Chemoselective protection of α-Ketoacids by direct annulations with oximes

Article abstract of DOI:10.1021/ol100467t

Figure presented Oximes and α-ketoacids undergo an unexpectedly facile and chemoselective annulation to afford 2,5-dihydrooxazole 3-oxides. The resulting cyclic nitrones serve as chemically and configurationally stable masked α-ketoacids that can be easil

Full text of DOI:10.1021/ol100467t

ORGANIC
LETTERS
2010
Vol. 12, No. 9
1924-1927
Chemoselective Protection of r-Ketoacids
by Direct Annulations with Oximes
Melissa A. Flores and Jeffrey W. Bode*,†
Roy and Diana Vagelos Laboratories, Department of Chemistry, UniVersity of
PennsylVania, Philadelphia, PennsylVania 19104
bode@org.chem.ethz.ch
Received February 24, 2010
ABSTRACT
Oximes and r-ketoacids undergo an unexpectedly facile and chemoselective annulation to afford 2,5-dihydrooxazole 3-oxides. The resulting
cyclic nitrones serve as chemically and configurationally stable masked r-ketoacids that can be easily elaborated and manipulated. Deprotection
is achieved by mild reduction with zinc metal and hydrolysis. This methodology allows for the protection, elaboration, and deprotection of
enantiopure peptide derived r-ketoacids, which are the key starting materials for the chemoselective ketoacid-hydroxylamine peptide ligation.
R-Ketoacids are uniquely reactive functional groups utilized
in both biological processes and in synthetic organic chem-
istry.1 While maintaining some characteristics of both ketones
and carboxylic acids, they possess distinct reactivity that
limits the application of the well-known transformations of
the functional groups inherent to R-ketoacids. Unprotected
R-ketoacids are also labile toward decarboxylation, restricting
conditions for their synthesis, purification, and handling.2
Our recent discovery of the highly chemoselective amide-
forming ligation reaction of R-ketoacids and hydroxylamines
renews interest in synthetic methods for the preparation and
manipulation of R-ketoacids.3 Chemoselective amide liga-
tions are in great demand for the preparation of peptides,
proteins, and related structures but are currently limited to
very specific amino acid residues.4 Despite the potential
power of the ketoacid-hydroxylamine ligation for complex
molecule synthesis, limitations in the preparation and
handling of the requisite R-ketoacids and hydroxylamines
have slowed the adoption of this chemistry.5,6 A critical
inadequacy is the lack of suitable protecting groups for
R-ketoacids. Common strategies such as protection as esters
or amides promote epimerization or accentuate the electro-
philic properties of the ketone.7 In this report, we disclose
an unexpectedly simple strategy for the chemoselective
(3) (a) Bode, J. W.; Fox, R. M.; Baucom, K. D. Angew. Chem., Int. Ed.
2006, 45, 1248–1252. (b) Carrillo, N.; Davalos, E. A.; Russak, J. A.; Bode,
J. W. J. Am. Chem. Soc. 2006, 128, 1452–1453
.
(4) (a) Dawson, P. E.; Muir, T. W.; Clark-Lewis, I.; Kent, S. B. H.
Science 1994, 266, 776–779. (b) Hackenberger, C. P. R.; Schwarzer, D.
† Current address: Laboratorium fu¨r Organische Chemie, ETH-Zu¨rich,
Wolfgang Pauli Strasse 10, CH-8093 Zu¨rich.
Angew. Chem., Int. Ed. 2008, 47, 10030–10074.
(5) For recent work on the preparation of N-terminal peptide hydroxy-
(1) (a) Adang, A. E. P.; et al. J. Med. Chem. 2002, 45, 4419–4432. (b)
Li, Z.; Patil, G. S.; Golubski, Z. E.; Hori, H.; Tehrani, K.; Foreman, J. E.;
Eveleth, D. D.; Bartus, R. T.; Powers, J. C. J. Med. Chem. 1993, 36, 3472–
3480. (c) Malandrinos, G.; Louloudi, M.; Hadjiliadis, N. Chem. Soc. ReV.
2006, 35, 684–692.
lamines, see: Fukuzumi, T.; Bode, J. W. J. Am. Chem. Soc. 2009, 131,
3864–3865
.
(6) We have recently disclosed a sulfur ylide-based linker for the solid
phase synthesis of C-terminal R-ketoacid peptides. While this protocol works
well for many complex, unprotected peptides it is not appropriate for simpler
ketoacids or for peptides with cysteine or methionine residues: Ju, L.; Bode,
(2) Cooper, A. J. L.; Ginos, J. Z.; Meister, A. Chem. ReV. 1983, 83,
321–358.
J. W. Org. Biomol. Chem. 2009, 7, 2259
.
10.1021/ol100467t  2010 American Chemical Society
Published on Web 03/31/2010

protection of R-ketoacids via direct annulations with oximes
(eq 1). The resulting 2,5-dihydrooxazole 3-oxides are chemi-
cally and configurationally stable masked R-ketoacids that
can be deprotected under mild reductive conditions.
Scheme 1.
Annulations of Oximes and R-Ketoacidsa
Our studies began by considering strategies to simulta-
neously protect both the acid and ketone functionalities of
the R-ketoacid. Although certain cyclic imines have been
employed as precursors to R-ketoacids, we found such
compounds to be labile to most common purification methods
and chemical transformations.8 The corresponding N-oxides,
that is, cyclic nitrones, were known to be stable compounds,
but the reported methods for their preparation relied on
cycloadditions of nitrosoketenes and ketones, a protocol that
was limited to the preparation of glyoxylic acid derivatives.9
Furthermore, we were primarily interested in identifying a
method for the protection of an existing R-ketoacid rather
than a de novo synthesis of this functionality.
In the course of our investigations of the properties and
reactions of R-ketoacids, we were therefore surprised to find
that R-ketoacids and aliphatic oximes undergo a spontaneous,
chemoselective annulation to afford 2,5-dihydrooxazole
3-oxides in good yields under a variety of conditions.10 The
structure of the annulation product was confirmed by single
crystal X-ray analysis of 8. The reaction of oximes and
R-ketocids under aqueous conditions has been previously
studied as an effective method for oxime hydrolysis, but no
reports of this annulation have appeared.11,12 A screen of
reaction parameters identified nonpolar solvents as preferred,
with reaction temperatures ranging from room temperature
to 40 °C. Oximes derived from either aliphatic aldehydes or
aliphatic ketones were suitable for annulations with a variety
of R-ketoacids (Scheme 1). The reaction usually failed for
oximes adjacent to an sp2 hybridized carbon, such as those
derived from benzaldehyde or cinnamaldehyde (not shown).
Bulky groups on the oxime did not significantly retard the
reaction, while R-ketoacids containing ꢀ-branching coupled
more slowly but were still viable substrates.13
a All reactions were preformed at 0.2 M CH2Cl2 for 12-16 h. Yields refer
to mass yields of isolated, pure products b Reaction performed at 40 °C.
A key feature of the annulation of R-ketoacids and oximes
is the chemoselectivity; selective protection of R-ketoacids
in the presence of unprotected carboxylic acids is possible
(Scheme 2). This enables the transient protection and
Scheme 2
.
Chemoselective Annulation, Elaboration, and
Deprotection of R-Ketoacids
(7) Brady, S. F.; Sisko, J. T.; Stauffer, K. J.; Colton, C. D.; Qiu, H.;
Lewis, S. D.; Ng, A. S.; Shafer, J. A.; Bogusky, M. J.; Veber, D. F.; Nutt,
R. F. Bioorg. Med. Chem. 1995, 3, 1063–1078.
(8) Barnish, I. T.; Cross, P. E.; Danilewicz, J. C.; Dickinson, R. P.;
Stopher, D. A. J. Med. Chem. 1981, 24, 399–404.
(9) (a) Katagiri, N.; Sato, H.; Kurimoto, A.; Okada, M.; Yamada, A.;
Kaneko, C. J. Org. Chem. 1994, 59, 8101–8106. (b) Katagiri, N.; Okada,
M.; Kaneko, C.; Furuya, T. Tetrahedron Lett. 1996, 37, 1801–1804. (c)
Baldwin, S. W.; Long, A. Org. Lett. 2004, 6, 1653–1656.
(10) Afagh, N. A.; Yudin, A. K. Angew. Chem., Int. Ed. 2010, 49, 262–
310.
(11) (a) Hershberg, E. B. J. Org. Chem. 1948, 13, 542–546. (b) Kim,
J. N.; Ryu, E. K. Bull. Korean Chem. Soc. 1992, 13, 184–187
.
(12) We have confirmed that hydrolysis of the oxime occurs upon
treatment with an a-ketoacid under aqueous conditions (tBuOH/H2O, rt).
We do not detect the 2,5-dihydrooxazole 3-oxide as an intermediate.
(13) The use of additives such as Lewis acids or dehydrating reagents
did not significantly improve yield.
elaboration of bifunctional R-ketoacids, a finding that will
extend the utility of the ketoacid-hydroxylamine amide-
forming ligation.
Org. Lett., Vol. 12, No. 9, 2010
1925

The cyclic nitrones derived from the annulation proved
remarkably inert. Unlike other nitrones, they do not undergo
cycloadditions even under forcing conditions or with highly
reactive dipolarophiles.14 They are to some extent hydro-
lytically unstable; prolonged exposure to aqueous acid or
base results in release of the oxime-derived aldehyde along
with formation of an R-oximino acid. They readily survive
standard laboratory operations including aqueous workup,
purification by column chromatography, reverse phase-
HPLC, and common organic transformations including amide
couplings, and deprotections of acids and amines. They resist
reduction with reagents such as NaCNBH3 but are readily
deoxygenated at room temperature with zinc metal in
aqueous NH4Cl.15,16 The resulting cyclic imine can be
hydrolyzed to the unprotected R-ketoacid. In the case of
nitrone 13, deprotection results in an intermediate ketoacid
that readily cyclizes and exhibits ring chain tautomerization.17
Scheme 3. Stereoretentive Protection of R-Peptide Ketoacids
An outstanding need for the chemoselective protection and
deprotection of R-ketoacids is in the application of the
ketoacid-hydroxylamine ligation to the synthesis of R-oli-
gopeptides. We have reported a stereoretentive method for
the preparation of C-terminal peptide R-ketoacids by oxida-
tion of cyanosulfur ylides.18 This method provides a reliable
route to enantiomerically pure R-ketoacids and longer
peptides, but has a number of limitations including an
incompatibility with sulfur containing side chains and the
potential for overoxidation of the R-ketoacid if the reaction
is not carefully monitored.
thionine-containing peptide 19b, no interference from the
thioether was detected at any stage.
A probable reaction pathway would involve either nu-
cleophilic attack of the oxime nitrogen onto the R-ketoacid
ketone (path a) or by the carboxylate of the R-ketoacid onto
the oxime carbon (path b). Either process would be followed
Anticipating the utility of the 2,5-dihydrooxazole 3-oxides
as protecting groups for the preparation and elaboration of
R-peptide ketoacids, we performed the following studies.
Fmoc-Alanine R-ketoacid was prepared using the sulfur ylide
method and annulated with the oxime derived from cyclo-
hexanone. We elected to employ an oxime that would not
give diastereomeric annulation products so that we could
rigorously establish the stereochemical integrity of the amino
acid during the protection, deprotection, and elaboration
steps. Analysis of annulation product 18 by SFC on chiral
columns demonstrated that the annulation proceeded without
epimerization. Subsequent Fmoc deprotection and peptide
coupling with either phenylalanine or methionine were
achieved without difficulty giving dipeptide nitrone 19a and
19b, respectively. Reduction with zinc metal afforded the
oxazolone in good yield; acid hydrolysis revealed the
R-ketoacid poised for amide bond formation with a dipeptide
N-alkylhydroxylamine. The ligation product 21a was found
to be 99:1 dr at the relevant stereocenter, confirming that
epimerization does not occur during the protection, manipu-
lation, deprotection, or ligation steps. In the case of me-
Scheme 4. Stereoretentive Protection of R-Peptide Ketoacids
(14) Eguchi, S.; Ohno, M.; Yu, Y. J. Chem. Soc., Chem. Commun. 1994,
3, 331–332.
(15) Aoyagi, Y.; Abe, T.; Ohta, A. Synthesis 1997, 891–894
(16) For similar deprotection conditions on a side-chain unprotected
peptide, see: Veber, D. F.; Paleveda, W. J., Jr.; Lee, Y. C.; Hirschmann, R.
.
J. Org. Chem. 1977, 42, 3286–3288
.
(17) Despite the ring-chain tautomerization, this R-ketoacid may be used
for decarboxylative condensation with an N-alkylhydroxylamine. See
Supporting Information.
(18) Ju, L.; Lippert, A. R.; Bode, J. W. J. Am. Chem. Soc. 2008, 130,
4253–4255.
1926
Org. Lett., Vol. 12, No. 9, 2010

to nucleophilic attack and are considerably less basic than
other amine derivatives, also making path b unlikely.19
In summary, we have disclosed a novel, chemoselective
annulation of oximes and R-ketoacids that proceeds without
reagents or catalysts and affords synthetically valuable 2,5-
dihydrooxazole 3-oxides. We have exploited this finding for
the development of a selective method for the protection and
deprotection of R-ketoacids, which will find utility in the
preparation of substrates for the ketoacid-hydroxylamine
amide-forming ligation. Efforts to extend these findings to
the solid-phase synthesis of peptide-ketoacids without the
need for our previously reported late stage oxidation are
currently underway.
Scheme 5. Postulated Reaction Pathways
Acknowledgment. This work was supported by the NIH
NIGMS [R01-GM076320]. JWB is a fellow of the Beckman
Foundation, the Packard Foundation, the Sloan Foundation,
and a Research Corporation Cottrell Scholar. Unrestricted
support from Amgen, Bristol Myers Squibb, and Roche is
appreciated.
by ring closure and elimination of water to give the final
2,5-dihydrooxazole 3-oxide. (Scheme 5). Our initial efforts
to distinguish these two pathways have proven inconclusive;
however we currently favor path a based on several observa-
tions. Oximes derived from benzaldehyde, cinnamaldehyde,
or any other aldehyde with an adjacent sp2 hydridized carbon
fail to give the annulation product, probably due to the
reduced nucleophilicity of the oxime nitrogen in the conju-
gated system. Likewise, oximes are known to be resistant
Supporting Information Available: Experimental pro-
cedures and characterization data for all compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL100467T
(19) Kalia, J.; Raines, R. T. Angew. Chem., Int. Ed. 2008, 47, 7523–
7526.
Org. Lett., Vol. 12, No. 9, 2010
1927