Synthesis of an enantiopure isoxazolidine monomer for β3-aspartic acid in chemoselective β-oligopeptide synthesis

Article abstract of DOI:10.1016/j.tetlet.2009.02.045

The synthesis of an enantiopure isoxazolidine monomer for the incorporation of β³-aspartic acid residues into β³-oligopeptides via chemoselective α-ketoacid-hydroxylamine amide formation is disclosed. This route involves nitrone cycl

Full text of DOI:10.1016/j.tetlet.2009.02.045

Tetrahedron Letters 50 (2009) 3258–3260
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Tetrahedron Letters
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Synthesis of an enantiopure isoxazolidine monomer for b3-aspartic acid
in chemoselective b-oligopeptide synthesis
*
Hiroshi Ishida, Nancy Carrillo, Jeffrey W. Bode
Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, United States
a r t i c l e i n f o
a b s t r a c t
The synthesis of an enantiopure isoxazolidine monomer for the incorporation of b3-aspartic acid residues
a-ketoacid–hydroxylamine amide formation is disclosed. This
route involves nitrone cycloaddition of 3-thiophenylpropanal and circumvents limitations of other
Article history:
into b3-oligopeptides via chemoselective
Received 22 January 2009
Revised 4 February 2009
Accepted 5 February 2009
Available online 11 February 2009
potential starting materials.
Ó 2009 Elsevier Ltd. All rights reserved.
O
R1
O
We have recently reported a highly chemoselective amide-bond
forming reaction between
addition to its potential for the fragment coupling of unprotected
HN
R2
O
OMe
OMe
a
-ketoacids and hydroxylamines.1 In
OH
+
N
H
O
O
1) H2O, no reagents
a
-peptide fragments, we have explored its application to the iter-
ative, aqueous synthesis of oligo-b3-peptides,2 an exciting class of
peptidomimetics of contemporary interest in medicinal and bioor-
ganic chemistry.3
2) LiOH
O
R1
O
R2
O
HN
R3
O
OMe
OMe
OH
R2
+
N
H
N
H
Our novel approach to the synthesis of these oligopeptides
takes advantage of the chemoselective coupling of a growing b-
peptide chain with chiral isoxazolidines. The amide formation re-
O
O
1) H2O, no reagents
2) LiOH
sults in N–O bond cleavage and the formation of an
a-ketoester.
O
R1
O
O
R3
O
Following hydrolysis, the chain is poised for iterative elongation
(Scheme 1). The key amide forming reaction requires no reagents,
produces only carbon dioxide and methanol as byproducts
(Scheme 2), and operates in the presence of unprotected functional
groups including amines and carboxylic acids. As such, it offers
great potential as an alternative route to the preparation of b-oligo-
peptides that circumvents challenges of the established methods
including difficult couplings and deprotections in longer b-peptide
chains and improved access to the requisite b-peptide monomers.
The major challenge to the widespread adoption of this method
for b3-peptide synthesis is the need to prepare the isoxazolidine
monomers as single enantiomers. We have reported the use of
Vasella’s mannose-derived nitrones4 as chiral auxiliaries for their
asymmetric synthesis via 1,3-dipolar cycloaddition with 2-meth-
oxymethacrylate (Scheme 3). Following purification of the cycload-
ducts and removal of the auxiliary, the desired isoxazolidine
monomers are obtained, in most cases, in enantiomerically pure
form. This approach has proven successful for the synthesis of isox-
azolidine monomers bearing most of the common amino acid side-
chains, including those found in leucine, valine, phenylalanine, ly-
sine, glutamic acid, glycine, and numerous others. Unfortunately,
this procedure fails for the most logical synthetic approaches to
OH
N
H
N
H
N
H
O
Scheme 1. Synthesis of b3-oligopeptides by iterative, aqueous synthesis via
a-
ketoacid–isoxazolidine couplings.
O
O
R1
NH
O
HN
R2
O
OMe
OMe
OH
R1
+
OMe
R2
O
O
O
OH
OH
HO
R1
R1
N
O
O
N
O
OMe
OMe
OMe
R2
R2
O
O
Scheme 2. Reaction pathway for chemoselective amide formation.
the monomer containing the side chain found in aspartic acid. This
is particularly disappointing as some of the most exciting advances
in the properties of longer b3-oligopeptides are with sequences rich
in this amino acid residue.5
* Corresponding author. Tel./fax: +1 215 573 1953.
E-mail address: bode@sas.upenn.edu (J.W. Bode).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.045

H. Ishida et al. / Tetrahedron Letters 50 (2009) 3258–3260
3259
Me Me
logue of our target, we successfully employed an omega-tert-
butylester aldehyde in the nitrone cycloaddition. In the case of
aspartic acid, however, the necessary aldehyde was difficult to pre-
pare and employ due to its propensity to adopt the enol form
(Scheme 4). The obvious alternatives, protected-b-hydroxyaldehy-
des, could be prepared but underwent elimination or other unpro-
ductive pathways during the attempted cycloaddition step. Brief
attempts to employ acrolein as a starting aldehyde were not ini-
tially successful.
In contrast to the b-oxo-substituted counterparts, we were
pleased to find that b-mercaptoaldehydes were easily prepared6
and suitable for use in the chiral auxiliary-directed cycloaddition
(Scheme 5). Importantly, cycloadduct 6 could be obtained as a sin-
gle enantiomer following purification by column chromatography
and recrystallization. Although the overall yield for the cycloaddi-
tion and subsequent removal of the minor diastereomer were
modest, the two-step sequence from acrolein could be easily exe-
cuted on a preparative scale.
O
O
O
D-Mannose
+
R
H
NHOH
(2 steps)
O
O
O
Me
1
Me
OMe
CO2Me
via:
toluene
O
R
D-Mannose
90-120 o
12-24 h
C
N
2
H
Me Me
O
O
O
O
aq. HClO4
or NH2OH
O
HN
O
N
O
O
OMe
O
OMe
OMe
R
Me
OMe
R
Me
20-60% yield
In order to convert the sulfide of 6 to the desired carboxylic
acid, we effected a Pummerer oxidation by oxidation of the sulfide
to the sulfoxide followed by treatment with trifluoroacetic anhy-
dride and workup in the presence of mercury chloride to afford
aldehyde 8 in 95% overall yield.7 This aldehyde is, in itself, a valu-
able scaffold for the preparation of unnatural b-amino acid side
chains. It can also be readily oxidized to carboxylic acid 9 under
standard conditions and in excellent yield.8
Deprotection of the sugar can be pursued at this stage of the
synthesis, but we found it advantageous in terms of handling and
purity of the final product to first protect the carboxylic acid side
chain. A number of standard protocols for tert-butyl ester forma-
tion failed, but we found success with tert-butyl N,N0-diisopropyli-
sourea.9 Removal of the chiral auxiliary was best effected with
aqueous hydrazine to afford 11.
Scheme 3. General approach to the synthesis of enantiopure isoxazolidine mono-
mers via nitrone cycloaddition.
O
O
O
O
HN
O
O
H
OtBu
H
O
OMe
OMe
O
HO
3
H
OTBS
H
OBn
Scheme 4. Target aspartic acid monomer
3 and aldehyde starting materials
deemed unsuitable for its preparation via nitrone cycloaddition.
In this Letter, we document a versatile workaround to the syn-
thesis of an enantiopure isoxazolidine monomer 3 containing the
aspartic acid side chain. This approach is high yielding, readily exe-
cuted on a preparative scale, and offers an entry into isoxazolidine
monomers bearing functionality poised for further elaboration.
Our studies began by considering the most direct methods to
access the aspartic acid-derived monomer 3. In our previous syn-
thesis of the glutamic acid side chain, which is a one carbon homo-
Although 11 serves as a suitable monomer for b3-oligopeptide
synthesis, we have often found it advantageous to employ the side
chain-unprotected monomers for our ongoing studies on the appli-
cation of these reagents for aqueous peptide synthesis. The tert-bu-
tyl group of 11 can be easily removed by treatment with
trifluoroacetic acid, providing TFA salt 3, which can be used di-
rectly in the amide forming reaction with a-ketoacids .
Me Me
O
O
Me Me
O
Me Me
O
O
O
NHOH
1 (1.0 equiv.)
O
O
O
Me
O
O
O
O
PhSH, cat. Et3N
oC, 30 min
99% yield
MCPBA
Me
N
O
N
O
O
O
O
O
PhS
O
O
OMe
OMe
OMe
-20 oC, 30 min
Me
Me
0
OMe
OMe
Me
Me
4
5 (1.2 equiv.)
6
7
97% yield
PhS
PhS
CO2Me
O
2 (1.2 equiv.)
120 oC, 12 h, 32% yield
Me Me
Me Me
Me Me
OtBu
O
O
O
O
O
O
1) TFAA, 2,6-lutidine
0 oC, 20 min
iPrN
NHiPr
NaClO2, NaH2PO4
O
O
O
O
O
O
N
H
O
N
O
O
N
O
O
O
O
O
2-methyl-2-butene
0 oC, 20 min
2) HgCl2, H2O
r.t. 2 h
O
O
O
r.t. 10 h
58% yield
OMe
OMe
OMe
OMe
OMe
OMe
Me
Me
Me
Me
Me
Me
O
HO
tBuO
10
95% yield
96% yield
8
9
O
O
NH2NH2 H2SO4
NaOAc
HN
O
TFA H2N
O
50% TFA
oC, 3 h
99% yield
OMe
OMe
OMe
OMe
MeOH/H2O (3/1)
70 oC, 20 h
0
tBuO
O
HO
O
11
3
66% yield
Scheme 5. Syntheis of aspartic acid side chain isoxazoline monomer 3.

3260
H. Ishida et al. / Tetrahedron Letters 50 (2009) 3258–3260
O
Acknowledgments
1:1
O
O
tBuOH/
Ph
HN
O
O
NH
O
O
pH 7.4 buffer
Ph
OH
This work was supported by the National Institutes of Health
(R01GM076320 and F31GM078854) and the Arnold and Mabel
Beckman Foundation. We are grateful to Kyowa Hakko for support
of H.I. Acrylate 2 was kindly prepared by Bioblocks, Inc. J.W.B is a
fellow of the Packard Foundation, Sloan Foundation, and a Re-
search Corporation Cottrell Scholar. We appreciate insightful ad-
vice and synthetic protocols from Justin Russak.
OMe
OMe
+
OMe
40 °C, 20 h
O
O
tBuO
48% yield
tBuO
12
11
13
(ee = 99%)
Scheme 6. Amide-forming ligation of 11 and confirmation of enantiopurity. A small
amount of ent-11 was prepared using a different chiral auxiliary in order to analyze
the enantiopurity by HPLC on chiral columns.
Supplementary data
Supplementary data (experimental procedures, characteriza-
tion data, and 1H and 13C NMR spectra for all new compounds)
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2009.02.045.
1H and 13C NMR analyses of intermediates 6 and 10 showed
only a single diastereomer, suggesting that the final isoxazolidine
would be obtained as a single enantiomer. To confirm and better
quantify the enantiopurity, we first subjected 11 to amide forma-
t
References and notes
tion with phenylpyruvic acid in 1:1 BuOH/pH 7.4 buffer to afford
a
-ketoester 13 (Scheme 6). Analysis of this material by HPLC on
1. Bode, J. W.; Fox, R. M.; Baucom, K. D. Angew. Chem., Int. Ed. 2006, 45, 1248–1252.
2. Carrillo, N.; Davalos, E. A.; Russak, J. A.; Bode, J. W. J. Am. Chem. Soc. 2006, 128,
1452–1453.
3. For reviews of the chemistry and biology of b-oligopeptides, see: (a) Seebach, D.;
Beck, A. K.; Bierbaum, D. J. Chem. Biodiv. 2004, 1, 1111–1239; (b) Chang, P. R.;
Gellman, S. H.; DeGrado, W. F. Chem. Rev. 2001, 101, 3219–3232; (c) Goodman,
C. M.; Choi, S.; Shandler, S.; Degrado, W. F. Nat. Chem. Biol. 2007, 3, 252–262.
4. Vasella, A. Helv. Chim. Acta 1977, 60, 1273–1295.
5. Petersson, E. J.; Schepartz, A. J. Am. Chem. Soc. 2008, 130, 821–823.
6. Kim, Y.; Fuchs, P. L. Org. Lett. 2007, 9, 2445–2448.
7. Nakamura, R.; Tanino, K.; Miyashita, M. Org. Lett. 2003, 5, 3579–3582.
8. Overman, L. E.; Paone, D. V. J. Am. Chem. Soc. 2001, 123, 9465–9467.
9. West, K. R.; Bake, K. D.; Otto, S. Org. Lett. 2005, 7, 2615–2618.
a chiral column established that 11 and by analogy 10 and 3 were
obtained in >99% ee.
In summary, we have disclosed an effective synthetic route to
isoxazolidine monomer 3, which allows incorporation of a b3-
aspartic acid residue into a growing peptide chain via the keto-
acid–hydroxylamine amide ligation. In addition to providing ac-
cess to an important monomer, this route will allow access to
other important monomers including asparagine, methionine,
and unnatural side chains derived from
intermediates.
3 or its synthetic