Iterative, aqueous synthesis of β3-oligopeptides without coupling reagents

Article abstract of DOI:10.1021/ja057706j

The chemoselective synthesis of amides by decarboxylative couplings of α-ketoacids and isoxazolidines makes possible an iterative approach to poly-β³-peptides. Peptide assembly occurs under aqueous conditions and requires no coupling reagents.

Full text of DOI:10.1021/ja057706j

Published on Web 01/17/2006
Iterative, Aqueous Synthesis of â3-Oligopeptides without Coupling Reagents
Nancy Carrillo, Eric A. Davalos, Justin A. Russak, and Jeffrey W. Bode*
Department of Chemistry and Biochemistry, UniVersity of California at Santa Barbara,
Santa Barbara, California 93106-9510
Received November 12, 2005; E-mail: bode@chem.ucsb.edu
Scheme 2a
The discovery that oligomers of â-amino acids fold into discrete,
stable secondary structures has fuelled an explosion in the synthesis,
study, and application of these peptidomimetics.1 Reported proper-
ties, including membrane translocation,2 antimicrobial activity,3
somastatin antagonism,4 and disruption of protein-protein interac-
tions,5 attest to their diverse therapeutic potential. Further advances,
however, are limited by the relative difficulty of preparing â-oligo-
a Conditions: (a) C6H6, 80 °C, cat. Bu2SnO, 12-36 h; (b) HClO4, MeOH
peptides.6,7 The constituent monomers, â-amino acids, often require
or NH2OH‚HCl, MeOH.
multiple steps for their preparation, and the known routes are not
readily scaled.8 Furthermore, the assembly of â-oligopeptides by
standard deprotection/activation peptide coupling approaches is
plagued by sluggish reactivity, necessitating the use of several
equivalents of the precious protected â-amino acids. These synthetic
challenges conspire to make the preparation of biologically interest-
ing poly-â-peptides a time-consuming and expensive endeavor,
thereby limiting the further advancement of these highly promising
peptidomimetics.
Chart 1
We recently developed a novel route to the construction of amide
bonds by the decarboxylative condensation of R-ketoacids and N-
alkylhydroxylamines.9 This powerful, mechanistically unique ami-
dation requires no reagents and produces no byproducts. However,
in order for it to be applied to the stepwise synthesis of peptide
oligomers, challenges in the design, synthesis, and reactivity of
suitable monomers must be addressed. We selected the preparation
of â3-oligopeptides as an initial target and now disclose an approach
for their synthesis by iterative couplings of isoxazolidines under
aqueous conditions (Scheme 1).
lectivity simply by heating a mixture of the D-mannose-derived
hydroxylamine 2, acrylate 3, and the appropriate aldehyde. The
isoxazolidines were isolated with >99% enantiomeric purity by
chromatography or crystallization.13 Auxiliary removal with acid
or hydroxylamine proceeded in good yield with a small, but
inconsequential, amount of epimerization at the acetal center. This
practical approach was applied to the synthesis of monomers for
common â-peptides on multigram scales (Chart 1).14
After an initial screen of monomer classes suitable for the
iterative synthesis of â-oligopeptides, we were pleased to find that
isoxazolidine acetals 1a-f could be readily prepared in enantio-
merically pure form and were exceptional substrates in peptide
forming reactions with R-ketoacids. Their synthesis takes advantage
of Vasella’s diastereoselective nitrone cycloaddition with carbo-
hydrate-derived chiral auxiliaries,10,11 a protocol that was easily
extended to cycloaddition reactions with methyl 2-methoxyacrylate
(3).12 Cycloadditions occurred with high regio- and diastereose-
Isoxazolidines 1a-f undergo facile decarboxylative peptide
couplings with R-ketoacids (Table 1). Our initial efforts with
isoxazolidine 1f revealed an interesting solvent effect on the amide
formation. Surprisingly, DMF, which was the ideal solvent for the
coupling of O-unsubstituted hydroxylamines with R-ketoacids,9 was
unproductive. In contrast, nonpolar solvents, including CH2Cl2 and
toluene, gave the desired amides in good yield. Protic solvents were
also effective, and water was particularly beneficial for the reactions.
Table 1.
Scheme 1. Iterative Synthesis of â-Oligopeptides by
Ketoacid-Isoxazolidine Peptide Formations in Water
entry
R1
Isox.
conditions
time (h)
yielda (%)
1
2
3
4
5
6
7
8
Bn
Bn
Bn
Bn
Bn
Me
Me
Me
1f
1f
1f
1f
1f
1a
1a
1a
DMF (0.1 M)
21
21
21
21
21
8
nr
75
82
73
92
92
72
93
toluene (0.1 M)
CH2Cl2 (0.1 M)
pH 5 Ac buffer (0.1 M)b
1:1 tBuOH/H2O (0.1 M)
1:1 tBuOH/H2O (0.2 M)
1:1 tBuOH/H2O (0.2 M), rt
1:1 tBuOH/H2O (0.5 M)
8
1
a Yield of pure product following chromatography. b Heterogeneous
reaction mixture.
9
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J. AM. CHEM. SOC. 2006, 128, 1452-1453
10.1021/ja057706j CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
From these studies, we selected a 1:1 mixture of tert-BuOH/H2O
for reasons of substrate solubility and reactivity. At higher
concentrations (0.5 M), the reactions were done within minutes and
proceeded with visible loss of carbon dioxide. The isoxazolidines
also coupled with R-ketoacids at room temperature with syntheti-
cally useful reaction rates (entry 7).
The ketoester products could be directly converted to the
corresponding carboxylic acids by oxidative decarboxylation in the
presence of basic hydrogen peroxide (eq 3).
The amide formation is chemoselective. Unprotected glutamic
acid substrate 8 reacted cleanly with pyruvic acid to give amino
acid derivative 9 in excellent yield (eq 1). In the context of solution
phase synthesis, reactions with unprotected amines were most
conveniently performed in MeOH, which upon removal of the
solvent afforded the corresponding amide-acetals (eq 2).
The ketoacid-hydroxylamine peptide ligation uses a unique set
of functional groups to achieve chemoselective amide bond forma-
tion in the presence of unprotected functionalities and without re-
agents. Further applications, including the identification of new mono-
mers suitable for other amide-based targets, will contribute to new
methods for the preparation of peptidic structures and materials.
Acknowledgment. This work was supported by the University
of California, a Camille and Henry Dreyfus New Faculty Award
(to J.W.B.), the donors of the Petroleum Research Fund (admin-
istered by the American Chemical Society), and the Society of
Japanese Pharmacopeia. E.A.D. was a 2005 Pfizer AIR Minority
Summer Fellow. We are grateful to Chris Day and Kenneth Chow
for preliminary studies.
The products of the reaction of isoxazolidines with ketoacids
were methyl R-ketoesters, which were easily saponified to R-ke-
toacids, and the peptide chain was extended by reaction with another
isoxazolidine (Scheme 1). This two-step iteration could be carried
out without purification or isolation of the intermediate acid. Using
this approach, we have prepared a number of di- and tripeptides
(Table 2). While there are some decreases in coupling rates as larger
molecules and lower concentrations are used, in all cases, the pep-
tide couplings proceed cleanly. For preliminary studies, we em-
ployed side-chain-protected substrates and isolated the intermediate
peptides. Less than optimal yields for some substrates, especially
those containing âh-valine residues, reflect the insolubility of the
â-oligopeptides during workup and silica gel chromatography and
not limitations of the amide formation step. Further studies employ-
ing solid phase synthesis and optimization of protocols for multiple,
iterative couplings without isolating intermediates are in progress.
Supporting Information Available: Experimental procedures and
characterization data for all compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
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Table 2. Selected â-Oligopeptides Prepared by Isoxazolidine
Couplings (see Scheme 1)
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(11) For other recent applications of carbohydrate-derived chiral auxiliaries
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(13) Enantiomeric purities and absolute configurations were confirmed by
conversion to known compounds and by chiral HPLC studies.
(14) For this study, we utilized a D-mannose-derived chiral auxiliary that
afforded the “unnatural” peptide enantiomers. We have also prepared the
“natural” enantiomers by using a chiral auxiliary derived from D-gulose.
Further studies on the syntheses of these and other isoxazolidine monomers
will be reported separately.
a Overall yield of pure, isolated product for (i) ketoester hydrolysis, (ii)
isoxazolidine coupling, and (iii) purification. See Supporting Information
for details.
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