Facile amide formation via S -nitrosothioacids

Article abstract of DOI:10.1021/ol1031393

Here we report a novel amide bond formation strategy from simple thioacid and amine starting materials. The reaction is mediated by unstable but very reactive S-nitrosothioacid intermediates. This fast reaction under mild conditions should be useful in synthesis.(Figure Presented)

Full text of DOI:10.1021/ol1031393

ORGANIC
LETTERS
2011
Vol. 13, No. 5
1092–1094
Facile Amide Formation via
S-Nitrosothioacids
Jia Pan, Nelmi O. Devarie-Baez, and Ming Xian*
Department of Chemistry, Washington State University, Pullman, Washington 99164,
United States
mxian@wsu.edu
Received December 26, 2010
ABSTRACT
Here we report a novel amide bond formation strategy from simple thioacid and amine starting materials. The reaction is mediated by unstable but
very reactive S-nitrosothioacid intermediates. This fast reaction under mild conditions should be useful in synthesis.
Due to the importance in biology and drug discovery,
amide or peptide bond formation is an active area in
organic chemistry. In the past decade, a number of new
strategies for the construction of amide bonds have been
discovered.1 In particular, thioacid or thioester derivatives
are attractive starting materials. Recent studies have revea-
led some unique reactivity of these sulfur-based compounds
and demonstrated some advantages of them compared to
carboxylic acid derivatives in amide and peptide bond form-
ing sequences.2 In our recent efforts to study new chemistry
of thiol S-nitrosation,3 we envisioned that if thioacids were
subjected to nitrosation (Scheme 1), the corresponding
S-nitrosothioacids (NTA) could be formed. Such a sulfur-
nitrosation process may activate thioacids and lead to a
facile acylation with certain nucleophiles. Herein, we report
a very efficient amide bond formation mediated by NTA.
It is known that S-nitrosothiols are unstable moieties.
Their chemistry, especially synthetically useful reactions,
has not been well studied.4 NTA type molecules have never
been clearly identified, although such compounds may be
involved in some thiyl radical formation processes.5 In our
study, wefirsttestedthepreparation of NTA. Oneexample
using thiobenzoic acid 1 is shown in Scheme 2. Compound
1 was treated with organonitrite (RONO) or HCl/NaNO2
in organic solutions at rt or 0 °C. The resulted species,
presumably NTA 2, showed a deep green color (UV
spectra of 2 were shown in the Supporting Information),
which is the characteristic color of tertiary S-nitrosothiols.
The NTA 2 appeared to be unstable as the green color
readily faded when we attempted to isolate compound 2.
The final isolated product was disulfide 3, which is the
(1) For selected reviews, see: (a) Bode, J. W. Curr. Opin. Drug
Discovery Dev. 2006, 9, 765. (b) Han, S.; Kim, Y. Tetrahedron 2004,
60, 2447. (c) Nilsson, B. L.; Soellner, M. B.; Raines, R. T. Annu. Rev.
Biophys. Biomol. Struct. 2005, 34, 91. (d) Kimmerlin, T.; Seebach, D.
J. Peptide Res. 2005, 65, 229.
(2) For selected examples, see: (a) Bao, Y.; Li, X.; Danishefsky, S. J.
J. Am. Chem. Soc. 2009, 131, 12924. (b) Wang, P.; Danishefsky, S. J.
J. Am. Chem. Soc. 2010, 132, 17045. (c) Crich, D.; Sasaki, K. Org. Lett.
2009, 11, 3514. (d) Crich, D.; Sharma, I. Angew. Chem., Int. Ed. 2009, 48,
2355. (e) Crich, D.; Sharma, I. Angew. Chem., Int. Ed. 2009, 48, 7591.
(f) Crich, D.; Sana, K.; Guo, S. Org. Lett. 2007, 9, 4423. (g) Dawson,
P. E.; Muir, T. W.; Clark-Lewis, I.; Kent, S. B. H. Science 1994, 266, 776.
(h) Dawson, P. E.; Churchill, M. J.; Ghadiri, M. R.; Kent, S. B. H. J. Am.
Chem. Soc. 1997, 119, 4325. (i) Liu, R.; Orgel, L. E. Nature 1997, 389, 52.
(j) Shangguan, N.; Katukojvala, S.; Greenberg, R.; Williams, L. J.
J. Am. Chem. Soc. 2003, 125, 7754. (k) Sheehan, J. C.; Johnson, D. A.
J. Am. Chem. Soc. 1952, 74, 4726.
(3) (a) Wang, H.; Xian, M. Angew Chem., Int. Ed. 2008, 47, 6598.
(b) Zhang, J.; Wang, H.; Xian, M. J. Am. Chem. Soc. 2009, 131, 3854.
(c) Wang, H.; Xian, M. J. Am. Chem. Soc. 2009, 131, 13238. (d) Zhang,
J.; Li, S.; Zhang, D.; Wang, H.; Whorton, A. R.; Xian, M. Org. Lett.
2010, 12, 4208. (e) Zhang, D.; Devarie-Baez, N. O.; Pan, J.; Wang, H.;
Xian, M. Org. Lett. 2010, 12, 5674. (f) Devarie-Baez, N. O.; Xian, M.
Org. Lett. 2010, 12, 752.
(4) For selected reviews on S-nitrosothiols, see: (a) Williams, D. L. H.
Acc. Chem. Res. 1999, 32, 869. (b) Szacilowski, K.; Stasicka, Z. Prog.
React. Kinet. Mech. 2001, 26, 1. (c) Al-Sadoni, H. H.; Ferro, A. Curr.
Med. Chem. 2004, 11, 2679. (d) Wang, P. G.; Xian, M.; Tang, X.; Wu, X.;
Wen, Z.; Cai, T.; Janczuk, A. J. Chem. Rev. 2002, 102, 1091.
(5) Potapenko, D. I.; Bagryanskaya, E. G.; Tsentalovich, Y. P.;
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r
10.1021/ol1031393
Published on Web 01/27/2011
2011 American Chemical Society

Table 1. NTA Mediated Amide Coupling
Scheme 1. Proposed NTA Coupling
Scheme 2. Formation of NTA and the Reaction of NTA
Scheme 3. Solvent Effects of NTA Mediated Amide Formation
expected decomposition product from S-nitrosothiols.
Although NTA 2 was unstable, we tested the idea to trap
NTA in situ with some nucleophiles. Amines proved to be
excellent substrates, and the formation ofamide bonds was
achieved in a very effective way (see table 1 below). It was
also remarkable that NTA, unlike other activated carbo-
xylic acid derivatives, did not show any reactivity toward
hydroxyl groups (such as benzyl alcohol, phenol, and N-
hydroxysuccinimide). This is promising for further study
of selective N-acylation.
We then optimized the conditions for this NTA media-
ted coupling between thioacids and amines. The best pro-
cedure was to mix the thioacid (1.0 equiv) and the amine
(1.1 equiv) at 0 °C. Commercially available amyl nitrite
(2.0 equiv) was then added dropwise into the solution. In
this process, no additional base was needed. The formation
of the desired amide product was observed immediately
and in high yields (monitored by TLC). As shown in
Scheme 3 (entries 1-4), this reaction worked nicely in a
number of common solvents including THF, DCM, DMF,
and CH3CN. Water seemed to have little effect on the coupl-
ing as the reaction gave similar results in buffer containing
systems (entries 5 and 6). This process proved to be a very
fast process, as in all the solvents the reaction completed
within minutes at 0 °C.
To prove the reaction was indeed involving NTA, we
carried out several control experiments (Scheme 4). The
reaction between thioacid 1 and R-methyl-benzylamine
Org. Lett., Vol. 13, No. 5, 2011
1093

The results shown above suggested that NTA act as an
effective activating group to facilitate amide formation. To
test the generality of this reaction, a series of thioacids and
amines were employed under the optimized conditions
(Table 1). The reaction proved to be very efficient with
not only primary amines but also sterically hindered
secondary amines (entries 1-8). We also tested several
amino acid substrates. As expected, the corresponding
dipeptide products were obtained in good yields in all
substrates (entries 9-14). As shown in entry 10, a free
hydroxyl group did not interfere with the reaction, which
was consistent with our previous selectivity results. In all
the cases, the reaction was able to be complete within 10
min at 0 °C.
Scheme 4. Control Experiments
In summary, we presented here a novel amide bond
formation strategy from simple thioacids and amines. This
process was mediated by reactive NTA intermediates. It
revealed nitrosation as a novel strategy for thioacid activa-
tion. Compared to other amide formation methods, this
reaction only utilized readily available organonitrite as the
activation reagent. It took place under very mild reaction
conditions, and the reaction rate was extremely fast. The
chemistry is easily executed. It also showed excellent
selectivity toward amines over hydroxyls. In our opinion,
this method should be promising for peptide coupling/
ligation and selective N-acylation. Further studies on the
detailed reaction mechanism, the application of this
method in synthesis and biology, and the exploration of
new chemistry of NTA are currently ongoing in our
laboratory.
only led to the formation of amide 4a in trace amounts at
rt, even when the amine was used in large excess (10 equiv).
A previous report by Orgel et al. also suggested that
thioacids should not directly react with amines to form
amides.2i As NTA are unstable species and could easily
decompose to form disulfides, we then tested the reaction
between disufide 3 and R-methyl benzylamine. Again, we
did not observe the formation of the amide product
(Scheme 4B). The addition of amyl nitrite into disulfide 3
did not lead to any amide formation either (Scheme 4C).
Finally, we tried to capture the unstable NTA intermediates
using the reductive ligation,3a which is a specific reaction of
SNO groups. After several attempts, we were able to
obtain the desired ligation product 6 using substrate 1a
(Scheme 4D). Although the yield of 6 was only 23%, the
formation of this sulfenamide product strongly supported
the presence of an NTA intermediate in the reaction.
Acknowledgment. This work is supported in part by
NIH (R01GM088226) and a CAREER award from the
NSF (0844931).
Supporting Information Available. Spectroscopic and
analytical data and selected experimental procedures.
This material is available free of charge via the Internet
at http://pubs.acs.org
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Org. Lett., Vol. 13, No. 5, 2011