Epimerization- and protecting-group-free synthesis of peptidomimetic conjugates from amphoteric amino aldehydes

Article abstract of DOI:10.1021/ja076155p

We have developed a novel protecting-group-free strategy for the synthesis of peptidomimetic conjugates. The high degrees of stereocontrol and chemoselectivity achieved in this chemistry hinge upon unusual preferences of the amphoteric amino aldehydes. Th

Full text of DOI:10.1021/ja076155p

Published on Web 10/27/2007
Epimerization- and Protecting-Group-Free Synthesis of Peptidomimetic
Conjugates from Amphoteric Amino Aldehydes
Xinghan Li and Andrei K. Yudin*
DaVenport Research Laboratories, Department of Chemistry, UniVersity of Toronto, 80 St. George Street,
Toronto, Ontario, Canada, M5S 3H6
Received August 15, 2007; E-mail: ayudin@chem.utoronto.ca
Peptide bonds are ubiquitous components of numerous bioactive
molecules that play important roles in regulating cellular function.
Replacement of the peptide bonds by their isosteres can give rise
to molecules with superior stability and specificity. For example,
the so-called reduced amide bond isosteres contain aminomethylene
functional groups in place of the selected amide linkages (Figure
1a).1 This structural substitution is present in a wide range of
aspartyl protease inhibitors.2 The aminomethylene fragment is
isosteric with the tetrahedral transition state formed during amide
bond hydrolysis. This ensures that the peptidomimetic inhibitor
binds to the protease target tighter than the substrate. At the same
time, the reduced amide bond analogue is not cleaved by the
protease and often displays better binding than its peptide proto-
type.3 Many different modes of interaction between proteases and
their inhibitors have been observed by X-ray crystallography.4 The
diversity of recognition mechanisms underscores the importance
of modulating the target/peptidomimetic ligand interactions in the
vicinity of the active site. Herein, we report a general strategy that
addresses three critical issues in methodology directed toward
peptidomimetic molecules: (1) the reaction sequence can be used
in order to selectively attach an unprotected aziridine electrophile
to an amino-acid-containing molecule; (2) it delivers a peptidomi-
metic connection without epimerization on either side of the reduced
amide bond; and (3) it allows for a late-stage peptidomimetic
ligation.
sequence has been used in numerous academic and industrial
applications, there are significant challenges that face this chemistry.
The amino aldehydes as well as their immediate precursors are
notoriously sensitive to epimerization.6 In addition, the imine/
enamine equilibrium triggered during the reductive amination can
lead to further epimerization on both the amine and the aldehyde
sides of the peptidomimetic fragment (Figure 1b).7 Last but not
least, reliance on protecting groups at nitrogen in amino aldehydes
diminishes the synthetic efficiency of these operations.
We became interested in exploring the potential of the recently
discovered unprotected amino aldehydes in constructing peptido-
mimetic conjugates.8a The kinetic amphoterism has been coined in
order to describe the coexistence of an unprotected aziridine and
aldehyde groups in such molecules.8b We reasoned that if one were
to attach an unprotected aziridine unit to an amino acid residue via
a non-peptide bond,9 one would not only gain access to an
electrophilic peptidomimetic conjugate but also facilitate synthesis
of both natural and unnatural amino-acid-based peptidomimetics
via aziridine ring-opening chemistry (eq 1). Standard reductive
amination conditions (NaBH3CN, MeOH, 1% HOAc) on aziridine
aldehyde dimers and amino acid derivatives delivered poor conver-
sions and yields. Further experiments revealed that ZnCl2/NaBH3-
CN combination gives optimal selectivity. Most importantly, the
reductive amination was not accompanied by overalkylation or
epimerization on either side of the peptidomimetic connection. A
mechanistic investigation uncovered the salient features of this
process (Scheme 1). According to our data, the monomeric amino
aldehyde-derived imine formation is not taking place during the
reaction. Instead, the adduct 3, formed upon condensation between
the amino aldehyde dimer 1 and amine 2, participates in an
unfavorable equilibrium with its “half-opened” form 4, which is
rapidly reduced by the hydride transfer agent. The short lifetime
of 4 ensures that tautomerization and, therefore, epimerization are
negligible. Using this protocol, a variety of unprotected amino
aldehydes can be cleanly conjugated with R-amino acid derivatives
(Table 1).
Figure 1. (a) Amide bond hydrolysis and its aminomethylene isostere; (b)
the challenges of peptidomimetic construction.
The absence of epimerization on the aldehyde side of the
aminomethylene linkage is secured through energetically uphill
enolization of the strained aziridine aldehyde. Another key feature
of this process is that the equilibrium concentration of the free
The most widely employed strategy toward reduced amide bond
isosteres is based on N-protected amino aldehydes (Figure 1b).5
Typically, a peptide or a nitrogen-protected amino acid is converted
into the corresponding aldehyde by first forming an ester or a
Weinreb amide, which is subsequently reduced by a hydride transfer
reagent. These steps are followed by reductive amination with an
appropriate amine component. Although this valuable reaction
aldehyde is unobservably low, resulting in no overalkylation.10
A
presently unexplained fidelity with regard to homochiral dimer re-
formation during the reaction must be responsible for the low
concentration of the free aldehyde. A crossover between two
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10.1021/ja076155p CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Mechanistic Underpinnings of Epimerization-Free
Synthesis of Peptidomimetic Conjugates
concentration of the free aldehyde species in that solvent. Impor-
tantly, the reductive amination is not occurring in trifluoroethanol.
Instead, the reaction leads to preferential aldehyde reduction,
providing further evidence for the dimer-driven mechanism depicted
in Scheme 1. The utility of amino acid conjugates is demonstrated
by a thioacid-triggered process (eq 2). This sequence offers a
possibility for a peptidomimetic ligation of two fragments such that
a reduced amide bond isostere is specifically introduced at the
cysteine residue with complete stereocontrol of the nearby chiral
centers.11
In closing, we have developed a protecting-group-free strategy
for replacing amide bonds with versatile aziridine-containing
templates for the synthesis of peptidomimetic molecules. A high
degree of stereocontrol achieved during reductive amination hinges
upon unusual preferences of the amphoteric amino aldehydes. One
can anticipate straightforward construction of structurally diverse
affinity probes using this chemistry.12 The resulting conjugates also
offer a possibility for peptidomimetic ligation. Taken together, these
findings should allow access to templates for introducing both
natural and unnatural amino acid residues in close proximity to
the reduced amide bond isosteres. Studies along these lines are being
actively pursued in our laboratories.
Table 1. The Scope of Peptidomimetic Conjugation Chemistrya
Acknowledgment. We thank NSERC and CIHR for financial
support of this work. Mr. Ryan Hili is thanked for helpful
discussions.
Supporting Information Available: Experimental procedures and
characterization data for all unknown compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) Babine, R. E.; Bender, S. L. Chem. ReV. 1997, 97, 1359.
(2) (a) Maly, D. J.; Huang, L.; Ellman, J. A. ChemBioChem 2002, 3, 17. (b)
Leung, D.; Abbenante, G.; Fairlie, D. P. J. Med. Chem. 2000, 43, 305.
(3) Szelke, M.; Leckie, B.; Hallett, A.; Jones, D. M.; Sueiras, J.; Atrash, B.;
Lever, A. F. Nature 1982, 299, 555.
(4) Wlodawer, A.; Erickson, J. Annu. ReV. Biochem. 1993, 62, 543.
(5) Gryko, D.; Chalko, J.; Jurczak, J. Chirality 2003, 15, 514.
(6) Potetinova, J. V.; Milgotina, E. I.; Makarov, V. A.; Voyushina, T. L. Russ.
J. Bioorg. Chem. 2001, 27, 141.
(7) Epimerizations on both the amine and the aldehyde sides during
peptidomimetic synthesis have been documented: (a) Aurelio, L.;
Brownlee, R. T. C.; Hughes, A. B. Chem. ReV. 2004, 104, 5823. (b)
Wasserman, H. H.; Berger, G. D.; Cho, K. R. Tetrahedron Lett. 1982,
23, 465. (c) Jensen, K. J.; Alsina, J.; Songster, M. F.; Vagner, J.; Albericio,
F.; Barany, G. J. Am. Chem. Soc. 1998, 120, 5441. (d) Giannis, A.; Kolter,
T. Angew. Chem., Int. Ed. Engl. 1993, 32, 1244. (e) Ho, P. T.; Chang,
D.; Zhong, J. W. X.; Musso, G. F. Peptide Res. 1993, 6, 10.
(8) (a) Hili, R.; Yudin, A. K. J. Am. Chem. Soc. 2006, 128, 14772. (b) Hili,
R.; Yudin, A. K. Chem.sEur. J. 2007, 13, 6538.
(9) For recent applications of aziridine carboxylic acids, see: (a) Vicik, R.;
Busemann, M.; Baumann, K.; Schrimeister, T. Curr. Top. Med. Chem.
2006, 6, 331. (b) Galonic´, D. P.; Ide, N. D.; van der Donk, W. A.; Gin,
D. Y. J. Am. Chem. Soc. 2005, 127, 7359.
(10) Overalkylation during reductive amination is a recognized problem: Abdel-
Magid, A. F.; Carson, K. G.; Harris, B. D.; Maryanoff, C. A.; Shah, R.
D. J. Org. Chem. 1996, 61, 3849.
a Unless stated otherwise, the reactions were carried out using 0.5 equiv
of the dimer (1 equiv of aldehyde), 1.2 equiv of amine, 1.5 equiv of
NaBH3CN, and 1 equiv of ZnCl2 in THF and MeOH (1/1) at room
temperature. b The corresponding monomer. c Isolated yield.
(11) For natiVe chemical ligation, see: Johnson, E. C. B.; Kent, S. B. H. J.
Am. Chem. Soc. 2006, 128, 6640.
(12) (a) Evans, M. J.; Cravatt, B. F. Chem. ReV. 2006, 106, 3279. (b) Fonovic´,
M.; Bogyo, M. Curr. Pharm. Des. 2007, 13, 253.
different amino aldehyde dimers has been detected by ESI MS only
in trifluoroethanol (pKa ) 12.4), definitively suggesting appreciable
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