Published on Web 04/07/2006
Mechanism of Thio Acid/Azide Amidation
Robert V. Kolakowski, Ning Shangguan, Ronald R. Sauers, and
Lawrence J. Williams*
Contribution from the Department of Chemistry and Chemical Biology, Rutgers,
The State UniVersity of New Jersey, Piscataway, New Jersey 08854
Received November 3, 2005; E-mail: ljw@rutchem.rutgers.edu
Abstract: A combined experimental and computational mechanistic study of amide formation from thio
acids and azides is described. The data support two distinct mechanistic pathways dependent on the
electronic character of the azide component. Relatively electron-rich azides undergo bimolecular coupling
with thiocarboxylates via an anion-accelerated [3+2] cycloaddition to give a thiatriazoline. Highly electron-
poor azides couple via bimolecular union of the terminal nitrogen of the azide with sulfur of the thiocarboxylate
to give a linear adduct. Cyclization of this intermediate gives a thiatriazoline. Decomposition to amide is
found to proceed via retro-[3+2] cycloaddition of the neutral thiatriazoline intermediates. Computational
analysis (DFT, 6-31+G(d)) identified pathways by which both classes of azide undergo [3+2] cycloaddition
with thio acid to give thiatriazoline intermediates, although these paths are higher in energy than the
thiocarboxylate amidations. These studies also establish that the reaction profile of electron-poor azides is
attributable to a prior capture mechanism followed by intramolecular acylation.
Introduction
capture may be significantly faster than conventional amidation.
Native peptide ligation,5 a powerful amidation reaction, is
It is of considerable interest to develop new amidation
reactions that can accommodate a broader range of substrates
than conventional amidation and that are compatible with a
wider range of reaction conditions, including in vitro and in
vivo studies.1,2 Systematic mechanistic and methodological
investigations have shown that chemoselectivity, solvent com-
patibility, and rate of intermolecular coupling limit conventional
intermolecular amidation. One alternative, and the basis for most
new methods, is to apply a nonacylation reaction to bring the
amine, or amine equivalent, and the acyl group into proximity.3
To be broadly effective, the nonacylating reaction must be
chemically orthogonal to the functionality of the coupling
partners and the solvent. This prior-capture strategy has the
potential to facilitate amide bond formation by rendering the
amidation reaction intramolecular. If the prior-capture reaction
is faster than conventional intermolecular amidation and leads
to a favorable geometry for a subsequent intramolecular
amidation,4 then the overall rate of amide formation by prior-
chemoselective, can be performed on unprotected peptide
segments, and is effective in water. This and related methods
rely on amino acid side-chain functionality to facilitate the prior-
capture step.6 Recent reports suggest that methods for the
chemical synthesis of fully elaborated post-translationally modi-
fied protein targets may soon be within reach.7,8 Among the
(5) C-terminal thio ester peptide segments couple with N-terminal cysteine
peptide segments to form an amide. Thus, trans-thioesterification is a
suitably fast prior-capture reaction. Intramolecular S- to N-acyl transfer
via a five-membered transition state constitutes a geometry that is not overly
congested. Consequently, the chemical synthesis of proteins and protein-
like compounds of approximately one hundred residues is feasible, see:
(a) Dawson, P. E.; Muir, T. W.; Clark-Lewis, I.; Kent, S. B. Science 1994,
266, 776. (b) Muir, T.; Dawson, P. E.; Kent, S. B. H. Methods Enzymol.
1997, 289, 266. See also, ref 2c. Compare with: (c) Gutte, B.; Merrifield,
R. B. J. Am. Chem. Soc. 1969, 91, 501. (d) Denkewalter, R. G.; Veber, D.
F.; Holly, F. W.; Hirschmann, R. J. Am. Chem. Soc. 1969, 91, 502. (e)
Yajima, H.; Fujii, N. J. Chem. Soc., Chem. Commun. 1980, 3, 115.
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2000, 2, 23. (b) Hacking, T. M.; Griffin, J. H.; Dawson, P. E. Proc. Natl.
Acad. Sci. USA 1999, 96, 10068. (c) Canne, L. E.; Bark, S. J.; Kent, S. B.
H. J. Am. Chem. Soc. 1996, 118, 5891. Alternatives to native amide linkages
have been explored and reviewed, see ref 2d and (d) Liu, C.-F.; Tam, J. P.
Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 6584. See also: (e) Tolbert, T. J.;
Wong, C. H. J. Am. Chem. Soc. 2000, 122, 5421. (f) Hohsaka, T.; Ashizuka,
Y.; Sasaki, H.; Murakami, H.; Sisido, M. J. Am. Chem. Soc. 1999, 121,
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(7) (a) Nilsson, B. L.; Hondal, R. J.; Soellner, M. B.; Raines, R. T.; J. Am.
Chem. Soc. 2003, 125, 5268. (b) Galonic, D. P.; Van der Donk, W. A.;
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V. Y.; Lyon, G. J.; Muir, T. W.; Danishefsky, S. J. Angew. Chem., Int. Ed.
Engl. 2003, 42, 431.
(8) (a) Nilsson, B. L.; Kiessling, L. L.; Raines, R. T. Org. Lett. 2000, 2, 1939.
(b) Nilsson, B. L.; Kiessling, L. L.; Raines, R. T. Org. Lett. 2001, 3, 9. (c)
Saxon, E.; Armstrong, J. I.; Bertozzi, C. R. Org, Lett. 2000, 2, 2141. (d)
Merkx, R.; Rijkers, D. T. S.; Kemmink, J.; Liskamp, R. M. J.; Tetrahedron
Lett. 2003, 44, 4515. (e) David, O.; Meester W. J. N.; Bieraugel, H.;
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(1) Wieland, T., Bodanszky, M.; World of Peptides: A Brief History of Peptide
Chemistry; Springer-Verlag: New York, 1991.
(2) (a) Prescher, J. A.; Bertozzi, C. R. Nat. Chem. Biol. 2005, 1, 13. (b) Saxon,
E.; Bertozzi, C. R. Science 2000, 287, 2007. (c) Kochendeoerfer, G. G.;
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(3) Prior Capture: (a) Wieland, T.; Bokelmann, E.; Bauer, L.; Lang, H.; Lau,
H.; Schafer, W. Liebigs Ann. 1953, 583, 129. (b) Brenner, M.; Zimmerman,
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(4) Avoidance of steric congestion in the transition state has been shown to be
critically important for prior-capture strategies, see (a) Kemp, D. S.; Carey,
R. I. J. Org. Chem. 1993, 58, 2216, reviewed in: (b) Coltart, D. M.
Tetrahedron 2000, 56, 3449.
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10.1021/ja057533y CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 5695-5702
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