
(Scheme 2). As such, the nucleophilic nitrogen atom gener-
Recently, our laboratory developed a reductive ligation
of RSNOs which converts RSNOs to stable sulfenamide
products in one step (Scheme 1).4 We believe the reaction
(1) For selected reviews, see: (a) Lancaster, J. R., Jr. Nitric Oxide 2008,
19, 68. (b) Foster, M. W.; McMahon, T. J.; Stamler, J. S. Trends Mol.
Med. 2003, 9, 160. (c) Zhang, Y.; Hogg, N. Free Radical Biol. Med. 2005,
38, 831.
(4) Wang, H.; Xian, M. Angew Chem. Int. Ed. 2008, 47, 6598.
(5) (a) Saxon, E.; Bertozzi, C. R. Science 2000, 287, 2007. (b) Lin,
F. L.; Hoyt, H. M.; van Halbeek, H.; Bergman, R. G.; Bertozzi, C. R. J. Am.
Chem. Soc. 2005, 127, 2686. (c) Koehn, M.; Breinbauer, R. Angew. Chem.,
Int. Ed. 2004, 43, 3106.
(2) For recent reviews on RSNO detection, see: (a) Gow, A.; Doctor,
A.; Mannick, J.; Gaston, B. J. Chromatogr. B 2007, 851, 140. (b)
Kettenhofen, N. J.; Broniowska, K. A.; Keszler, A.; Zhang, Y.; Hogg, N.
J. Chromatogr. B. 2007, 851, 152. (c) MacArthur, P. H.; Shiva, S.; Galdwin,
M. T. J. Chromatogr. B 2007, 851, 93. (d) Jaffrey, S. R. Methods Enzymol.
2005, 396, 105.
(6) Haake, M. Tetrahedron Lett. 1972, 33, 3405
.
(7) (a) Nilsson, B. L.; Kiessling, L. L.; Raines, R. T. Org. Lett. 2000,
2, 1939. (b) Saxon, E.; Armstrong, J. I.; Bertozzi, C. R. Org. Lett. 2000, 2,
2141. (c) Soellner, M. B.; Tam, A.; Raines, R. T. J. Org. Chem. 2006, 71,
9824. (d) Soellner, M. B.; Nilsson, B. L.; Raines, R. T. J. Am. Chem. Soc.
2006, 128, 8820. (e) Tam, A.; Soellner, M. B.; Raines, R. T. J. Am. Chem.
Soc. 2007, 129, 11421.
(3) (a) Giustarini, D.; Milzani, A.; Dalle-Donne, I.; Rossi, R. J. Chro-
matogr. B 2007, 851, 124. (b) Gladwin, M. T.; Wang, X.; Hogg, N. Free
Radical Biol. Med. 2006, 41, 557.
10.1021/ol802663q CCC: $40.75
Published on Web 12/19/2008
2009 American Chemical Society

Scheme 1
.
Reductive Ligation of RSNOs
Scheme 3. Traceless Reductive Ligation Substrates
products 17a-c in moderate yield (35-65%). Thioester 14
is the most efficient substrate for the traceless Staudinger
ligation discovered by Raines et al.7 However, the reaction
between t-BuSNO and 14 only provided the ligation product
in 22% yield. When substrates 13 and 14 were treated with
other relatively stable tertiary RSNOs, corresponding ligation
products were obtained in similar yields (see the Supporting
Information for details). Under the same conditions, no
ligation product was observed for unstable primary RSNOs
such as BnSNO and S-nitrosocysteine derivatives. We
noticed that these traceless ligations were much slower when
compared to the original ligation.4 It usually takes more than
12 h for the aza-ylide intermediates (or other possible
intermediates such as compound 23 in Scheme 4) to be
completely consumed (monitored by TLC or 31P NMR).
Therefore, the hydrolysis of these intermediates may pre-
dominate the reaction.
ated from the reaction between RSNO and the ligation
substrate 7 should attack the carbonyl group. Upon hydroly-
sis, it should produce the sulfenamide 108 and liberate the
phosphine oxide 11. Here we report the first example of the
“traceless” reductive ligation.
Scheme 2. Traceless Reductive Ligation of RSNOs
Table 1. Traceless Ligation of t-BuSNO
Four suitable phosphines (compounds 12-15, Scheme 3)
were used to examine the traceless reductive ligation of
RSNOs. These compounds were prepared following the
procedures reported previously.7 In each case, intramolecular
transfer of the acetyl group from the phosphine to SNO-
bearing compound would indicate a successful ligation
reaction.
t-Butyl SNO 16 was used as the model in this study (Table
1). Since water is necessary for the ligation process, we
screened a number of water-containing solvent systems such
as THF/water, dioxane/water, CH3CN/water, DMF/water, etc.
The best conditions were found to be a mixture of THF,
CH3CN, and water (1.5/1.5/1). When tert-butyl SNO was
treated with 12, no detectable ligation product was observed.
We only isolated corresponding phosphine oxide. The
flexibility of the methylene bridge in 12 may reduce the rate
of cyclization such that the hydrolysis of aza-ylide intermedi-
ate predominated. In contrast, the treatment of t-BuSNO with
ester phosphines 13a-c led to the desired traceless ligation
Interestingly, when thioester 15 was used in the reaction,
we not only obtained the desired ligation product 17a in 23%
(8) For a review on the chemistry of sulfenamides, see: Craine, L.;
Raban, M. Chem. ReV. 1989, 89, 689.
478
Org. Lett., Vol. 11, No. 2, 2009

yield but also isolated a stable thioimidate product 18 as the
major product (59% yield, Table 1, entry 6). Similar imidate
compounds were also observed in some Staudinger ligation
processes.9 We noticed the presence of compound 18 under
the reaction conditions (THF/CH3CN/H2O, with or without
phosphine 15) did not lead to any detectable 17a even after
3 days. Therefore, we suspected that sulfenamide 17a was
not the hydrolysis product of thioimidate 18. We propose
the reaction between RSNO 16 and thioester phosphine 15
involves two different pathways (Scheme 4): in pathway A,
the aza-ylide intermediate 19 undergoes desired ligation
process to form amidophosphonium salt 21. Then, hydrolysis
of the P-N bond of the amidophosphonium salt forms
sulfenamide 17a and phosphine oxide 22. In path B, the aza-
ylide intermediate 19 undergoes an intramolecular aza-Wittig
reaction10 to produce the stable thioimidate 18.
Table 2. Thioimidate Formation Results
Scheme 4. Proposed Reaction Mechanism of RSNOs
(such as LiOH) H2O/CH3OH mixtures at 25 °C for 24 h does
not lead to the formation of any detectable sulfenamide
products, nor was any decomposition observed. However, it
is possible to convert them to sulfenamides under some
conditions. For example, the treatment of compound 18 with
We envisioned the absence of water in the reaction
medium might lead to better yield of the thioimidate
products. To test this hypothesis, as well as to examine the
generality of this process, we tested the reactions of a series
of tertiary RSNOs with thioester phosphine 15a and 15b in
pure THF. As shown in Table 2, in all cases, the thioimidate
products were obtained in good to excellent yields. Interest-
ingly, when these tertiary RSNOs were treated with ester
phosphine substrates (13a-c) in anhydrous THF, we did not
observe the formation of any O-imidate products.
11
CuCl2 in the presence of water led to sulfenamide 17a in
excellent yield (Scheme 5).
Scheme 5. Hydrolysis of Thioimidates
The thioimidate products obtained here appear to be quite
stable. Incubation of them in acidic (such as AcOH) or basic
(9) (a) Rose, M. W.; Xu, J.; Kale, T. A.; O’Doherty, G.; Barany, G.;
Distefano, M. D. Peptide Sci. 2005, 80, 164. (b) Restituyo, J. A.; Comstock,
L. R.; Petersen, S. G.; Stringfellow, T.; Rajski, S. R. Org. Lett. 2003, 5,
4357.
In conclusion, the first example of traceless reductive
ligation of RSNOs has been demonstrated. We also found
(10) (a) Palacios, F.; Alonso, C.; Aparicio, D.; Rubiales, G.; de los
Santos, J. M. Tetrahedron 2007, 63, 523. (b) Fresneda, P. M.; Molina, P.
Synlett 2004, 1. (c) Molina, P.; Vilaplana, M. J. SynthesissStuttgart 1994,
1197.
(11) Kopylova, B. V.; Bragina, I. O.; Kandror, I. I.; Freidlina, R. K.
IzVest. Akade. Nauk SSSR 1980, 719.
Org. Lett., Vol. 11, No. 2, 2009
479

that stable thioimidate products could be obtained from the
reaction between thioester ligation substrates and RSNOs.
Refinements of these preliminary studies may lead to new
methods for the detection of S-nitrosothiols.
Prof. R. Ronald in the Department of Chemistry for helpful
discussions.
Supporting Information Available: Synthetic procedures,
spectroscopic data, and experimental procedures. This mate-
rialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
Acknowledgment. This work has been supported by
Washington State University. We thank Prof. P. Garner and
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Org. Lett., Vol. 11, No. 2, 2009