Orthogonally protected thiazole and isoxazole diamino acids: An efficient synthetic route

Article abstract of DOI:10.1002/chem.201001492

An efficient strategy has been developed for the synthesis of heteroaromatic amino acids (HAAs). These methods generate mono-or orthogonally protected diamino acids from bamino acids (see scheme). Their synthetic reliability and biological potential was d

Full text of DOI:10.1002/chem.201001492

DOI: 10.1002/chem.201001492
Orthogonally Protected Thiazole and Isoxazole Diamino Acids:
An Efficient Synthetic Route
Jeffrey D. Butler, Keith C. Coffman, Kristin T. Ziebart, Michael D. Toney, and
Mark J. Kurth*[a]
Heterocyclic and heteroaromatic amino acids (HAAs) are
central to the motifs of peptide antibiotics, including micro-
cin B17, nostocyclamide, telomestatin, and thiostrepton.[1] a-
Amino acids undergo cyclization and oxidation to form het-
eroaromatic rings, notably, thiazoles, oxazoles, indoles, and
pyridines, which give rise to well-documented antibiotic ac-
tivity.[1] Few of these targets have succumbed to total synthe-
sis due, in large part, to the demand for orthogonally pro-
tected HAA building blocks.[1b] In contrast, commercial or-
thogonally protected natural amino acids, most commonly
lysine and aspartic acid, are routinely used as the branch
point in the synthesis of branched or cyclic peptide and oli-
gosaccharide mimetics[2–6] (Figure 1a). Similarly, these
agents see action in the ligation of imaging agents (Fig-
ure 1b) and in diversity-oriented syntheses (e.g., I!II, Fig-
ure 1c).[7,8] However, the stringent orthogonal chemistry re-
quirements, especially in solid-phase synthesis, make optimi-
zation at this branch-point region challenging.
Surprisingly, methods to generate new heterocyclic non-
natural amino acids with an additional orthogonally protect-
ed amino group (e.g., diamino acids), are still rare.[6,9d] Non-
natural conformationally restrictive amino acids have poten-
tial in the discovery of new peptidomimetics and in efforts
to improve the pharmacological and protease resistant prop-
erties of bioactive peptides.[1b,9] There is demand for practi-
cal HAA syntheses that deliver orthogonally protected dia-
mino acids compatible with the traditional solid and solution
phase 9-fluorenylmethoxycarbonyl (Fmoc) protection strat-
egy. Thus our focus herein is on the development of short,
high yielding syntheses delivering heteroaromatic mono-
and diamino acids from readily available starting materials.
Herein, we report an efficient synthesis yielding thiazole-
and isoxazole-based HAAs from b-amino acids. This strat-
egy allows for orthogonal carbamate protection that permits
independent synthetic manipulation (Figure 1). Further, the
viability of the synthesized HAAs as branch-point amino
acids is demonstrated in the solid-phase synthesis of an in-
hibitor of two chorismate utilizing enzymes, anthranilate
synthase (AS) and isochorismate synthase (IS). This inhibi-
tor shows two- and threefold better activity than its lysine
predecessor in the inhibition of AS and IS, respectively.
A wide variety of b-amino acids are commercially avail-
able and considerable synthetic effort has been focused on
producing novel optically active b-amino acids.[10] This avail-
ability makes b-amino acids an attractive starting material
for this work. As outlined in Figure 2, our synthetic method
began by carbamate protection (Teoc, Boc, Cbz, and Alloc)
of b-alanine following literature procedures.[11] These pro-
tected acids were subjected to coupling conditions to install
the Meldrum acid moiety in 94–98% yield. Intramolecular
cyclization of 1a–d!2a–d is accomplished quantitatively in
EtOAc at reflux via a presumed ketene intermediate.[12] In a
modification of Suzukiꢀs general method of cyclocondensa-
Figure 1. Common motifs and methods employing an orthogonally pro-
tected diamino acid, which include a) branched peptides, b) ligated imag-
ing agents, and c) diversity-oriented methods.
[a] Dr. J. D. Butler, K. C. Coffman, Dr. K. T. Ziebart, Prof. M. D. Toney,
Prof. M. J. Kurth
Department of Chemistry, University of California, Davis
One Shields Avenue, Davis, CA 95616 (USA)
Fax : (+1)530-752-8995
E-mail: mjkurth@ucdavis.edu
Homepage: http://chemgroups.ucdavis.edu/~kurth/
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001492.
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COMMUNICATION
To deliver orthogonally protected thiazole HAAs, inter-
mediates 1a–c (Figure 4) were heated in methanol at reflux
to quantitatively deliver methyl esters 18a–c.[12b] These b-
Figure 2. General synthetic method of isoxazole-based amino acids.
Boc=tert-butyloxycarbonyl, Teoc=2-(trimethylsilyl)ethoxycarbonyl,
Cbz=carbobenzyloxy, Alloc=allyloxycarbonyl.
Figure 4. General synthetic method of thiazole-based orthogonally pro-
tected diamino acids.
tion,[13] compounds 2a–d were treated with strong base
(NaH) and a-chlorobenzaldo oximes to generate isoxazole
adducts 3–9 in 83–90% yield. These mixed imides (3–9)
were efficiently hydrolyzed to yield amino acids 3’–9’ in 72–
81% overall yield without carbamate deprotection.
Our approach to diaminoisoxazole and thiazole HAAs
began similarly to the monoamino acids of Figure 2. As de-
lineated in Figure 3, zinc reduction of intermediates 3, 8,
and 9 (when Y=p-NO2) delivered aniline analogues 10–12.
These anilines were carbamate protected with Alloc and
Cbz; subsequent imide hydrolysis delivered a collection of
orthogonally protected bis-carbamate isoxazole-based
HAAs (13–17) in 16–45% overall yield as tabulated in
Figure 3.
keto esters were a-chlorinated by using sulfuryl chloride
and, without purification, condensed with thiourea to give
thiazoles (19a–c) in two-step yields of 75–79%.[14] These 2-
amino thiazoles were carbamate protected to give orthogo-
nally protected esters that were subsequently saponified to
deliver a collection of thiazole-based orthogonally protected
diamino acids (20–28) as summarized in Figure 4 in an effi-
cient 48–61% overall yield. In addition, these methods have
been successful in preparing >5 g of 25 in comparable over-
all yield.
To explore how efficiently our diamino HAAs perform in
a typical branched synthesis, a solid-phase synthesis of a
staged inhibitor analogue (29) was undertaken, as depicted
in Figure 5. These inhibitors are of particular interest to our
group[8] and are an ideal test case for the employment of
these novel orthogonal diamino acids.
To synthesize 29, Rink amide resin was Fmoc deprotected
and N-Fmoc-3-chloro-l-phenylalanine was coupled by using
standard solid-phase peptide chemistry conditions. After
subsequent Fmoc deprotection, the Teoc/Alloc orthogonal
HAA 25 was coupled by using DIC and HOBt. Treatment
with TBAF in DMF removed the Teoc protecting group[11]
revealing a free amine that was subsequently coupled using
dehydrative conditions with 3-hydroxy-4-methyl-2-nitroben-
zoic acid. The Alloc protecting group was removed under
Pd0 conditions.[11] Finally, DIC- and HOBt-mediated cou-
pling of 3-[(tert-butoxycarbonyl)methoxy]benzoic acid com-
pleted the independent functionalization of both amines.
Resin cleavage and HPLC purification gave 29 in 91%
purity and 39% overall yield, which confirms that these or-
Figure 3. General synthetic method of isoxazole-based orthogonally pro-
tected diamino acids.
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M. J. Kurth et al.
starting materials has been realized. Our strategy delivers
orthogonally carbamate protected diamino HAAs that are
of value for diversity-oriented methods and allow for
branched peptide/imaging agent synthesis. Their viability as
branch-point amino acids has been successfully demonstrat-
ed in the solid-phase synthesis of an inhibitor (29) of anthra-
nilate synthase (AS) and isochorismate synthase (IS) with
improved potency over its lysine predecessor.
Acknowledgements
We thank the National Science Foundation (CHE-0910870) for generous
financial support.
Keywords: amino acids · inhibitors · isoxazoles · protecting
groups · thiazoles
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Orthogonally Protected Thiazole and Isoxazole Diamino Acids
COMMUNICATION
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Received: May 28, 2010
Published online: July 9, 2010
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